Lhermitte-Duclos disease (LDD) is a very rare, benign (non-cancerous) brain tumor, called a dysplastic gangliocytoma of the cerebellum, that is characterized by abnormal development and enlargement of the cerebellum, and an increased intracranial pressure. LDD manifests most commonly in the third and fourth decades of life. Symptoms may include headache, nausea, cerebellar dysfunction, hydrocephalus, ataxia (problems with movement and coordination), and visual disturbances. Other features may include an enlarged brain (megalencephaly), hydromyelia, extra fingers or toes (polydactyly), partial gigantism, and/or a large tongue (macroglossia). Lhermitte-Duclos disease can occur as an isolated condition; it is also associated with a hereditary cancer syndrome called Cowden disease. Although the exact cause is unknown, genetic changes in the PTEN gene have been identified in some individuals with LDD.