Differential Diagnosis of Mendelian and Mitochondrial Disorders in Patients With Suspected Multiple Sclerosis

James D. Weisfeld-Adams; Ilana B. Katz Sand; Justin M. Honce; Fred D. Lublin

Disclosures

Brain. 2015;138(3):517-539. 

In This Article

Abstract and Introduction

Abstract

Several single gene disorders share clinical and radiologic characteristics with multiple sclerosis and have the potential to be overlooked in the differential diagnostic evaluation of both adult and paediatric patients with multiple sclerosis. This group includes lysosomal storage disorders, various mitochondrial diseases, other neurometabolic disorders, and several other miscellaneous disorders. Recognition of a single-gene disorder as causal for a patient's 'multiple sclerosis-like' phenotype is critically important for accurate direction of patient management, and evokes broader genetic counselling implications for affected families. Here we review single gene disorders that have the potential to mimic multiple sclerosis, provide an overview of clinical and investigational characteristics of each disorder, and present guidelines for when clinicians should suspect an underlying heritable disorder that requires diagnostic confirmation in a patient with a definite or probable diagnosis of multiple sclerosis.

Introduction

Multiple sclerosis is a common, chronic demyelinating neurological disease primarily affecting young adults, with a prevalence of ~0.1% in the Caucasian population (Miller and Leary, 2007). Early, accurate diagnosis is critical to effective patient management and counselling, but assignment of an incorrect diagnosis of multiple sclerosis remains a frequent concern.

Patients with multiple sclerosis are classified according to their clinical phenotype, with ~85% following a relapsing-remitting course (relapsing-remitting multiple sclerosis) characterized by recurrent, acute neurological deficits punctuating periods of latency or remission (Lublin and Reingold, 1996). Typical symptoms of relapses may be referable to demyelinating pathology involving the optic nerves (e.g. optic neuritis), brainstem (e.g. internuclear ophthalmoplegia) or spinal cord (e.g. partial myelitis), although non-specific symptoms referable to the cerebral hemispheres or other brain regions can also occur (Katz Sand and Lublin, 2013). Approximately 15% of patients follow a primary progressive or progressive relapsing course from disease onset, usually characterized by symptoms of progressive myelopathy (gait instability, spasticity, bladder symptoms) and cognitive impairment. Approximately half of relapsing-remitting patients develop a secondary progressive course within 20 years of disease onset (secondary progressive multiple sclerosis; Tremlett et al., 2008).

McDonald Criteria (Polman et al., 2011) for multiple sclerosis integrate data from neurological history, physical examination, and MRI appearances of the brain and cord. Diagnostic criteria and classification of multiple sclerosis subtypes have evolved in recent decades, and, although successive versions have differed in emphasis, all have required dissemination of disease in space (requiring involvement of multiple areas of the CNS) and in time (requiring ongoing disease activity over time). Unmatched oligoclonal bands are observed in the CSF of >80% of patients with multiple sclerosis and may be considered as supporting evidence, but are neither completely sensitive nor specific for multiple sclerosis. In the absence of pathognomonic clinical findings or a definitive laboratory test, the diagnosis of multiple sclerosis remains challenging in many patients, and diagnostic criteria emphasize the caveat of 'no better explanation' for a patient's clinical presentation and MRI findings. If the presentation is atypical, alternative diagnoses should be explored and investigated, and patients subjected to a diligent search for clinical, imaging or laboratory 'red flags'; in 2008, a scheme and 'red flag' scoring instrument was presented as a tool to be used in the differential diagnosis of multiple sclerosis (Miller et al., 2008). Patients with adolescent or adult-onset inherited disorders, as well as those with non-multiple sclerosis idiopathic inflammatory demyelinating disease entities such as neuromyelitis optica and acute disseminated encephalomyelitis, or infectious, neoplastic, or vascular disorders, can all present similarly to multiple sclerosis, resulting in misdiagnoses (Miller et al., 2008; Rudick and Miller, 2012).

The spectrum of disorders mistaken for multiple sclerosis have likely changed over time as a result of evolution of diagnostic criteria for multiple sclerosis (Solomon et al., 2012). These disorders share characteristics with multiple sclerosis but have quite different clinical courses and prognostic implications. The incidence of multiple sclerosis misdiagnosis probably varies between individual clinicians and institutions, and depends upon efforts used to investigate for other disorders in patients with 'multiple sclerosis-like' presentations (Engell, 1988; Solomon and Weinshenker, 2013). In a recent editorial concerned with multiple sclerosis misdiagnosis, the authors suggested that multiple sclerosis diagnosis is based on symptoms, signs and MRI findings, clinical judgement, experience, and, crucially, the capacity needed to identify an alternative diagnosis (Rudick and Miller, 2012). They also discussed over-reliance on MRI appearances in multiple sclerosis diagnosis, as well as the phenomenon of 'cessation of critical thinking about alternative diagnoses once the multiple sclerosis diagnosis has been made'.

Multiple historical case reports describe clinical and diagnostic confusion between multiple sclerosis and various single gene disorders. Diagnosis of some of these rare disorders is likely to be subject to ascertainment bias, with classic presentations occurring during early childhood, and later-onset variants going unrecognized and conferring more subtle presentations. Several such disorders can present in adolescence or adulthood with progressive spastic paraparesis and accompanying white matter lesions on MRI, mimicking primary progressive multiple sclerosis. Less commonly patients with such disorders can display relapsing-remitting or insidiously progressive neurological symptoms. The overlap may at times result in misdiagnosis of multiple sclerosis, ultimately leading to inappropriate treatment with immunomodulatory therapies and, in certain disorders, a missed opportunity for other potentially helpful management interventions.

