Focal Dermal Hypoplasia (FDH) is a rare ectomesodermal genetic disorder that mainly affects development of the skin, skeletal system and eyes. Typical dermatological features of the condition include localized patches of hypoplastic skin; presence of papillomas in sensitive areas like gums, tongue, armpits, genital organs, and anus; hyperkeratosis in the palms and soles resulting in excessive sweating; and patchy hair loss. In addition, patients may have dental problems (malformed teeth and cavities), webbed digits, syndactyly, spinal curvature, hearing loss, facial deformities, and ocular abnormalities (anophthalmia, coloboma, and strabismus). The disorder can also affect the stomach, intestines, heart, lungs, and kidneys. About 15% of affected patients also display intellectual disability. With rare exceptions, FDH is seen only in females. This is because the condition is lethal in males, and affected males die in utero.

Diagnosis of FDH is based on the clinical symptoms of the condition. The lack of pigmentation and abnormal sweating seen in FDH are useful indicators in differentiating it from other ectodermal dysplasias. Affected female patients can be properly managed with the help of dermatological treatments, dental work, orthopedic surgery, and if needed, respiratory therapies.

FDH is an X-linked dominant disorder, which explains the lethality observed in males. It is caused by mutation in PORCN gene, the protein product of which modulates the Wnt signalling pathway that is involved in skin organogenesis. Loss of function mutations in the PORCN gene interferes with the normal functioning of Wnt pathway and is believed to affect dermal tissue development.