Focal dermal hypoplasia is a multisystem disorder characterized primarily by involvement of the skin, skeletal system, eyes, and face. Skin manifestations present at birth include atrophic and hypoplastic areas of skin; cutis aplasia; fat nodules in the dermis manifesting as soft, yellow-pink cutaneous nodules; and pigmentary changes. Verrucoid papillomas of the skin and mucous membranes may appear later. The nails can be ridged, dysplastic, or hypoplastic; hair can be sparse or absent. Limb malformations include oligo-/syndactyly and split hand/foot. Developmental abnormalities of the eye can include anophthalmia/microphthalmia, iris and chorioretinal coloboma, and lacrimal duct abnormalities. Craniofacial findings can include facial asymmetry, notched alae nasi, cleft lip and palate, and pointed chin. Occasional findings include dental anomalies, abdominal wall defects, diaphragmatic hernia, and renal anomalies. Psychomotor development is usually normal; some individuals have cognitive impairment.

Incontinentia pigmenti (IP) is a disorder that affects the skin, hair, teeth, nails, eyes, and central nervous system; it occurs primarily in females and on occasion in males. Characteristic skin lesions evolve through four stages: I. Blistering (birth to age ~4 months). II. Wart-like rash (for several months). III. Swirling macular hyperpigmentation (age ~6 months into adulthood). IV. Linear hypopigmentation. Alopecia, hypodontia, abnormal tooth shape, and dystrophic nails are observed. Neovascularization of the retina, present in some individuals, predisposes to retinal detachment. Neurologic findings including seizures, intellectual disability, and developmental delays are occasionally seen.

Darier-White disease (DAR), also known as keratosis follicularis, is an autosomal dominant skin disorder characterized by warty papules and plaques in seborrheic areas (central trunk, flexures, scalp, and forehead), palmoplantar pits, and distinctive nail abnormalities (Sakuntabhai et al., 1999). Onset is usually before the third decade, and penetrance is complete in adults, although expressivity is variable. Involvement may be severe, with widespread itchy malodorous crusted plaques, painful erosions, blistering, and mucosal lesions. Secondary infection is common. Sun, heat, and sweating exacerbate the symptoms. Darier disease never remits, but oral retinoids may reduce hyperkeratosis. Neuropsychiatric abnormalities, including mild mental retardation and epilepsy, have been described in association with Darier disease in a few families (Burge and Wilkinson, 1992); whether this is an association based on pleiotropism of the mutant gene or reflects coincidence is not clear. Histologic findings are (1) mild nonspecific perivascular infiltration in the dermis; (2) dermal villi protruding into the epidermis; (3) suprabasal detachment of the spinal layer leading to the formation of lacunae containing acantholytic cells; (4) in the more superficial epidermis, dyskeratotic round epidermal cells ('corps ronds'), the most distinctive feature; and (5) in the stratum corneum, 'grains' that resemble parakeratotic cells embedded in a hyperkeratotic horny layer. Electron microscopy reveals loss of desmosomal attachments, perinuclear aggregations of keratin filaments, and cytoplasmic vacuolization. Ultrastructural and immunologic studies suggest the disease results from an abnormality in the desmosome-keratin filament complex leading to a breakdown in cell adhesion.

Nail-patella syndrome (NPS) (previously referred to as Fong's disease), encompasses the classic clinical tetrad of changes in the nails, knees, and elbows, and the presence of iliac horns. Nail changes are the most constant feature of NPS. Nails may be absent, hypoplastic, or dystrophic; ridged longitudinally or horizontally; pitted; discolored; separated into two halves by a longitudinal cleft or ridge of skin; and thin or (less often) thickened. The patellae may be small, irregularly shaped, or absent. Elbow abnormalities may include limitation of extension, pronation, and supination; cubitus valgus; and antecubital pterygia. Iliac horns are bilateral, conical, bony processes that project posteriorly and laterally from the central part of the iliac bones of the pelvis. Renal involvement, first manifest as proteinuria with or without hematuria, occurs in 30%-50% of affected individuals; end-stage renal disease occurs up to 15% of affected individuals. Primary open-angle glaucoma and ocular hypertension occur at increased frequency and at a younger age than in the general population.