No comprehensive review of single-gene aetiologies of 'multiple sclerosis-like' presentations has been written since that by Natowicz and Bejjani (1994), 20 years ago. Since then, neuroimaging technologies have evolved unrecognizably, and have brought an abundance of knowledge regarding MRI characteristics in multiple sclerosis and other conditions. In the same period, the molecular genetic and/or biochemical basis of many 'multiple sclerosis mimics' has been elucidated, and much has been learned about the phenotypic spectrum of these disorders. Most inherited mimics of multiple sclerosis are now amenable to molecular genetic testing of specific genes, or interrogation of multiple genes using next-generation, whole exome, or whole genome sequencing. The current article, based on an extensive review of the literature of cases of Mendelian diseases misdiagnosed as multiple sclerosis, and the authors' own clinical experiences, presents a scheme for neurologists and medical geneticists to follow for differential diagnosis of several single-gene disorders that may mimic multiple sclerosis and seem to meet McDonald criteria, and outlines strategies for evaluation of such patients.

Disorders of Lysosomal Storage and Peroxisomal Function

Lysosomal storage diseases are inherited metabolic disorders characterized by defective catabolism of complex molecules, including lipids, glycoproteins, and oligosaccharides, and by resultant toxic accumulation of cellular debris. Several lysosomal disorders have CNS white matter involvement in patterns that may overlap with those observed in multiple sclerosis, and attenuated forms of Krabbe disease (globoid cell leukodystrophy), metachromatic leukodystrophy, Niemann-Pick disease type C (a disorder of cholesterol trafficking), Fabry disease, and variant forms of X-linked adrenoleukodystrophy and adrenomyeloneuropathy, also have the potential to show clinical overlap with multiple sclerosis in adult patients, and are probably underdiagnosed. Several published single cases or case series in the literature report individuals with confirmed lysosomal storage disease or peroxisomal disease who were originally diagnosed with 'suspected multiple sclerosis' or clinically definite multiple sclerosis (Zwetsloot et al., 1992; Stöckler et al., 1993; Kazibutowska et al. 2002; Chebel et al., 2009; Godeiro-Junior et al., 2009; Carra-Dalliere et al., 2013; Weisfeld-Adams et al., 2013). The biochemical defects underlying specific lysosomal disorders and peroxisomal disorders that can potentially mimic multiple sclerosis, together with corresponding clinical and radiologic features and guidelines for diagnostic testing, are summarized in Table 1.

Various newborn screening initiatives for several lysosomal and peroxisomal disorders are now underway (Matern et al., 2013) and may help answer several unsolved questions relating to their natural histories and phenotypic spectra. Over 7 years of experience with newborn screening for Krabbe disease in New York State suggests that late-onset Krabbe disease is significantly more common than previously recognized. This is indicated by large numbers of asymptomatic infants with low galactosylceramidase (GALC) activity and biallelic, pathogenic GALC mutations. Specific GALC missense alleles (p.G57S, p.W115R and p.G286D) are associated with late-onset Krabbe disease (Lissens et al., 2007). Many infants with very low GALC activity on newborn screening have remained asymptomatic over several years (Wang et al., 2011) and at what age, if ever, these individuals will develop symptomatic Krabbe disease is unclear. Late-onset Krabbe disease typically presents in adolescence or adulthood with insidiously progressive paraparesis and gait disturbance which can appear similar to progressive multiple sclerosis. MRI 'red flags' to suggest the diagnosis of Krabbe disease over multiple sclerosis include prominent involvement of the pyramidal tracts and/or symmetric or confluent involvement of the posterior white matter, splenium of the corpus callosum, and corticospinal tracts (Fig. 1A–C; Farina et al., 2000; Sedel et al., 2008).

Figure 1.

Selected magnetic resonance images from patients with adult-onset lysosomal and peroxisomal disorders. (A) Axial T2-weighted image of a 24-year-old female with Krabbe disease (globoid cell leukodystrophy) demonstrating confluent T2 hyperintensity in the bilateral centrum semiovale, centred in the fronto-parietal lobes and involving the corticospinal tracts. The patient presented with a 2-year history of worsening spastic paraparesis and symmetrical lower extremity sensory disturbance. (B and C) Axial T2 and proton-density weighted images of the same patient showing symmetric hyperintensity in the posterior limbs of the internal capsule and pons, corresponding to the corticospinal tracts. (D and E) Axial FLAIR images from a 35-year-old Taiwanese male with an 18-year history of progressive spastic paraplegia, diagnosed with adrenomyeloneuropathy. Images demonstrate asymmetric T2 hyperintensity within the internal capsules with corticospinal tract involvement, and asymmetric T2 hyperintensity in cerebellar white matter and dentate nuclei. (F) Post-contrast T1-weighted image of the same patient demonstrating patchy contrast enhancement in and around corresponding regions of signal hyperintensity (reproduced with permission, Mo et al., 2004). (G and H) Axial FLAIR images of a 64-year-old female with a confirmed diagnosis of Fabry disease and with a 40-year history of ataxia and intermittent right-sided hemiparesis and hemisensory disturbance. There is focal and confluent T2 hyperintensity in the periventricular, subcortical and deep white matter, in a pattern mimicking multiple sclerosis (reproduced with permission, Böttcher et al. 2013). Although the patient met McDonald criteria for multiple sclerosis, oligoclonal bands were absent from CSF. (I) Axial T1-weighted image of a 46-year-old male with Fabry disease demonstrating classic, intrinsic T1 hyperintensities in the pulvinar nuclei bilaterally, a classic imaging feature of Fabry disease (reproduced with permission, Burlina et al., 2008).

