Congenital hypotrichosis refers to a group of inherited conditions that present with sparse hair as a result of abnormalities in hair structure, differentiation, or growth. Numerous distinct genetic mutations, inherited in either an autosomal dominant or autosomal recessive inheritance pattern, may underlie hypotrichotic phenotypes. Hypotrichosis may be isolated (nonsyndromic), or it may occur as part of a syndrome. Depending on the mutation, scalp hair only; scalp, eyelash, and eyebrow hair; all terminal hair growth sites, including pubic and axillary hair; or all hair on scalp and body globally, may be affected.

Congenital hypotrichosis most commonly presents at birth, but with some genotypes, hypotrichosis is noticed in the first year of life or in early childhood. In Marie Unna hereditary hypotrichosis, hair is sparse or absent at birth, grows in and becomes wiry and coarse in childhood, and this is followed by progressive hair loss after puberty.

Hair may be dry, fine, brittle, coarse, twisted, or tightly coiled. Known associated hair shaft abnormalities may accompany the hypotrichosis. In monilethrix-like hypotrichosis (hypotrichosis type 6), hairs shafts display monilethrix-like features. The woolly hair syndromes (eg, type 1, type 2, type 3) and uncombable hair syndromes may also be accompanied by hypotrichosis. Pili torti is a feature of hypotrichosis with juvenile macular dystrophy, a distinct genodermatosis with progressive macular dystrophy.

Additional clinical features associated with nonsyndromic hypotrichosis, besides the macular dystrophy already mentioned, include hyperkeratotic follicular papules (monilethrix-like hypotrichosis), vesicles on the scalp and other skin surfaces (hypotrichosis and recurrent skin vesicles), and dental anomalies (autosomal recessive woolly hair type 3 with hypotrichosis).

Congenital hypotrichosis may accompany genetic syndromes, including hypohidrotic ectodermal dysplasia, Clouston syndrome, ectrodactyly-ectodermal dysplasia-clefting (EEC) syndrome, acro-dermato-ungual-lacrimal-tooth (ADULT) syndrome, Hallerman-Streiff syndrome, Treacher-Collins syndrome, Conradi-Hunnermann syndrome and popliteal pterygium syndrome, and Bartsocas-Papas type, among others.