Mononeuritis Multiplex Workup

The suspected cause of mononeuritis multiplex, as suggested by the patient’s history, symptoms, and pattern of symptom development, helps to determine which tests to perform. (Imaging studies are not indicated for the diagnosis of mononeuritis multiplex.) Laboratory tests ordered include the following:

• Complete blood count (CBC) with a differential
• Fasting blood glucose levels
• Borrelia burgdorferi antibody titer - If Lyme disease is suspected
• Human immunodeficiency virus (HIV) blood tests - If HIV infection is suspected
• Hepatitis screen - If hepatitis is suspected as a causative agent
• Erythrocyte sedimentation rate (ESR) and C-reactive protein level - If a systemic inflammatory process is suspected
• Autoimmune profile
• Herpes viridae serology
• Parvovirus B-19 antibodies

Procedures

The performance of any procedure, such as a nerve biopsy, is dictated by the patient’s history and physical examination results. The purpose of any procedure is to determine the primary pathologic process.^[3] A nerve biopsy may be performed to help distinguish between demyelination (destruction of parts of the myelin sheath covering the nerve) and axonal degeneration (destruction of the axon portion of the nerve cell), to identify inflammatory nerve conditions (neuropathies), or to confirm specific diagnoses.

Electrodiagnostic studies are used only in conjunction with an accurate and complete history and physical examination. The lesion or lesions are distal to the motor and sensory cell bodies and result in either axonal disruption/degeneration or abnormal axonal conduction.

Sensory nerve conduction studies

Sensory nerve conduction studies (NCSs) show abnormalities of decreased amplitude in the presence of axonal disruption. Physical examination will direct the electromyographic examination. Sensory NCSs are beyond the reference range for amplitude and/or latency only if a large enough percentage of the sensory axons are damaged. A lesion that eliminates conduction in less than 10% of the sensory axons produces a loss of amplitude that may not be detectable. H-reflex latencies may be prolonged or absent in mononeuritis multiplex.

Motor nerve conduction studies

Abnormalities are similar to those seen in axonal polyneuropathies, with the exception of the anatomic distribution. A reduction in the motor action potential amplitudes and minimal alterations in nerve conduction velocity will be seen.

Velocity may be slightly reduced compared with the reference range, but it does not usually decrease below 70-80% of the lower limit of the reference range. Abnormal findings directly depend on the severity and aggressiveness of the underlying disease. A decrement of motor amplitude may be seen if there is significant denervation.

Needle electrode examination

Findings on the needle electrode examination can vary, depending on the severity and time course of the disorder. Findings are typically neuropathic and may include abnormal spontaneous membrane activity (positive sharp waves and fibrillation potentials) and increases in motor unit action potential (MUAP) duration, amplitude, and polyphasia. Additionally, the loss of motor axons should generate a reduced number of MUAPs firing at high rates (ie, reduced recruitment).

In some cases, a nerve biopsy may be appropriate to determine the underlying cause of mononeuritis multiplex (eg, a combination of perivascular mononuclear inflammatory cells and multifocal axonal loss and axonal loss with multinucleated inflammatory cells).^[3] A pattern of necrotizing vasculitis of epineural arteries may be observed in HIV-related mononeuritis.

Some studies have indicated that combining a nerve biopsy with a muscle biopsy can help clinicians to better diagnose peripheral nerve vasculitis, because it appears that in many cases, individuals with peripheral nerve vasculitis also have vasculitis in striated muscle.^[3] A 1988 study, for example, found that among patients known to had peripheral nerve vasculitis, up to 45% of them did not have demonstrable evidence of the condition in their superficial peroneal nerve but did show evidence of vasculitis in their peroneus brevis muscle.^[15]

However, a 2008 study of patients with proven peripheral nerve vasculitis (most of whom presented with either asymmetrical axonal polyneuropathy or mononeuritis multiplex) found that the benefits of combining nerve and muscle biopsies for the diagnosis of peripheral nerve vasculitis seem to depend on which muscle and nerve are chosen.^[3] When compared with results from sural nerve biopsies alone, the report's authors found no significant increase in diagnoses of peripheral nerve vasculitis derived from combined biopsies of the sural nerve and the vastus lateralis muscle.