Based on their extensive clinical overlap with each other and with acquired neurologic conditions, it is probably reasonable to extrapolate the under-ascertainment phenomenon suggested by early experiences with newborn screening for Krabbe and Fabry diseases to other disorders including metachromatic leukodystrophy and X-linked adrenoleukodystrophy/adrenomyeloneuropathy (X-ALD/AMN). Among European patients with adult-onset metachromatic leukodystrophy, the p.I179S allele (associated with a predominant psychiatric phenotype) and p.P426L, account for the majority of ARSA alleles (Ługowska et al., 2005). Non-pathogenic pseudodeficiency alleles occur at relatively high frequencies in ARSA, and may complicate interpretation of ARSA enzymology for metachromatic leukodystrophy; the best overall initial screening test for metachromatic leukodystrophy is therefore urinary sulphatides. A complex ARSA pseudodeficiency allele (c.[1049A>G; *96A>G]) has been recognized at higher frequency among patients with multiple sclerosis compared to matched controls (Kappler et al., 1991; Baronica et al., 2011). Genotype–phenotype correlations have also been observed in adult-onset forms of Chédiak-Higashi disease, a disorder of lysosomal trafficking with associated pigmentary abnormalities, progressive spastic paraplegia and progressive spinocerebellar degeneration and leukoencephalopathy on MRI, with most cases occurring in the setting of biallelic missense mutations or in-frame deletions in LYST (Karim et al., 2002; Weisfeld-Adams et al., 2013).

Patients with the adrenomyeloneuropathy phenotype of X-linked adrenoleukodystrophy can be confused with primary progressive multiple sclerosis, and classically present with progressive spastic paraparesis and bladder disturbance in early or middle adulthood. Families with ABCD1 mutations exhibit extreme intra and interfamilial heterogeneity, and very long chain fatty acid profiles are poorly predictive of clinical phenotypes. More than 80% of female heterozygotes harbouring pathogenic mutations in ABCD1 develop adult-onset spastic paraparesis, mild myeloneuropathy, and/or peripheral neuropathy usually after 40 years of age, and have a high incidence of bladder spasticity and faecal incontinence. Prevalence of symptoms in female heterozygotes seems to correlate strongly with advancing age (Engelen et al., 2014). Cerebral disease is highly unusual in females. Most hemizygous males develop adult-onset adrenomyeloneuropathy (accounting for 40–45% of males) or childhood-onset cerebral adrenoleukodystrophy (30–35% of males), whereas a much smaller proportion of adult males develop clinical and radiological features of cerebral disease without preceding myeloneuropathy symptoms (Engelen et al., 2012). Although the prevalence of cerebral disease in males with adrenomyeloneuropathy was previously estimated at 20% within 10 years of onset (van Geel et al., 2001), a more recent Dutch study evaluating 27 males with adrenomyeloneuropathy with mean symptomatic onset at 35.3 years of age showed that >60% of patients developed evidence of cerebral demyelination a mean of 10.2 years after onset of adrenomyelomeuropathy (de Beer et al., 2014). Furthermore, survival after onset of cerebral demyelinating symptoms in these patients was limited to a mean of 3.4 ± 2.9 years, suggesting that cerebral disease in adult males, once underway, may be as devastating and as rapidly progressive as is the case for childhood-onset cerebral X-linked adrenoleukodystrophy. Head trauma and stroke may precipitate the onset of cerebral disease (Raymond et al., 2010). Several different MRI patterns have been described in adults and children; these patterns have utility in predicting clinical course and, together with extent of disease and age of onset, may be helpful in determining candidacy for haematopoietic stem cell transplantation. The classic pattern seen in childhood-onset cerebral X-linked adrenoleukodystrophy encompasses symmetric T2 hyperintensity of parieto-occipital white matter, classically periatrially (so-called 'Pattern 1' disease); most cases are of early-onset and are characterized by rapid progression (Loes et al., 2003). Males with the adrenomyeloneuropathy phenotype can have normal brain MRI, changes that appear in advance of clinical disease, or changes that appear several years or decades later (Loes et al., 2003; de Beer et al., 2014). Abnormalities restricted to the pyramidal tracts, or diffuse MRI abnormalities involving the cerebral hemispheres, posterior limbs of the internal capsules, brainstem and cerebellum can also occur (Fig. 1D–F; Kumar et al., 1995).

Pilot newborn screening studies for Fabry disease, which frequently presents with intermittent acroparaesthesias and abnormal white matter appearances on MRI, have also revealed an unexpectedly high incidence among male newborns (Spada et al., 2006; Inoue et al., 2013). Relapsing-remitting symptoms sharing characteristics with relapsing-remitting multiple sclerosis have been observed in Fabry disease. In a retrospective study of 187 confirmed patients with Fabry disease, 11 patients had concomitant diagnoses of clinically definite multiple sclerosis or possible multiple sclerosis; Fabry disease was diagnosed a mean of 8.2 years after the multiple sclerosis diagnosis was assigned (Böttcher et al., 2013) and some patients with 'true' Fabry disease had unmatched oligoclonal bands in CSF. 'Red flag' characteristics suggesting an alternative diagnosis of Fabry disease were suggested and included patients with asymmetric, confluent white matter T2 hyperintensity on MRI (Fig. 1G and H), normal spinal MRI, ectatic vertebrobasilar arteries, proteinuria and absence of unmatched CSF oligoclonal bands (Böttcher et al., 2013). Additionally, T1 hyperintensity in the pulvinar nuclei of the thalami, seen in 25% of patients with Fabry disease, are a specific feature of the condition (Fig. 1I; Moore et al., 2003; Burlina et al., 2008).

Further complicating the differentiation of these disorders from multiple sclerosis, unmatched oligoclonal bands have been reported in confirmed cases of Fabry disease, X-linked adrenoleukodystrophy and adrenomyeloneuropathy, metachromatic leukodystrophy, and Niemann-Pick disease type C (Böttcher et al., 2013). Clinical clues to lysosomal or peroxisomal disease in patients with 'multiple sclerosis-like' presentations include peripheral neuropathy (frequently observed in Krabbe disease, metachromatic leukodystrophy, and X-linked adrenoleukodystrophy spectrum disorders), extra-neurological manifestations, such as cardiac, renal and dermatological manifestations of Fabry disease, splenomegaly in Niemann-Pick disease type C and more rarely in Fabry disease, and immunodeficiency and pigmentary abnormalities in Chédiak-Higashi disease. Phenotypic heterogeneity is a well-known feature of Niemann-Pick disease type C, and data from an international patient registry demonstrated that 27% of participants experienced symptomatic onset in late adolescence or adulthood, and that a majority presented with ataxia, dysarthria, and cognitive impairment, features often seen in progressive multiple sclerosis. Many patients also had clinical features more specific to Niemann-Pick disease type C and less typical of multiple sclerosis, such as vertical supranuclear gaze palsy and gelastic cataplexy (Patterson et al., 2013).

