Key Clinical Features of Acute Flaccid Myelitis Identified

Marcia Frellick

November 30, 2018

Researchers have identified distinguishing clinical features of the polio-like disease acute flaccid myelitis (AFM), a new report shows. They propose a more restrictive definition of the disease to help identify cases more quickly and to differentiate such cases from those of other illnesses that can cause acute weakness.

The current definition from the Centers for Disease Control and Prevention (CDC) was designed to catch all possible cases for epidemiologic surveillance of the emerging disease, which was first recognized in 2014. But the definition is so broad that some cases of illness will be misdiagnosed as AFM.

In the current case series of 45 patients, 24% were found to have a definable alternate diagnosis, primarily transverse myelitis and spinal cord stroke. Other alternate diagnoses included demyelinating syndromes, Guillain-Barré syndrome, Chiari I myelopathy, and meningitis.

Matthew J. Elrick, MD, PhD, in the Department of Neurology at Johns Hopkins University School of Medicine in Baltimore, Maryland, and colleagues report their findings in an article published online November 30 in JAMA Pediatrics.

Elrick and colleagues studied 45 patients younger than 18 years who were diagnosed on the basis of CDC criteria as having AFM between 2012 and 2016. The investigators divided the patients into two groups. Group 1 comprised patients from the United States and Canada whose conditions met the CDC definition of AFM. Group 2 included patients referred to Johns Hopkins Transverse Myelitis Center for evaluation of suspected AFM.

Patient records and imaging data were reviewed by three neurologists to identify patients for whom there were possible alternate diagnoses. The remaining patients were considered to have AFM on the basis of a more restrictive definition (rAFM).

The researchers then compared clinical characteristics of those with rAFM to those with other diagnoses to develop a case description for rAFM.

Elements of the Narrower Definition

The restrictive definition requires that the following elements be present:

  • Preceding fever or viral syndrome.

  • Weakness in a lower motor neuron pattern involving one or more limbs, the neck, the face, and/or bulbar muscles. In addition, weak limbs should have less muscle tone, and tendon reflexes should be reduced or absent.

  • At least one of the following: gray matter–predominant T2-hyperintense lesion on MRI of the spinal cord spanning many levels, with or without ventral nerve root enhancement; evidence from electromyography and nerve conduction studies of a motor neuronopathy with intact sensory nerve conductions; or cerebrospinal fluid (CSF) studies indicating pleocytosis (white blood cell count >5 cells/μL).

In addition, the definition specifies that the following factors or characteristics should be absent:

  • Sensory deficits

  • Supratentorial white matter or cortical lesions >1 cm

  • Encephalopathy that cannot be explained by fever, illness, respiratory distress, or metabolic abnormalities

  • Elevation of CSF protein greater than two times the upper limit of normal in the absence of CSF pleocytosis

  • A definable alternative diagnosis

The authors emphasize that the restrictive definition "is intentionally designed with high specificity as a goal and acknowledge that sensitivity will be less than that of the CDC case definition."

The narrower definition is likely to be most useful for research by enabling better identification of pathogens and potential treatments and helping with the differential diagnosis, although more research is needed to confirm the findings.

AFM appears to occur in waves every 2 years, the authors say. The next wave is expected in 2020.

"Clinicians should be prepared to recognize cases presenting in the late summer and fall, often following a viral respiratory infection, and presenting with asymmetric flaccid weakness that progresses over hours to days, associated with characteristic MRI and CSF findings," the authors write.

As reported recently by Medscape Medical News, as of November 13, 90 CDC-confirmed cases of AFM had been reported across 27 states in 2018.

Keys to Confirmation

"The clues to the diagnosis lie in the patient history and physical examination, and keys to confirmation come from lumbar puncture and magnetic resonance imaging (MRI) of the spinal cord," write Sarah E. Hopkins, MD, MSPH, from the Division of Neurology at Children's Hospital of Philadelphia, Pennsylvania, and colleagues in an accompanying editorial.

When clinicians suspect AFM, hospitalization is recommended, they write, because the patient may weaken quickly. Particularly at risk are patients with upper extremity and bulbar weakness; these patients may need to be admitted to a pediatric intensive care unit.

"All patients require respiratory monitoring with measures such as negative inspiratory force and forced vital capacity. Constipation is prominent, and aggressive management is warranted," they add.

They note that AFM may involve asymmetric areas and mainly proximal muscles. At presentation, patients' signs can range from barely discernible weakness in one limb to quadriparesis that necessitates respiratory support and a feeding tube.

"Spinal cord stroke should be considered in cases with vascular risk factors, recent trauma, or hyperacute onset," write Hopkins and colleagues.

In Guillain-Barré syndrome, the pattern of weakness differs. Typically, distal muscles are affected, rather than proximal muscles, with symmetric ascending progression, the authors write.

With AFM, pain in extremities may be extreme, and treatment of neuropathic pain is helpful, the editorialists say.

The study authors and the editorialists emphasize the need to rapidly collect samples. CSF should be assessed, and nasopharyngeal and oropharyngeal swabs, serum, and stool or rectal swabs should be taken. "Prompt sample procurement with submission of specimens and data to the CDC will maximize identification of associated infections and help establish next steps for prevention, including vaccine development," the editorialists write.

The study was partially funded by the Johns Hopkins University Provost Catalyst Award with support from the Bart McLean Fund for Neuroimmunology Research and Project Restore. The authors' and editorialists' relevant financial relationships are listed in the original articles.

JAMA Pediatr. Published online November 30, 2018. Full text, Editorial

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