Diencephalic syndrome of infancy, or Russell's syndrome, is a rare cause of failure to thrive. Infants present with severe emaciation despite adequate feeding and linear growth. Hydrocephalus, skin pallor, hyperactivity, euphoria, nystagmus, and vomiting are also sometimes observed. Symptoms typically begin during the first year of life.

Diencephalic syndrome is secondary to a neoplasm growing near the hypothalamus or optic chiasm. The physiologic cause of the failure to thrive is not fully understood, but is hypothesized to be associated with abnormal growth hormone secretion.

Diencephalic syndrome will resolve with treatment of the underlying tumor.