Of all the groups of heritable disorders that have the potential to mimic multiple sclerosis, lysosomal storage disorders are likely to be among the most amenable to pharmacotherapeutic interventions that can effectively modify disease outcomes, illustrating the importance of accurate and timely diagnosis of these conditions. Available treatments for Krabbe disease, metachromatic leukodystrophy, and childhood cerebral X-linked adrenoleukodystrophy are currently limited to haematopoietic stem cell transplantation, and are generally reserved for pre- or early symptomatic disease, and outcomes have only been reported in any detail in paediatric patients (Escolar et al., 2005; Biffi et al., 2013). There are currently limited data to support the efficacy of haematopoietic stem cell transplantation in adults with cerebral X-linked adrenoleukodystrophy (Engelen et al., 2012) and other adult-onset leukodystrophies. Trials of gene therapies, stem cell therapies, or intrathecal enzyme replacement therapy are underway or in development for Krabbe disease, metachromatic leukodystrophy, and X-linked adrenoleukodystrophy, and are showing promise in animal subjects as well as in small-scale human studies (Cartier et al., 2009; Stroobants et al., 2011; Rafi et al., 2012; Patil et al., 2013).

The efficacy of systemic enzyme replacement therapy is well established for individuals with Fabry disease and significantly improves survival and quality of life (Pisani et al., 2012), reasserting the need for accurate and expedient diagnosis. In a randomized clinical trial of Niemann-Pick type C patients treated with oral miglustat, a reversible inhibitor of glycosphingolipid synthesis, miglustat successfully stabilized key neurological manifestations of the disease in paediatric and adult patients (Patterson et al., 2007). Other chemical chaperone approaches, substrate reduction therapies, and stop-codon read-through strategies have been used in several other lysosomal storage diseases or are in development (Boustany, 2013), and are beyond the scope of this review.

Mitochondrial Disorders

Dysfunctional mitochondria play a central role in the pathogenesis of various neurological disorders including multiple sclerosis, Alzheimer's disease, Parkinson's disease and stroke. Mitochondrial dysfunction increases production of reactive oxygen species which may contribute to oxidative stress, potentially leading to neuronal injury, neurodegeneration and, ultimately, clinical disability (Witte et al., 2010; Friese et al., 2014). Biochemical defects in respiratory chain complex I activity have been proposed as contributory to the pathogenesis of multiple sclerosis, and the lack of complex IV activity in demyelinated axons supports axonal dysfunction as a contributor to disease progression in multiple sclerosis (Mahad et al., 2009). The capacity for remyelination and axonal repair, critical factors in delaying or preventing progressive disease, also seem to be dependent on healthy mitochondria.

Given these indicators that mitochondrial dysfunction is integral to multiple sclerosis pathology, it is not surprising that several mitochondrial respiratory chain defects, caused by defined mutations in nuclear or mitochondrial DNA (mtDNA) may present with neurologic syndromes sharing characteristics with multiple sclerosis (Iñiguez et al., 1998). Clinical features, MRI appearances and diagnostic testing strategies for these disorders are outlined in Table 2. Prominent myopathy, cardiomyopathy or cardiac conduction abnormalities, external ophthalmoplegia, early severe optic neuropathy that does not recover, seizures, cerebrovascular disease, peripheral neuropathy, pigmentary retinopathy, and diabetes mellitus may all serve as clinical clues differentiating this group of disorders from multiple sclerosis. Myoclonic epilepsy with lactic acidosis and stroke-like episodes (MELAS), myoclonic epilepsy with ragged red fibres (MERRF), and Leber's hereditary optic neuropathy (LHON) are caused by specific mutations or structural rearrangements in mtDNA, and can present with spasticity, ataxia and optic neuropathy. Leigh disease encompasses a heterogeneous group of conditions, some of which are caused by autosomal recessively-inherited mutations in nuclear genes that are disruptive to mitochondrial function or maintenance, and some by mutations in mtDNA itself. Non-Mendelian/maternal transmission, if observed, is strong evidence for a mitochondrially-inherited condition; however, given the epidemiology of multiple sclerosis (females affected more frequently than males; increased risk of multiple sclerosis in first-degree relatives) it is not unusual to see multiple sclerosis in a patient with a similarly affected mother. Further, the absence of family history does not exclude the diagnosis of a mtDNA-encoded disorder, owing to the phenomena of incomplete penetrance and heteroplasmy in mtDNA disorders. POLG is a nuclear gene encoding polymerase gamma, a DNA polymerase in humans that effects mtDNA replication and repair.

LHON, caused by specific point mutations in mtDNA (typically at positions 3460, 11778, or 14484) encoding complex I of the respiratory chain, merits special consideration because of its particularly strong associations with multiple sclerosis, occurring with ~50 times higher frequency in patients with multiple sclerosis than would be expected by chance (Palace, 2009), an association referred to as 'LHON-plus' or 'Harding disease'. Subpopulations of multiple sclerosis patients (those with early/severe optic neuritis) appear to harbour characteristic LHON mutations at high frequencies (Leuzzi et al., 1997). Curiously, although LHON is of lower penetrance among females harbouring mtDNA mutations, the coexistence of the disorder with multiple sclerosis-like illness occurs with a similar female preponderance as is observed in isolated multiple sclerosis. Harding reported three females with LHON harbouring m.11778G>A mutations who did not have extraocular symptoms but had white matter lesions on MRI mimicking multiple sclerosis (Harding et al., 1992), and MRI appearances in LHON commonly closely mimic those observed in multiple sclerosis (Inglese et al., 2001; Lerman-Sagie et al., 2005; Küker et al., 2007; Fig. 2A–C). Unmatched oligoclonal bands have been detected in multiple patients (Palace, 2009). Some authors have proposed that specific mtDNA mutations traditionally associated with LHON are associated with and predispose to multiple sclerosis; an inflammatory process and clinical course similar to that observed in 'LHON-plus' has also been described in patients with MELAS (Maruyama et al., 1998), although MELAS is predominantly a grey matter disorder.

