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

Eye 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.