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Finger Extension Weakness and Downbeat Nystagmus Motor Neuron Disease Syndrome: A Series of Two Cases.

E ye movement abnormalities are rare in motor neuron disease (MND). Ophthalmoparesis is eventually found in patients with amyotrophic lateral sclerosis (ALS) who chose invasive ventilation, but is otherwise typically not observed. There are a few case reports documenting slowing of vertical saccades, gaze-evoked horizontal or rotatory nystagmus, impairment of smooth pursuit, and progressive ophthalmoplegia and eyelid apraxia (1–4). These eye movement abnormalities are typically seen in ALS patients with bulbar onset or frontal lobe impairment. In 2006, Thakore et al (5) reported 3 cases of a novel MND with early and disproportionately severe involvement of finger and wrist extensors, downbeat nystagmus (DBN), relative sparing of bulbar and respiratory musculature, prolonged survival, and lack of family history. In 2016, they described an additional 3 patients with the same clinical phenotype, and designated the disorder as Finger Extension Weakness and Downbeat Nystagmus Motor Neuron Disease Syndrome with an acronym of FEWDON-MND (6). We document 2 patients with FEWDON-MND adding clinical and genetic insights into this novel entity. A 31-year-old woman reported an eleven-year history of slowly progressive left hand weakness years after the onset of left hand weakness, she experienced oscillopsia worse with downgaze, and 3 years after that cramping in her calves and left forearm muscles with generalized intermittent fasciculations. There were no associated sensory symptoms. Her medical history was significant for anxiety, and she was on sertraline and oral contraceptive pill. There was no family history of neurological or neuromuscular disorders. On examination, visual acuity was 20/20 in each eye with intact pupillary responses and color vision. Ocular motility showed full versions and ductions with DBN in primary gaze, which worsened with lateral gaze (See Supplemental Digital Content, video, http://links.lww.com/WNO/A358) and downgaze. Pursuit was interrupted by DBN, which did not dampen with convergence. Automated visual fields and the appearance of the anterior and posterior segments of both eyes were normal. Neurological examination revealed tongue fasciculations and atrophy without slowing of speech or dysarthria. There were both upper and lower motor neuron signs present in the limbs with predominant finger extensor weakness bilaterally. Nerve conduction studies (NCS) showed a reduced right peroneal motor amplitude, and needle electromyography (EMG) revealed large motor units and reduced recruitment in the left cervical and lumbosacral myotomes with fibrillations. MRI brain showed a Chiari I malformation without cerebellar atrophy, and MRI of the cervical spine was unremarkable. A 39-year-old woman experienced oscillopsia at the age of 18 years. Nine years later, after her second pregnancy, she developed persistent fatigue. At age 32 years, the patient presented to our clinic with slowly progressive weakness. She had no significant medical history and was not taking any medications. There was no family history of neurological or neuromuscular disorders. On examination, visual acuity was 20/20 in the right eye and 20/30 in the left eye. Pupillary responses and color vision were normal. Ocular motility showed full versions and ductions with DBN in primary gaze, worsened with lateral gaze and downgaze, and slightly decreased in upgaze. Pursuit was interrupted by the DBN, which did not dampen with convergence. Automated perimetry, slitlamp examination, and funduscopy were normal. Neurological examination revealed both upper and lower motor neuron signs in the limbs with predominant finger extensor weakness bilaterally. NCS were normal and needle EMG showed very large motor units with reduced recruitment typical of a very longstanding slowly progressive neurogenic process. Brain MRI was normal. A trial of 3, 4-diaminopyridine had resulted in some symptomatic relief. However, she independently stopped this medication. Department of Neurology (AM), University of Toronto, Toronto, Canada; Department of Neurology (LZ, ANES), Sunnybrook Health Sciences Centre, Toronto, University of Toronto, Canada; Department of Neurology (PA), Toronto Western Hospital, University of Toronto, Toronto, Canada; and Department of Ophthalmology and Vision Sciences (ANES), University of Toronto, Toronto, Canada.