Genetic testing and counseling in the diagnosis and management of young-onset dementias.

Young-onset dementia is hereditary, multifactorial, or sporadic. The most common hereditary dementias include Alzheimer disease, frontotemporal degeneration, Huntington disease, prion diseases, and cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy. Careful attainment of family history assists with diagnosis and determining the likelihood of a genetic cause, and can direct genetic testing. The type of genetic testing depends on confidence of the diagnosis, patient's and affected relatives' symptoms, and the number of disease genes. Single gene, disease-specific gene panels, and large dementia panels are available. Genetic counseling should be given and informed consent obtained. Predictive testing follows the Huntington disease protocol.