Subacute combined degeneration of the spinal cord masking motor neuron disease: a case report

BACKGROUND: Subacute combined degeneration of the spinal cord is a potentially reversible myelopathy typically associated with vitamin B12 deficiency. There is predominant involvement of spinal cord posterior and lateral tracts, and manifestations include peripheral paraesthesia, impaired proprioception, gait disturbance, neuropathy and cognitive changes. Motor neuron disease (MND) is an unremittingly progressive neurodegenerative disorder involving upper and lower motor neurons with an average prognosis of 2–3 years. The diagnosis is clinical and may be supported by electromyography. A subset of MND occurs concurrently with frontotemporal dementia (FTD-MND) and may be initially misdiagnosed as a primary psychotic disorder. CASE PRESENTATION: We describe a 57-year-old Caucasian woman who presented with confusion and dysarthria. Low vitamin B12 levels and MRI findings led to an initial diagnosis of subacute combined degeneration of the spinal cord. Despite treatment, persistent dysarthria and presence of both upper and lower motor neuron signs on clinical examination prompted further assessment. Electromyography supported the diagnosis of MND. Comorbid chronic paranoid schizophrenia complicated the diagnostic process. We discuss overlapping features between B12 deficiency and MND as well as the neuropsychiatric overlap of B12 deficiency, FTD-MND and chronic schizophrenia. CONCLUSIONS: Firstly, variability in neurocognitive and imaging manifestations of B12 deficiency can limit delineation of other pathologies. Failure to improve following correction of nutritional deficiencies warrants further investigation for an alternate diagnosis. Secondly, re-evaluation of patients with comorbid mental health conditions is important in reaching timely and accurate diagnoses.