Distal Myasthenia Gravis: A Missed Clinical Diagnosis

Abstract

Myasthenia gravis (MG) is an immune mediated, post-synaptic neuromuscular transmission disorder, presenting with predominant ocular, bulbar, and/or proximal fluctuating weakness and fatiguability. Distal and asymmetric onset of symptoms in MG is rare and could lead to delay in diagnosis especially when fatiguability or fluctuation is not clearly reported as a prominent initial symptom – as seen with our patients. We report three cases of distal MG who presented with predominant distal, asymmetric hand weakness. Their clinically diagnoses for referral to the electrodiagnostic (EDx) laboratory were: Chronic inflammatory demyelinating peripheral neuropathy, C8T1 motor radiculopathy, and multifocal motor neuropathy. It was in the EDx laboratory that the diagnosis of MG was suspected in these patients. Since the routine EDx tests were normal in all three patients – A repetitive nerve stimulation test (RNST) was done – which was positive in the affected muscles and hence a diagnosis of distal MG made. (This is the protocol followed in our laboratory) These patients were then found to have high serum levels of antibodies to acetyl choline receptors – confirming the diagnosis of MG. RNST is not routinely done in all patients sent for EDx testing and none of these patients had a reference for the same – however, using a protocol based method of EDx testing, lead to the diagnosis of a treatable condition. Electrodiagnostician need to be aware of this presentation of MG and follow protocols or the diagnosis could be missed.