Downbeat nystagmus secondary to familial hemophagocytic lymphohistiocytosis.

Hemophagocytic lymphohistiocytosis is a rare autosomal recessive disorder characterized by severe inflammation induced by defective natural killer cell function, which triggers a state of highly stimulated but ineffective immune response. This disorder can affect multiple organ systems, and neurologic manifestations include irritability, seizures, impaired consciousness, meningismus, and cranial nerve palsies. We describe a unique case of hemophagocytic lymphohistiocytosis in which downbeat nystagmus developed due to cerebellar swelling with compression of the cervicomedullary junction.