Craniofrontonasal syndrome is an X-linked developmental disorder that shows paradoxically greater severity in heterozygous females than in hemizygous males. Females have frontonasal dysplasia, craniofacial asymmetry, craniosynostosis, bifid nasal tip, grooved nails, wiry hair, and abnormalities of the thoracic skeleton, whereas males typically show only hypertelorism (Twigg et al., 2004; Wieland et al., 2004).

Pycnodysostosis is characterized by short-limbed short stature, typical facial appearance (convex nasal ridge and small jaw with obtuse mandibular angle), osteosclerosis with increased bone fragility, acroosteolysis of the distal phalanges, delayed closure of the cranial sutures, and dysplasia of the clavicle. In affected individuals, the facial features become more prominent with age, likely due to progressive acroosteolysis of the facial bones, but can usually be appreciated from early childhood, particularly the small jaw and convex nasal ridge. Additional features include dental and nail anomalies. Intelligence is typically normal with mild psychomotor difficulties reported in some individuals.

Acrokeratosis verruciformis of Hopf (AKV) is a localized disorder of keratinization affecting the distal extremities. Onset occurs early in life (Dhitavat et al., 2003).

Kindler syndrome (KS), a rare subtype of inherited epidermolysis bullosa, is characterized by skin fragility and acral blister formation beginning at birth, diffuse cutaneous atrophy, photosensitivity (most prominent during childhood and usually decreasing after adolescence), poikiloderma, diffuse palmoplantar hyperkeratosis, and pseudosyndactyly. Mucosal manifestations are also common and include hemorrhagic mucositis and gingivitis, periodontal disease, premature loss of teeth, and labial leukokeratosis. Other mucosal findings can include ectropion, urethral stenosis, and severe phimosis. Severe long-term complications of KS include periodontitis, mucosal strictures, and aggressive squamous cell carcinomas. Manifestations can range from mild to severe.

Some ectodermal dysplasias are here classified as congenital disorders characterized by abnormal development in 2 or more ectodermal structures (hair, nails, teeth, and sweat glands) without other systemic findings. Witkop syndrome is a rare autosomal dominant ectodermal dysplasia involving the teeth and nails. Although a few reported cases have sparse or fine hair, almost all affected individuals have normal hair, sweat glands, and ability to tolerate heat. Affected individuals have a variable number and variable types of congenitally missing permanent and/or primary teeth, which frequently results in lip eversion due to loss of occlusion in the vertical dimension. Nails are generally thin, slow-growing, brittle, and spoon-shaped (koilonychia). Toenails are usually more severely affected than fingernails. The nail defects are alleviated with age and may not be easily detectable during adulthood (summary by Jumlongras et al., 2001).

Odontoonychodermal dysplasia (OODD) is an autosomal recessive disorder characterized by dry hair, severe hypodontia, smooth tongue with marked reduction of fungiform and filiform papillae, onychodysplasia, hyperkeratosis of the palms and soles, hypo- and hyperhidrosis of the skin, and atrophic patches on the face (summary by Adaimy et al., 2007; Yu et al., 2019).

