Diencephalic syndrome is a condition caused by a tumor located in a portion of the brain above the brainstem called the diencephalon. The diencephalon includes the hypothalamus and the thalamus. This condition is usually found in infants and young children and may result in symptoms including failure to gain weight and grow normally (failure to thrive), progressive thinness and weakness (emaciation), and hyperactivity and restlessness (hyperkinesia). Other symptoms may include abnormal eye movements and vision issues, vomiting, and hydrocephalus. Diencephalic syndrome is most commonly caused by astrocytomas; however, other types of tumors may be associated including ependymoma, dysgerminoma, and ganglioma. Given their location, these tumors may cause symptoms by leading to abnormal functioning of the hypothalamus and optic nerve.