Figure 2.

Selected magnetic resonance images from miscellaneous hereditary conditions mimicking multiple sclerosis. (A) Axial FLAIR image of a 36-year-old female with spastic paraparesis and painless visual loss, known to harbour the m.11778G>A mtDNA mutation, diagnostic of LHON-plus, also known as Harding's disease. Numerous foci of hyperintensity of the periventricular and deep white matter strongly resemble appearances seen in multiple sclerosis (reproduced with permission, Küker et al., 2007). Four years after presentation, this patient developed bilateral optic atrophy, ataxia, and internuclear ophthalmoplegia. (B) Axial T1-weighted image of another patient with LHON showing white matter lesions with hypointense signal, indistinguishable from multiple sclerosis (reproduced with permission, Inglese et al., 2001). (C) Axial T2-weighted image demonstrating more confluent T2 hyperintense white matter disease in a third LHON patient (reproduced with permission, Lerman-Sagie et al., 2005). (D–F) Axial FLAIR and T2-weighted imaging through the brain (D), brainstem (E) and cervical cord (F) in three individual patients with adult-onset Alexander disease. The female patient in (D) presented at 43 years of age with bulbar weakness; patients (E) and (F) presented with lower extremity weakness. The images demonstrate confluent posterior predominant T2 hyperintensity in the periventricular and deep white matter (D) together with atrophy of medullary pyramids (arrow in E) and marked thinning and T2 hyperintensity in the cervical cord (F) (reproduced with permission, Pareyson et al., 2008). (G-I) Axial FLAIR imaging in a patient with CADASIL. Patchy and confluent hyperintensity in the periventricular white matter (G) with involvement of the external capsules (H) and subcortical white matter of the temporal poles (I) are characteristic. (J and K) Axial FLAIR imaging of a 49-year-old female patient with PLP1 disease (hereditary spastic paraplegia, SPG2 type) presenting with a phenotype mimicking primary progressive multiple sclerosis. She had a 10-year history of progressive spastic paraplegia, dysarthria, and ataxia. Her son had died in childhood of a previously undiagnosed severe neurodevelopmental disorder. Multifocal T2 hyperintensities are seen in the periventricular and deep white matter near the atria and occipital horns of the ventricles, together with and other scattered foci of T2 signal in the white matter, mimicking multiple sclerosis (reproduced with permission, Warshawsky et al., 2005). She had abnormal visual-evoked potentials bilaterally and CSF was positive for unmatched oligoclonal bands. (L) Axial T2-weighted image in a 22-year-old female with hereditary spastic paraplegia, SPG11 type, presenting with subacute gait disturbance, urge incontinence, and mild, subjective cognitive disturbance. Confluent T2 hyperintensity is seen in the posterior white matter together with small, scattered foci elsewhere in the brain (reproduced with permission, Romagnolo et al., 2014).

Late-onset autosomal recessive Leigh disease is clinically and genetically heterogeneous and has been associated with missense mutations in SURF1 as well as several other nuclear genes and mtDNA mutations, and is characterized by motor regression, dementia and dystonia. Although Leigh disease, like MELAS, is primarily a disorder of grey matter, its clinical course may be acute, gradually progressive, or relapsing, and there are historical reports of misdiagnosis with multiple sclerosis (Malojcic et al., 2004). Clues to Leigh disease or other mitochondrial disorders on MRI in suspected multiple sclerosis include symmetric bilateral T2 hyperintense lesions in the basal ganglia, or multifocal grey matter involvement. Common variants in two prominent mitochondrial Leigh disease genes, MT-ATP6 and MT-ND2, have been associated with multiple sclerosis (Vyshkina et al., 2008).

Mutations in POLG have also been associated with multiple sclerosis-like presentations in several patients. The clinical spectrum of POLG-related disorders comprise a continuum of overlapping phenotypes including a subset of ataxia-neuropathy disorders, which overlap with MELAS/MERRF phenotypes and presenting with optic neuritis, white matter hyperintensities, unmatched oligoclonal bands in CSF, and variable axonal polyneuropathy (Degos et al., 2014). Other POLG mutations result in infantile-onset hepatocerebral disease (Alpers-Huttenlocher syndrome) with no clinical resemblance to multiple sclerosis. Although heterozygous POLG mutations can cause ophthalmoplegia phenotypes, most other POLG-related phenotypes, including ataxia-neuropathy, are recessively inherited. Heterozygous mutations in OPA1, another nuclear gene, also cause dysfunction and misdirection of mitochondria in neural tissue; the most common phenotype is insidiously progressive childhood-onset optic nerve atrophy, although a proportion of patients have extraocular neurologic involvement including ataxia and/or spastic paraplegia. Some patients have diffuse white matter hyperintensities on MRI with prominent involvement of periventricular white matter (Yu-Wai-Man et al. 2010; Pretegiani et al., 2011). Several patients with pyruvate dehydrogenase complex deficiency, classically described as a severe infantile-onset disease, have been described with later-onset presentations including recurrent, episodic ataxia. These patients are usually males with specific missense mutations of PHDA1, a nuclear gene, and may have Leigh-like grey matter appearances and other basal ganglia changes on MRI (Barnerias et al., 2010) that are unlikely to mimic 'typical' MRI findings observed in multiple sclerosis.

