Purkinje cell cytoplasmic antibody type 1 (anti-Yo) autoimmunity in a child with Down syndrome.

IMPORTANCE Purkinje cell cytoplasmic antibody type 1 (PCA-1)-IgG (or anti-Yo) is characteristically detected in women with gynecological or breast adenocarcinoma. We describe 2 unique scenarios occurring in 1 patient: PCA-1 paraneoplastic autoimmunity in a child, and a paraneoplastic neurological disorder in the context of Down syndrome. OBSERVATIONS A child with Down syndrome and a history of adrenocortical carcinoma resected at age 1 year presented at age 7 years with cerebellar ataxia of subacute onset. Paraneoplastic serological and cerebrospinal fluid evaluations revealed PCA-1. Serological and biochemical studies also supported a diagnosis of subclinical autoimmune hypothyroidism. Extensive serum, urine, and radiological testing did not reveal a new or recurrent neoplasm. Neurological improvements after standard immunotherapy were lacking. CONCLUSIONS AND RELEVANCE Solid organ neoplasms are uncommon among patients with Down syndrome, but organ-specific autoimmune diseases are common. In our patient, Down syndrome-related impaired T regulatory lymphocyte function (previously reported) may have resulted in both enhanced immunity against an undetected solid neoplasm and paraneoplastic neurological (PCA-1) autoimmunity.