nevoid hypertrichosis ( nh ) is an uncommon disorder consisting of a solitary , circumscribed area of terminal hair growth present at or soon after birth . the involved hair may be normal or hypo - pigmented , with or without any associated cutaneous and extra - cutaneous abnormalities . we report a 12- year- old boy with localized hypertrichosis but without any underlying pigmentary or structural abnormality . a 12-year - old boy presented with a circumscribed linear patch of terminal hair over the middle of the forehead since birth . a patch of hair appeared over the forehead which gradually increased in size over the next few years . cutaneous examination revealed a localized 42 cm well - circumscribed patch of terminal hair present over the forehead , 1 cm left of the midline [ figure 1 ] along blaschko 's lines . the overlying hairs were black , coarse and measured 3 - 4 cm in length . 42 cm circumscribed patch of terminal hair over the forehead routine hematological and biochemical investigations were normal . the histological examination revealed an increased number of morphologically normal hair follicles [ figure 2 ] . skin biopsy with normal epidermis and increased number of normal hair follicles ( h and e 100 ) primary hypertrichosis has been defined based on the age of onset as congenital or acquired and on the extent of distribution as localized or generalized . nevoid hypertrichosis is an uncommon congenital disorder consisting of terminal hair growth in a localized distribution . the skin lesions are usually present at or soon after birth and are typically solitary , though cases of multiple patches developing after puberty have been reported . a few cases of multiple nh in which patches are either round , in a check - board or linear pattern , along blaschko 's lines have also been documented . the color of the terminal hair may be the same as the color of the scalp hair , rarely grey or white . the histopathology usually shows a normal epidermis with increased number of morphologically normal hair follicles in the dermis but is needed to rule out any underlying nevus . it has not yet been clarified whether nh is a malformation of unknown etiology or is genetically determined . lopez - barrantes et al . , concluded that nh may occur in a mosaic pattern and may be associated with other cutaneous diseases that may show a segmental pattern as a twin spotting phenomenon with severe mental , ocular or myoskeletal anomalies or without any associated abnormalities . twin spots consist of two genetically different clones of neighboring cells in a background of normal cells . if the mutations give rise to a clinically visible phenotype , the twin spot is evident as paired nevoid skin abnormalities . congenital disorders in which localized hypertrichosis occur are becker 's nevus , congenital melanocytic nevi and smooth muscle hamartoma . in our case , absence of pigmentary alteration of the underlying skin , absence of induration or pseudo darier 's sign rules out the other congenital disorders mentioned above . lumbo - sacral hypertrichosis or faun tail deformity is localized to the sacral midline and is associated with a variety of concurrent cutaneous malformations such as sacral dimple , lipoma , port - wine stain , or dermoid cyst . this developmental defect often overlies bone and spinal cord defects , most commonly spina bifida occulta and diastematomyelia . the cutaneous lesions may persist for life , however , spontaneous resolution has also been reported . the other associations which have been reported are hypertrichosis cubiti , an uncommon variety of congenital circumscribed hypertrichosis in which unexplained long vellus hair are present over the extensor surface of the upper extremities along with unexplained short stature in some cases . sotiriadis et al . , documented that nh is not often associated with systemic abnormalities . however , they mentioned partial lipoatrophy or bony abnormalities as reported in the literature . further , a review of the literature by us revealed that nh may be associated with ocular , cerebral , mental , myoskeletal , neurological , gastrointestinal , pulmonary and facial dysmorphia [ table 1 ] . patients with nh must be investigated to rule out any systemic involvement associated with the skin lesion(s ) .
nevoid hypertrichosis is a rare entity characterized by circumscribed patches of terminal hair . 
 it is associated with many cutaneous and extra - cutaneous abnormalities . in our case , 
 a single circumscribed patch of terminal hair growth was present without any underlying or associated cutaneous or systemic abnormalities .