Other Metabolic Leukodystrophies, Leukoencephalopathies and Myelopathies

'Leukodystrophy' refers to a group genetic disorders primarily involving myelin pathology and with prominent white matter involvement, while 'leukoencephalopathy' is a generic, largely radiological term encompassing many genetic (and acquired) disorders with diffuse white matter abnormalities on MRI and including some disorders where neuropathology is primarily axonal. Although many lysosomal disorders (discussed above) are classifiable as leukodystrophies, several other leukodystrophies and leukoencephalopathies potentially presenting in adulthood are not classifiable as disorders of lysosomal or peroxisomal function, or as mitochondrial disorders. Several of these disorders can present with spastic paraparesis, or with other neurological features that can mimic multiple sclerosis (Table 3). Other neurometabolic disorders that mimic multiple sclerosis and that are treatable or potentially treatable with pharmacotherapy include urea cycle disorders, acute intermittent porphyria, biotinidase deficiency, cerebrotendinous xanthomatosis, and some disorders of vitamin B12 or folate metabolism. Some patients with urea cycle disorders, acute intermittent porphyria, and some methylmalonic acidaemias may be candidates for liver transplantation.

Two primary urea cycle disorders (arginase deficiency and the hyperornithinaemia-hyperammonaemia-homocitrullinaemia syndrome) are known to onset with spasticity and diplegia of late-onset in rare patients. Both disorders can present with liver disease and life-threatening hyperammonaemia and are managed with dietary protein restriction and pharmacologic ammonia scavengers, and patients with diplegia may improve clinically with treatment (Prasad et al., 1997). Several historical reports attest to the potential of adult polyglucosan body disease, caused by defective glycogen branching and allelic to glycogen storage disease type IV, to be confused with multiple sclerosis. Although adult polyglucosan body disease is recessively inherited, progressive gait disturbance and cognitive decline have also been reported in heterozygotes (Paradas et al., 2014). Management for adult polyglucosan body disease is currently supportive only.

Neurologic phenotypes common to nutritional or autoimmune B12 deficiency and inherited disorders of B12 metabolism include paraesthesias, ataxia, impaired proprioception, and spasticity. Many patients also have megaloblastic anaemia. Several late-onset forms of many inherited disorders of B12 metabolism have been described, including methylmalonic acidaemia (MMA), mutase type, and some cobalamin defects, and may share characteristics with nutritional/autoimmune B12 deficiency and with multiple sclerosis. The cobalamin C defect has been reported in association with adult-onset myelopathy, neuropsychiatric disturbance and patchy white matter T2 hyperintensities on MRI, which may involve the cord (Thauvin-Robinet et al., 2008; Huemer et al., 2014). Optic atrophy presenting with acute visual loss has been reported in mutase deficiency methylmalonic acidaemia (Pinar-Sueiro et al., 2010). MTHFR deficiency, most often characterized by early-childhood clinical onset, also has late-onset forms with white matter disease on MRI and progressive myelopathic gait disturbance. Reported patients lacked a history of relapses and were negative for unmatched CSF oligoclonal bands (Haworth et al., 1993). Screening plasma B12 levels, as well as quantification of methylmalonic acid (MMA) in plasma and/or urine, are already recommended for all individuals for whom the diagnosis of progressive multiple sclerosis is considered; we also recommend additional screening of plasma homocysteine if there is any reasonable suspicion for these disorders, since B12 and methylmalonic acid will be normal in patients with MTHFR deficiency or cobalamin E or G defects.

Adolescents and adults with partial biotinidase deficiency with diffuse white matter disease and spastic paraparesis have also been described; this condition is subject to newborn screening in many countries, can be diagnosed with straightforward biochemical testing, and is treatable with oral biotin (vitamin B7) supplementation.

Cerebrotendinous xanthomatosis is an autosomal recessive lipid storage disease that is also classified as a leukodystrophy, and characterized by infantile-onset diarrhoea, cataracts, and adolescent or adult-onset tendon xanthomas and progressive neurological disease causing cerebellar signs, spasticity, dystonia, parkinsonism, and peripheral neuropathy. MRI may show diffuse cerebral and cerebellar atrophy with variable T2 abnormalities involving deep and subcortical white matter which may overlap with abnormalities seen in multiple sclerosis. When present, characteristic symmetric T2 hyperintensity in the dentate nuclei is suggestive of this disorder (De Stefano et al., 2001). Cerebrotendinous xanthomatosis is a potentially treatable condition with clinical symptoms stabilizing with administration of chenodeoxycholic acid (Yahalom et al., 2013).

Many patients with acute intermittent porphyria, an autosomal dominant defect of haem metabolism with limited penetrance, experience recurrent attacks of poorly localized abdominal pain; some patients experience acute neurologic deficits or psychiatric symptoms for which pathomechanisms are poorly characterized. Such patients may have evidence of both peripheral nerve and CNS disease. Many patients harbouring pathogenic HMBS mutations (and who have symptomatic family members) have 'latent' porphyria and remain asymptomatic throughout their lives. A proportion of patients have hyperintense white matter lesions on MRI that can occur in a multiple sclerosis-like distribution (Bylesjö et al., 2004). In relation to haem and iron metabolism, iron is known to accumulate in the CNS in multiple sclerosis, although the pathomechanisms for this phenomenon and its contribution to multiple sclerosis pathogenesis are incompletely understood. Iron may modify disease activity in multiple sclerosis by promoting production of pro-inflammatory mediators, mitochondrial dysfunction, and reactive oxygen species production (Williams et al., 2012). In one large multiple sclerosis cohort, there was no enrichment for mutations in HFE, the gene for haemochromatosis (Ramagopalan et al., 2008). Although other inherited disorders of metabolism of iron and other metals [including neurodegeneration with brain iron accumulation (NBIA), and Wilson disease] may rarely involve white matter, they do not typically mimic multiple sclerosis clinically and generally have distinct radiological features (van Wassenaer-van Hall et al., 1995; Hayflick et al., 2006).

