Electrophysiological Abnormalities in Finger Extension Weakness and DOwnbeat Nystagmus Motor Neuron Disease: Three New Patients and Review of the Literature

Abstract

Introduction/aims: Finger Extension Weakness and DOwnbeat Nystagmus Motor Neuron Disease (FEWDON-MND) is characterized by motor weakness predominantly affecting finger extension, accompanied by downbeat nystagmus. To date, only 11 patients have been reported. The present study adds a further three and aims to provide a more detailed description of the electrodiagnostic features of these patients.

Methods: We present the clinical and electrophysiological features of three French patients from specialized motor neuron centers and review the electrophysiological findings of previously reported patients.

Results: These three patients presented with pure motor weakness affecting finger extension and downbeat nystagmus. They exhibited a slowly progressive disease course without respiratory involvement. Nerve conduction studies showed decreased compound muscle action potential amplitudes in the extensor indicis muscles. Abnormal spontaneous activity on needle electromyography (EMG) was rare in two patients, absent in one, and otherwise limited to weak muscles. Additionally, chronic motor axon loss features suggestive of motor neuronopathy were seen in our patients. Importantly, they were also detected in distant asymptomatic muscles.

Discussion: The three patients reported here confirm the typical phenotype of FEWDON-MND, characterized by slowly progressive distal motor weakness initially affecting finger extension, associated with downbeat nystagmus. Although chronic motor axon loss features have been found in all reported patients, our three patients show that active denervation can be absent or rare. Thus, finger drop and diffuse chronic neurogenic changes on EMG should lead clinicians to look for downbeat nystagmus and to consider FEWDON-MND.

Keywords: FEWDON‐MND; amyotrophic lateral sclerosis; downbeat nystagmus; electrophysiology; motor neuron.