Objective:

To assess the role of serial electrophysiology in early identification of disease severity in anti-GQ1b spectrum disorders

Background:

Miller Fisher Syndrome and Bickerstaff Brainstem Encephalitis share anti-GQ1b antibody–mediated pathology affecting oculomotor nerves, muscle spindle afferents, peripheral nerves, and the brainstem reticular formation. Electrophysiological studies typically demonstrate a sensory-predominant axonal pattern with reduced sensory nerve action potentials (SNAPs) as the most frequent early finding. Motor and cranial nerve–predominant involvement with preserved sensory conduction is less well characterized, and its relationship to clinical severity remains poorly defined.

Design/Methods:

Case report

Results:

A previously healthy 36-year-old male presented with acute diplopia progressing within hours to ptosis, dysarthria, facial palsy, ophthalmoplegia, encephalopathy, and respiratory failure requiring intubation. Serial cerebrospinal fluid analyses and MRI brain remained unremarkable. Serial nerve conduction studies (NCS) and electromyography (EMG) demonstrated a motor- and cranial nerve–predominant axonal pattern with preserved sensory conduction. SNAPs were normal, while compound motor action potentials were reduced in the left ulnar and accessory nerves. EMG revealed absent motor unit potentials in facial and bulbar muscles, directly correlating with dysarthria, dysphagia, and respiratory failure. Low-amplitude H-reflexes suggested concurrent central involvement. Serial electrophysiology provided the only objective evidence of dual central and peripheral nervous system pathology and guided clinical decision-making prior to serological confirmation. Anti-GQ1b antibodies were markedly elevated (>1:12,800), confirming MFS–BBE overlap. Concurrent COVID-19 infection was identified as a likely immunological trigger. Despite IVIG therapy, the patient required tracheostomy and gastrostomy.

Conclusions:

This case demonstrates a motor- and cranial nerve–predominant axonal pattern with preserved sensory conduction, diverging from the classically described sensory-predominant profile in anti-GQ1b spectrum disorders. Absent bulbar motor unit potentials correlated with severe clinical involvement and prolonged respiratory dependence, suggesting that serial electrophysiology may serve as an early marker of disease severity and help identify patients at risk for severe outcomes.