To report an intriguing case of a patient with the rare Facial Onset Sensory and Motor Neuronopathy (FOSMN) syndrome who developed bulbar weakness prior to trigeminal sensory loss. 

FOSMN is a very rare neurodegenerative disease first described in 2006 and since then only 100 cases have been reported ¹. The clinical course is characterized by an initial trigeminal sensory loss followed by bulbar dysfunction and progressive muscle weakness in a cranial to caudal pattern. FOSMN usually presents with a predominantly lower motor neuron pattern of weakness with a smaller proportion of patients also exhibiting upper motor neuron signs. There is only one prior reported case of bulbar symptoms preceding sensory symptoms ². The pathophysiology remains unknown but it is postulated to be a TAR DNA-binding protein 43 (TDP-43) proteinopathy similar to amyotrophic lateral sclerosis and frontotemporal dementia¹. It may in fact be immune mediated as a handful of cases have responded to IVIG therapy ^3,4.

Literature review performed and case described.  

An otherwise healthy 65-year old female presented with a 12-month history of speech disturbance, dysphagia, tongue fasciculations, diffuse hyperreflexia and lower facial weakness without trigeminal sensory loss. Diagnostic studies including MRI-Brain, EMG and extensive autoantibody and neuropathy work-up were negative. Two months later she returned to clinic with numbness in the right V2 trigeminal distribution. Given the presence of trigeminal sensory loss in conjunction with facial onset progressive weakness, the patient was suspected to have FOSMN syndrome, with other differentials including progressive bulbar onset amyotrophic lateral sclerosis, however such prominent sensory symptoms would be unusual.

This case adds to the 100 documented FOSMN cases in the existing literature and illustrates a possible clinical course of FOSMN where bulbar weakness precedes trigeminal sensory loss.