Horizontal Diplopia at Distance and Right Gaze

Kelly A. Malloy, OD; Jean Marie Pagani, OD

Disclosures

June 23, 2016

Discussion

An amyloidoma is an extracellular aggregation of misfolded insoluble proteins and is considered a monoclonal immunoglobulin deposition disease.[1] Approximately 20 unrelated proteins are known to form amyloid fibrils in vivo; each is associated with a distinct clinical picture[2]:

Primary amyloidomas are benign localized amyloid deposits that rarely affect the central nervous system. Depending on their location, especially when they affect other parts of the body, surgical resection is the treatment of choice. Up to 95% of patients recover completely or partially from their symptoms if the tumor is resected.[3] Of course, if the amyloidoma involves a cranial nerve, surgical removal is not an option to restore function.

There are documented cases of amyloidomas affecting the central nervous system,[3] but only about 15-20 reportedly affect the trigeminal nerve or gasserian ganglion.[4] Current theories on amyloidoma formation in the gasserian ganglion include monotypic lambda-producing B cells reacting with latent antigens in the trigeminal ganglion or with microglia.[5,6]

An amyloidoma of the gasserian ganglion usually presents with facial numbness, neuralgia, and loss of the motor division innervated by the fifth cranial nerve. Cranial nerve VI palsy can occur either by perineural spread into the cavernous sinus or by enlargement of the mass. Amyloidoma expansion 6.5 mm inferior and temporal from the mouth of the Meckel cave would allow compression of cranial nerve VI as it enters the dura.[7]

Amyloid deposits are difficult to detect and differentiate from other tumors or infiltrative processes on brain imaging. However, trigeminal amyloidomas often appear isointense on T1-weighted MRI and can be a combination of hypointense and isointense on T2-weighted MRI.[2] This low signal intensity on T2-weighted imaging may help to differentiate amyloidoma from other lesions, such as schwannoma. Trigeminal amyloidomas enhance uniformly with contrast, as seen in this patient. A definitive diagnosis of amyloidoma is made microscopically after using Congo red staining and observing an apple-green birefringence under polarized light.[2]

Amyloidomas have an increased prevalence in middle-aged women,[3] but as this case demonstrates, they may also occur in men.

The Importance of Assessing Cranial Nerves

Amyloidoma is a rare cause of brain space-occupying lesions. This benign aggregation of protein must be considered in the differential diagnoses for atypical progressive hypoesthesia and neuralgia of the trigeminal dermatomes.

This patient presented for an eye examination because of diplopia. There are many etiologies of cranial nerve VI palsy, and even more etiologies of abduction deficit. For this reason, assessing other cranial nerves is helpful to localize the problem.

In this case, the combination of cranial nerve VI palsy and involvement of the trigeminal nerve may at first suggest cavernous sinus involvement. However, owing to the involvement of sensory and motor distributions of V3, which do not travel in the cavernous sinus, the lesion could not be contained solely to the cavernous sinus.

The involvement of V2 and V3, but not V1, localizes to the trigeminal or gasserian ganglion in the Meckel cave. Coinvolvement of cranial nerve VI can occur either by perineural spread to the cavernous sinus or by expansion of the mass to compress cranial nerve VI as it pierces the dura. The combination of nerves involved helped to localize the lesion and identify where to direct special attention on the neuroimaging studies.

As eye care providers, we routinely check the functions of cranial nerves II, III, IV, and VI. We need to remember the importance of assessing the remaining cranial nerves to help localize the problem and to ensure that we find the reason for the patient's symptoms.

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