Non-metabolic Leukodystrophies/Leukoencephalopathies and Small Artery Disorders

Several other heritable disorders of white matter can mimic multiple sclerosis, form a heterogeneous group, and are not strictly classifiable as metabolic diseases (Table 4). Adult-onset Alexander disease is caused by heterozygous, usually missense, mutations in GFAP; both sporadic cases (with de novo mutations) and familial cases with autosomal dominant transmission and incomplete penetrance have been described (Li et al., 2006; Balbi et al., 2010). Alexander disease is characterized by fibrinoid bodies in astrocytes and classically described as an infantile-onset disorder presenting with macrocephaly, growth failure, seizures, and progressive neurological impairment. In recent years, knowledge of the clinical spectrum has expanded, with late-onset disease now thought to account for at least a quarter of all cases of Alexander disease. The clinical course of late-onset disease entails progressive gait ataxia, spasticity and pyramidal signs, bulbar signs, and autonomic features, and can mimic multiple sclerosis (Tschampa et al., 2011); unlike infantile-onset Alexander disease, a very low incidence of macrocephaly and mental retardation is observed in adult-onset disease (Pareyson et al., 2008). Thirty-five per cent of adult patients in one series had palatal myoclonus, which is only rarely seen in other disorders and may be a useful diagnostic clue (Balbi et al., 2010). Adult patients may have T2 hyperintensities in periventricular and deep white matter (Fig. 2D), and atrophy and T2 signal abnormalities of the medulla oblongata and cervical cord on MRI (Fig. 2E and F), which may also be seen in multiple sclerosis. Sixty-five to 75% of patients with adult-onset Alexander disease in two series had white matter hyperintensities on MRI in varying distributions which have potential to be mistaken for multiple sclerosis (Farina et al., 2008; Yoshida et al., 2011). MRI can be normal in some adult patients.

Autosomal dominant leukodystrophy is associated with duplications of the lamin B1 (LMNB1) gene, causing lamin B1 overexpression, resultant defects in oligodendroglial differentiation, and subtotal demyelination. The disorder has been misdiagnosed as multiple sclerosis, and was originally identified as a distinct disorder in two large Irish-American kindreds sharing a common founder duplication allele (Padiath et al., 2006). Leukoencephalopathy with brainstem and cord involvement and lactate elevation (LBSL) is an autosomal recessive disorder of white matter caused by mutations in DARS2. It is characterized by slowly progressive pyramidal, cerebellar, and dorsal column dysfunction, manifesting as cerebellar ataxia and spasticity, with a predominance of lower extremity involvement, and dysarthria (Isohanni et al., 2010). Onset is typically in adolescence or early adulthood, but the disorder exhibits phenotypic heterogeneity and may be incompletely penetrant in some families (Labauge et al., 2011). MRI shows a pattern of homogeneous, often enhancing white matter disease, and involvement of brain stem and spinal tracts. Increased lactate is often present spectroscopically in abnormal white matter. One Finnish study screened for two common DARS2 mutations in 321 patients with multiple sclerosis; no enrichment for these mutations was observed (Isohanni et al., 2010). Hereditary diffuse leukoencephalopathy with spheroid cysts (HDLS) is an autosomal dominant disorder that may resemble primary progressive multiple sclerosis (Keegan et al., 2008), and is caused by mutations in CSF1R. Most cases are inherited from an affected parent, although de novo cases are also reported (Rademakers et al., 2011). This disorder typically becomes symptomatic in early adulthood and has a prominent psychiatric or cognitive impairment phenotype. White matter disease on MRI may be asymmetric and patchy and mimic multiple sclerosis, developing confluence later. Significant grey matter pathology, brainstem atrophy and cerebellar abnormalities are minimal or absent.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a progressive, debilitating cerebrovascular disorder that typically follows a stepwise course, with symptomatic onset in the fourth or fifth decade. More than 90% of individuals with CADASIL have autosomal dominantly-inherited heterozygous mutations in NOTCH3; de novo mutations are uncommon, most individuals having an affected parent. Missense mutations located distally in the NOTCH3 transcript may confer a less specific phenotype with predisposition to age-related progression of small vessel vasculopathy-associated white matter lesions and a high incidence of progressive disability compared to controls. CARASIL, a, similar autosomal recessive disorder, shares phenotypic characteristics with CADASIL and is caused by homozygous or compound heterozygous mutations in HTRA1 (Fukutake et al., 1995). The pathologic hallmark of CADASIL is electron-dense granules in the media of arterioles that can often be identified by electron microscopic evaluation of skin biopsies; these appearances are not observed in CARASIL. On MRI, CADASIL/CARASIL have a predilection for involving subcortical white matter of the anterior temporal lobes, frontoparietal regions near the vertex, and the bilateral external capsules, but may also exhibit diffuse leukoencephalopathy that may mimic multiple sclerosis (Fig. 2G–I; O'Riordan et al., 2002).

Exome sequencing recently identified several adult patients with autosomal recessive ClC-2 chloride channel deficiency with clinical and imaging features resembling multiple sclerosis. These patients had leukoencephalopathy with MRI signal abnormality in the posterior limb of the internal capsule and middle cerebellar peduncles, and presented with variable clinical features including optic neuropathy, spasticity, and cerebellar ataxia; some patients had chorioretinopathy. Among individuals with adult-onset disease, age of onset ranged from 30–57 years. All identified patients were homozygous or compound heterozygous for CLCN2 mutations (Depienne et al., 2013). Finally, several families with X-linked Charcot–Marie–Tooth disease (CMTX) have been described in which individuals have presented with, in addition to the mixed axonal demyelinating polyneuropathy that characterizes the disease, 'relapsing-remitting' neurological deficits (including transient limb weakness, hemiparesis, or dysarthria) and transient CNS white matter changes often with confluence and posterior or parietal predominance (Hanemann et al., 2003; Sato et al., 2012). In some individuals these episodes seemed to be triggered by physical exertion or high altitude (Paulson et al., 2002).

