A case of Homer-3 IgG cerebellar ataxia & literature review of 15 reported cases.

Homer-3-IgG autoimmunity is a rare diagnosis typically presenting with subacute cerebellar ataxia. Here, we report a patient presenting insidiously with cerebellar ataxia and mild cognitive impairment, referred to the Mayo Clinic for a second opinion of her initial diagnosis of parkinsonism. Considering autoimmune cerebellar ataxia as the correct diagnosis, the patient's cerebrospinal fluid (CSF) and serum samples were sent for Movement Disorders Autoimmune/Paraneoplastic Evaluation neural antibody testing. Murine tissue-based indirect immunofluorescence assay of serum and CSF revealed a unique cerebellar molecular layer-predominant 'medusa head' staining pattern. Protein microarray disclosed Homer-3 as high ranking hits. Serological diagnosis was confirmed by Homer-3-specific line blot assay. The patient was treated with a 12-week regimen of intravenous methylprednisolone. She had limited improvement, though at 6-month follow-up she had not progressed further off treatment. This patient is the third detected case with Homer-3-IgG in the Neuroimmunology laboratory of the Mayo Clinic, and the only one clinically evaluated at Mayo Clinic. Histories for the two other cases were unavailable. This case adds to the limited number of reports on Homer-3-IgG autoimmunity, with our literature review identifying 15 other cases reported to date. Neurological presentations include cerebellar ataxia (15/16), cognitive impairment (4/16), REM sleep behavior disorder (2/16), seizures (2/16), myeloradiculopathy (1/16), radiculoneuropathy (1/16), and psychosis (1/16). The most common treatment used was corticosteroids (15/16), followed by IVIg (7/16), mycophenolate mofetil (MMF) (4/16), plasma exchange (3/16), and rituximab (1/16). Reported outcomes have varied, with partial improvement being most common (7/15).