Dyskeratosis congenita and related telomere biology disorders (DC/TBD) are caused by impaired telomere maintenance resulting in short or very short telomeres. The phenotypic spectrum of telomere biology disorders is broad and includes individuals with classic dyskeratosis congenita (DC) as well as those with very short telomeres and an isolated physical finding. Classic DC is characterized by a triad of dysplastic nails, lacy reticular pigmentation of the upper chest and/or neck, and oral leukoplakia, although this may not be present in all individuals. People with DC/TBD are at increased risk for progressive bone marrow failure (BMF), myelodysplastic syndrome or acute myelogenous leukemia, solid tumors (usually squamous cell carcinoma of the head/neck or anogenital cancer), and pulmonary fibrosis. Other findings can include eye abnormalities (epiphora, blepharitis, sparse eyelashes, ectropion, entropion, trichiasis), taurodontism, liver disease, gastrointestinal telangiectasias, and avascular necrosis of the hips or shoulders. Although most persons with DC/TBD have normal psychomotor development and normal neurologic function, significant developmental delay is present in both forms; additional findings include cerebellar hypoplasia (Hoyeraal Hreidarsson syndrome) and bilateral exudative retinopathy and intracranial calcifications (Revesz syndrome and Coats plus syndrome). Onset and progression of manifestations of DC/TBD vary: at the mild end of the spectrum are those who have only minimal physical findings with normal bone marrow function, and at the severe end are those who have the diagnostic triad and early-onset BMF.

Split-hand/split-foot malformation is a limb malformation involving the central rays of the autopod and presenting with syndactyly, median clefts of the hands and feet, and aplasia and/or hypoplasia of the phalanges, metacarpals, and metatarsals. Some patients with SHFM3 have been found to have mental retardation, ectodermal and craniofacial findings, and orofacial clefting (Elliott and Evans, 2006). For additional phenotypic information and a discussion of genetic heterogeneity in this disorder, see SHFM1 (183600).

Schopf-Schulz-Passarge syndrome (SSPS) is an autosomal recessive disorder characterized by a constellation of multiple eyelid cysts, hypodontia, hypotrichosis, palmoplantar hyperkeratosis, and onychodystrophy (summary by Mallaiah and Dickinson, 2001).

T-cell immunodeficiency, congenital alopecia, and nail dystrophy (TIDAND) is an autosomal recessive primary immunodeficiency characterized by congenital thymic aplasia and severe T-cell immunodeficiency apparent at birth or soon thereafter. Affected individuals tend to have recurrent infections, oral candidiasis, and failure to thrive. Immunologic investigations show decreased numbers of T cells with poor proliferative response to phytohemagglutinin (PHA) and variable hypogammaglobulinemia. The phenotype is consistent with a T-/B+/NK+ form of severe combined immunodeficiency (SCID; see, e.g., 102700). Patients with FOXN1 mutations do not respond well to hematopoietic stem cell transplantation, as it is not curative; thymic transplantation offers a potential cure (Chou et al., 2014).

Hypotrichosis simplex refers to a group of hereditary isolated alopecias characterized by diffuse and progressive hair loss, usually beginning in early childhood (Pasternack et al., 2008). Localized autosomal recessive hypotrichosis (LAH) is characterized by fragile hairs that break easily, leaving short, sparse scalp hairs. The disorder affects the trunk and extremities as well as the scalp, and the eyebrows and eyelashes may also be involved, whereas beard, pubic, and axillary hairs are largely spared. In addition, patients can develop hyperkeratotic follicular papules, erythema, and pruritus in affected areas (summary by Schaffer et al., 2006). Woolly hair (WH) refers to a group of hair shaft disorders that are characterized by fine and tightly curled hair. Compared to normal curly hair that is observed in some populations, WH grows slowly and stops growing after a few inches. Under light microscopy, WH shows some structural anomalies, including trichorrhexis nodosa and tapered ends (summary by Petukhova et al., 2009). Several families have been reported in which some affected individuals exhibit features of hypotrichosis and others have woolly scalp hair (Khan et al., 2011). Woolly hair is also a feature of several syndromes, such as Naxos disease (601214) and cardiofaciocutaneous syndrome (115150) (Petukhova et al., 2009), or the palmoplantar keratoderma and cardiomyopathy syndrome (601214) (Carvajal-Huerta, 1998). Genetic Heterogeneity of Hypotrichosis and Woolly Hair For a discussion of genetic heterogeneity of nonsyndromic hypotrichosis, see HYPT1 (605389). For a discussion of genetic heterogeneity of localized hypotrichosis, see LAH1 (HYPT6; 607903). Another form of autosomal recessive woolly hair with or without hypotrichosis (ARWH2; 604379) is caused by mutation in the LIPH gene (607365) and is allelic to autosomal recessive localized hypotrichosis (LAH2). ARWH3 (616760) is caused by mutation in the KRT25 gene (616646) on chromosome 17q21. An autosomal dominant form of woolly hair with hypotrichosis (HYPT13; 615896) is caused by mutation in the KRT71 gene (608245) on chromosome 12q13. Another autosomal dominant form of woolly hair (ADWH; 194300) with normal hair density is caused by mutation in the KRT74 gene (608248) on chromosome 12q13, and is allelic to an autosomal dominant form of hypotrichosis simplex of the scalp (HYPT3; 613981) as well as an ectodermal dysplasia of the hair/nail type (ECTD7; 614929).

