To describe the existing literature on spinocerebellar syndromes associated with HTLV-I and present a new case of HTLV-1-associated downbeat nystagmus.

In patients from endemic regions, HTLV-1 is classically associated with a slowly progressive myelopathy (HAM), correlating with spinal cord atrophy on MRI, and adult T-cell leukemia/lymphoma (ATL). However, recent literature suggests that there is a much broader spectrum of disease involving the central nervous system, peripheral nervous system, and association with rheumatologic conditions. Posterior fossa presentations, also termed HTLV-1-associated spinocerebellar syndrome (HSCS), have been noted both early and late in the course of HAM, however they continue to be underrecognized. Early recognition of HTLV-1-associated syndromes is becoming crucial as novel therapies such as mogamulizumab seem to show greatest efficacy when given in low-disability states.

Twenty published cases of posterior fossa manifestations associated with HTLV-I were identified.  Two patients demonstrated acute cerebellar ataxia without other neurologic manifestations of HTLV-1. Otherwise, all patients demonstrated progressive spinal and cerebellar signs (13 with initial cerebellar, 3 with spinal, and 2 with simultaneous onset). Eight documented eye movement abnormalities, including 3 with downbeat nystagmus. Ten patients had cerebellar atrophy. We present a 54-year-old woman from the Dominican Republic with 18 years of symptoms whose examination was notable for spastic paraparesis, absent distal vibration sense, pathologic hyperreflexia, and downbeat nystagmus in all directions of gaze without dysmetria or other cerebellar deficits. MRI showed severe spinal cord atrophy without pontocerebellar atrophy. HTLV-1 antibodies were present in cerebrospinal fluid (CSF) and serum, and extensive testing for autoimmune, nutritional, and other mimics was negative. 

In addition to classic manifestations of HTLV-1, it is critical to recognize HSCS in patients from HTLV-endemic regions. Spinocerebellar signs can be the presenting feature of HTLV-1, and recognition could lead to earlier initiation of disease-modifying therapy.