Group of clinically, pathologically, and genetically heterogeneous disorders associated with atrophy in the frontal lobe and temporal lobe of the brain, with sparing of the parietal lobe and occipital lobe. FTLD is one of the most common causes of dementia after alzheimer’s disease, lewy body dementia, and vascular dementia. Three clinical subtypes of FTLD are described: frontotemporal dementia, semantic dementia, and progressive nonfluent aphasia.