Patients with this syndrome develop cutaneous telangiectases in infancy with patchy alopecia over areas of affected skin, thinning of the lateral eyebrows, and mild dental and nail anomalies. Affected individuals are at increased risk of developing oropharyngeal cancer, and other malignancies have been reported as well (Tanaka et al., 2012).

Epidermolysis bullosa simplex (EBS) is characterized by fragility of the skin (and mucosal epithelia in some instances) that results in non-scarring blisters and erosions caused by minor mechanical trauma. EBS is distinguished from other types of epidermolysis bullosa (EB) or non-EB skin fragility syndromes by the location of the blistering in relation to the dermal-epidermal junction. In EBS, blistering occurs within basal keratinocytes. The severity of blistering ranges from limited to hands and feet to widespread involvement. Additional features can include hyperkeratosis of the palms and soles (keratoderma), nail dystrophy, milia, and hyper- and/or hypopigmentation. Rare EBS subtypes have been associated with additional clinical features including pyloric atresia, muscular dystrophy, cardiomyopathy, and/or nephropathy.

Hypohidrotic ectodermal dysplasia (HED) is characterized by hypotrichosis (sparseness of scalp and body hair), hypohidrosis (reduced ability to sweat), and hypodontia (congenital absence of teeth). The cardinal features of classic HED become obvious during childhood. The scalp hair is thin, lightly pigmented, and slow growing. Sweating, although present, is greatly deficient, leading to episodes of hyperthermia until the affected individual or family acquires experience with environmental modifications to control temperature. Only a few abnormally formed teeth erupt, at a later-than-average age. Physical growth and psychomotor development are otherwise within normal limits. Mild HED is characterized by mild manifestations of any or all the characteristic features.

Dyskeratosis congenita and related telomere biology disorders (DC/TBD) are caused by impaired telomere maintenance resulting in short or very short telomeres. The phenotypic spectrum of telomere biology disorders is broad and includes individuals with classic dyskeratosis congenita (DC) as well as those with very short telomeres and an isolated physical finding. Classic DC is characterized by a triad of dysplastic nails, lacy reticular pigmentation of the upper chest and/or neck, and oral leukoplakia, although this may not be present in all individuals. People with DC/TBD are at increased risk for progressive bone marrow failure (BMF), myelodysplastic syndrome or acute myelogenous leukemia, solid tumors (usually squamous cell carcinoma of the head/neck or anogenital cancer), and pulmonary fibrosis. Other findings can include eye abnormalities (epiphora, blepharitis, sparse eyelashes, ectropion, entropion, trichiasis), taurodontism, liver disease, gastrointestinal telangiectasias, and avascular necrosis of the hips or shoulders. Although most persons with DC/TBD have normal psychomotor development and normal neurologic function, significant developmental delay is present in both forms; additional findings include cerebellar hypoplasia (Hoyeraal Hreidarsson syndrome) and bilateral exudative retinopathy and intracranial calcifications (Revesz syndrome and Coats plus syndrome). Onset and progression of manifestations of DC/TBD vary: at the mild end of the spectrum are those who have only minimal physical findings with normal bone marrow function, and at the severe end are those who have the diagnostic triad and early-onset BMF.