Hereditary Spastic Paraplegias

Hereditary spastic paraplegias are a heterogenous and expanding group of disorders characterized by slowly progressive paraparesis and spasticity. Historically, these disorders have been broadly classified as 'uncomplicated' if neurological impairment is limited to progressive spastic paraparesis, hypertonic urinary bladder disturbance, and diminution of lower-extremity vibration sensation and proprioception. The primary pathology is axonal degeneration, which is maximal in distal corticospinal tracts and dorsal column fibres (Harding, 1993). There may be a family history of similarly affected individuals, especially in dominantly-inherited subtypes; however, absence of a family history should not exclude hereditary spastic paraplegias, as many subtypes exhibit incomplete or variable penetrance in some families, result from de novo mutations, or follow an autosomal recessive inheritance mechanism. MRI is often normal, helping to differentiate hereditary spastic paraplegia from multiple sclerosis and other conditions, but may reveal mild global atrophy, atrophy of the corpus callosum, non-specific white matter lesions, T2 hyperintensity in the internal capsule, and cord atrophy (Casali et al., 2004; Hourani et al., 2009), making exclusion of a multiple sclerosis diagnosis challenging in some cases. A recent study revealed alterations in multiple diffusion-tensor indices in many patients, suggesting widespread disruption of myelin integrity (Aghakhanyan et al., 2014). Uncomplicated autosomal dominant forms include types 3A, 4, 6, 8, 10, 13, 17, 31 and 33; types 5A, 7 and 11 are autosomal recessive, and X-linked forms have been described. While many cases of hereditary spastic paraplegia have prominent clinical features that are unlike those seen in multiple sclerosis (e.g. amyotrophy, mental retardation), others may present with dysarthria, optic disc pallor or other features that overlap with multiple sclerosis.

PLP1 disease is classified as X-linked hereditary spastic paraplegia under many schemes, with segmental duplication of Xq22 the most common molecular mechanism described. The classic phenotype of PLP1 disease is early childhood-onset Pelizaeus-Merzbacher disease in young males, although numerous reports of attenuated disease exist, particularly in female heterozygotes with adult-onset spastic paraplegia phenotypes (Hurst et al., 2006), designated as the SPG2 subtype of hereditary spastic paraplegia. SPG2 has been documented in association with mutations involving specific domains of the PLP1 protein transcript or less-conserved gene regions, and some gene truncations. Adult patients with SPG2, as well as clinically asymptomatic obligate carrier mothers of males with classical Pelizaeus-Merzbacher disease, have been described with non-specific periventricular and subependymal white matter changes on MRI, and cystic changes in the basal ganglia (Fig. 2J and K; Battini et al., 2003; Warshawsky et al., 2005; Hurst et al., 2006). A young female presenting with subacute gait disturbance and periventricular white matter lesions was recently described with biallelic pathogenic mutations in SPG11 (Fig. 2L; Romagnolo et al., 2014). Using a whole exome sequencing approach in combination with bioinformatics and functional validation, several additional putative hereditary spastic paraplegia genes were recently identified (Novarino et al., 2014), although as yet, the potential for these hereditary spastic paraplegia subtypes to mimic multiple sclerosis has not been evaluated.

Other Miscellaneous Mendelian Disorders of Relevance to Multiple Sclerosis

Other Mendelian genes can harbour highly penetrant, rare variants implicated in specific disorders that overlap with multiple sclerosis. Familial haemophagocytic lymphohistiocytosis is a genetically heterogenous autosomal recessive disorder characterized by proliferation/infiltration of macrophages and T-lymphocytes; adult disease has been reported. Several reports exist of cases with severe CNS manifestations, and multiple sclerosis has been reported with higher frequency among patients harbouring PRF1 mutations and polymorphisms (Camiña-Tato et al., 2010). Neurological features may include spasticity, cranial nerve palsies, ataxia, quadriplegia and blindness. Other findings, including fever, cytopaenias, rash, lymphadenopathy, liver dysfunction, and haemophagocytosis are not universally present, and the overall picture can mimic acute disseminated encephalomyelitis as well as multiple sclerosis.

Friedreich ataxia usually results from biallelic trinucleotide expansions in FRDA, a nuclear gene, and causes degeneration of dorsal columns, spinocerebellar and pyramidal tracts, dorsal root ganglia, and peripheral nerves. On MRI, atrophy of the medulla and upper cervical cord is common, and in late onset forms, cerebellar atrophy and cerebellar white matter abnormalities become evident (Bhidayasiri et al., 2005). It and other hereditary ataxias can mimic multiple sclerosis, although this group of disorders typically have clinical characteristics that are clearly distinguishable from multiple sclerosis.

Familial Mediterranean fever (FMF), which typically presents with recurrent febrile episodes with or without abdominal pain, is caused by recessively-inherited mutations in MEFV, encoding pyrin and implicated in the regulation of the pro-inflammatory cytokine interleukin 1β. MEFV mutations and polymorphisms have been identified at higher frequencies in patients with multiple sclerosis compared to healthy control groups; it has been suggested that endothelial dysfunction in familial Mediterranean fever may disrupt blood–brain barrier integrity, an important element in the evolution of inflammatory lesions and plaques (Alpayci et al., 2012).

Other miscellaneous rare disorders merit consideration in a discussion of hereditary mimics of multiple sclerosis. Gerstmann-Straussler-Scheinker syndrome is a rare, usually autosomal dominant prion disease with a protracted course encompassing a progressive ataxia syndrome, with MRI and CSF findings suggestive of a demyelinating-inflammatory process. It has been noted that certain vascular malformation disorders (cerebral angiomatosis; von Hippel-Lindau disease) may cause progressive neurological features punctuated by acute clinical deteriorations, and radiological appearances that could potentially be confused with multiple sclerosis (Natowicz and Bejjani, 1994).

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