Pick disease refers to the neuropathologic finding of 'Pick bodies,' which are argyrophilic, intraneuronal inclusions, and 'Pick cells,' which are enlarged neurons. The clinical correlates of Pick disease of brain include those of frontotemporal dementia, which encompass the behavioral variant of FTD, semantic dementia, and progressive nonfluent aphasia (summary by Piguet et al., 2011). Kertesz (2003) suggested the term 'Pick complex' to represent the overlapping syndromes of FTD, primary progressive aphasia (PPA), corticobasal degeneration (CBD), progressive supranuclear palsy (601104), and FTD with motor neuron disease. He noted that frontotemporal dementia may also be referred to as 'clinical Pick disease,' and that the term 'Pick disease' should be restricted to the pathologic finding of Pick bodies.

In general, frontotemporal dementia (FTD) refers to a clinical manifestation of the pathologic finding of frontotemporal lobar degeneration (FTLD). FTD, the most common subtype of FTLD, is a behavioral variant characterized by changes in social and personal conduct with loss of volition, executive dysfunction, loss of abstract thought, and decreased speech output. A second clinical subtype of FTLD is 'semantic dementia,' characterized by specific loss of comprehension of language and impaired facial and object recognition. A third clinical subtype of FTLD is 'primary progressive aphasia' (PPA), characterized by a reduction in speech production, speech errors, and word retrieval difficulties resulting in mutism and an inability to communicate. All subtypes have relative preservation of memory, at least in the early stages. FTLD is often associated with parkinsonism or motor neuron disease (MND) resembling amyotrophic lateral sclerosis (ALS; 105400) (reviews by Tolnay and Probst, 2002 and Mackenzie and Rademakers, 2007). Mackenzie et al. (2009, 2010) provided a classification of FTLD subtypes according to the neuropathologic findings (see PATHOGENESIS below). Clinical Variability of Tauopathies Tauopathies comprise a clinically variable group of neurodegenerative diseases characterized neuropathologically by accumulation of abnormal MAPT-positive inclusions in nerve and/or glial cells. In addition to frontotemporal dementia, semantic dementia, and PPA, different clinical syndromes with overlapping features have been described, leading to confusion in the terminology (Tolnay and Probst, 2002). Other terms used historically include parkinsonism and dementia with pallidopontonigral degeneration (PPND) (Wszolek et al., 1992); disinhibition-dementia-parkinsonism-amyotrophy complex (DDPAC) (Lynch et al., 1994); frontotemporal dementia with parkinsonism (FLDEM) (Yamaoka et al., 1996); and multiple system tauopathy with presenile dementia (MSTD) (Spillantini et al., 1997). These disorders are characterized by variable degrees of frontal lobe dementia, parkinsonism, motor neuron disease, and amyotrophy. Other neurodegenerative disorders associated with mutations in the MAPT gene include Pick disease (172700) and progressive supranuclear palsy (PSP; 601104). Inherited neurodegenerative tauopathies linked to chromosome 17 and caused by mutation in the MAPT gene have also collectively been termed 'FTDP17' (Lee et al., 2001). Kertesz (2003) suggested the term 'Pick complex' to represent the overlapping syndromes of FTD, primary progressive aphasia (PPA), corticobasal degeneration (CBD), PSP, and FTD with motor neuron disease. He noted that frontotemporal dementia may also be referred to as 'clinical Pick disease' and that the term 'Pick disease' should be restricted to the pathologic finding of Pick bodies. Genetic Heterogeneity of Frontotemporal Lobar Degeneration Mutations in several different genes can cause frontotemporal dementia and frontotemporal lobar degeneration, with or without motor neuron disease. See FTD2 (607485), caused by mutation in the GRN gene (138945) on chromosome 17q21; FTDALS7 (600795), caused by mutation in the CHMP2B gene (609512) on chromosome 3p11; inclusion body myopathy with Paget disease and FTD (IBMPFD; 167320), caused by mutation in the VCP gene (601023) on chromosome 9p13; ALS6 (608030), caused by mutation in the FUS gene (137070) on 16p11; ALS10 (612069), caused by mutation in the TARDBP gene (605078) on 1p36; and FTDALS1 (105550), caused by mutation in the C9ORF72 gene (614260) on 9p21. In 1 family with FTD, a mutation was identified in the presenilin-1 gene (PSEN1; 104311) on chromosome 14, which is usually associated with a familial form of early-onset Alzheimer disease (AD3; 607822).

Mitochondrial complex III deficiency nuclear type 2 is an autosomal recessive severe neurodegenerative disorder that usually presents in childhood, but may show later onset, even in adulthood. Affected individuals have motor disability, with ataxia, apraxia, dystonia, and dysarthria, associated with necrotic lesions throughout the brain. Most patients also have cognitive impairment and axonal neuropathy and become severely disabled later in life (summary by Ghezzi et al., 2011). The disorder may present clinically as spinocerebellar ataxia or Leigh syndrome, or with psychiatric disturbances (Morino et al., 2014; Atwal, 2014; Nogueira et al., 2013). For a discussion of genetic heterogeneity of mitochondrial complex III deficiency, see MC3DN1 (124000).