An abnormality of histiocytes, in which the cells take on a sea blue appearance due to abnormally increased lipid content. Histiocytes are a type of macrophage. Sea-blue histiocytes are typically large macrophages from 20 to 60 micrometers in diameter with a single eccentric nucleus whose cytoplasm if packed with sea-blue or blue-green granules when stained with Wright-Giemsa.

Hidrotic ectodermal dysplasia 2, or Clouston syndrome (referred to as HED2 throughout this GeneReview) is characterized by a triad of major clinical features including partial-to-complete alopecia, nail dystrophy, and palmoplantar hyperkeratosis. Sweating is preserved and there are usually no dental anomalies. Sparse scalp hair and dysplastic nails are seen early in life. In infancy, scalp hair is fine, sparse, and brittle. Progressive hair loss may lead to total alopecia by puberty. The nails may be milky white in early childhood; they gradually become dystrophic, thick, and distally separated from the nail bed. Palmoplantar keratoderma may develop during childhood and increases in severity with age. Associated features may include cutaneous hyperpigmentation (particularly over the joints) and finger clubbing. The clinical manifestations are highly variable even within the same family.

Keratitis-ichthyosis-deafness (KID) syndrome is a rare ectodermal dysplasia characterized by sensorineural hearing loss, photophobia and corneal vascularization, hyperkeratosis of the palms and soles, erythrokeratoderma, follicular hyperkeratosis, and recurrent bacterial and fungal infections. A subset of patients with KID may develop multiple cystic pilar tumors, which are prone to malignant transformation and metastasis (Nyquist et al., 2007). Vohwinkel syndrome (124500) is an allelic disorder involving congenital deafness with keratopachydermia and constrictions of fingers and toes. Another similar disorder caused by mutation in GJB2 is palmoplantar keratoderma with deafness (148350). Genetic Heterogeneity of Keratitis-Ichthyosis-Deafness Syndrome An autosomal recessive form of KID syndrome (KIDAR; 242150) is caused by mutation in the AP1B1 gene (600157) on chromosome 22q12.

Syndrome with characteristics of psychomotor delay, brachycephaly with flat face, small nose, microstomia, cleft palate, cataract, hearing loss, hypoplastic scrotum and digital anomalies. Less than 10 patients have been described in the literature so far. Although the majority of reported cases were sporadic, the syndrome has been reported in one pair of siblings (a brother and sister) with an apparently autosomal recessive inheritance pattern.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

Alopecia universalis congenita (ALUNC) is a severe autosomal recessive form of alopecia characterized by a complete absence of hair development affecting all scalp and body hair (Nothen et al., 1998). This rare disorder is clearly distinct from alopecia areata (AA1; 104000), which has an autoimmune basis with probable genetic predisposition.

Hypotrichosis-5 (HYPT5), also known as Marie Unna hereditary hypotrichosis-2 (MUHH2), is a form of hereditary hypotrichosis characterized by twisting hair. Affected individuals have little or no scalp hair at birth, wiry and irregular scalp hair in childhood, and sparse or no forehead and parietal hair at puberty. Eyebrows and eyelashes are thin, and pubic and axillary hair fails to develop. Scarring alopecia is modest, and vertex hair is normal (summary by Zhang et al., 2012). For a general phenotypic description of Marie Unna hereditary hypotrichosis, see MUHH1 (146550). For a discussion of genetic heterogeneity of nonsyndromic hypotrichosis, see 605389.

Hypotrichosis-11 (HYPT11) is a form of isolated alopecia characterized by diffuse and progressive loss of hair starting in childhood. Affected individuals typically present with sparse to absent scalp hair, and may have brittle or absent eyebrows and eyelashes as well as sparse body hair, without hair shaft anomalies (summary by Pan et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of nonsyndromic hypotrichosis, see HYPT1 (605389).

Hypotrichosis-14 (HYPT14) is characterized by sparse to absent lanugo-like scalp hair, sparse and brittle eyebrows, and sparse eyelashes and body hair (Romano et al., 2018). For a discussion of genetic heterogeneity of hypotrichosis, see HYPT1 (605389).

Myoectodermal gonadal dysgenesis syndrome (MEGD) is characterized by 46,XY complete or partial gonadal dysgenesis, or 46,XX gonadal dysgenesis, in association with extragonadal anomalies, including low birth weight, typical facies, rod and cone dystrophy, sensorineural hearing loss, omphalocele, anal atresia, renal agenesis, skeletal abnormalities, dry and scaly skin, severe myopathy, and neuromotor delay. Dysmorphic facial features along with muscular habitus are the hallmarks of the syndrome. Abnormal hair patterning with frontal upsweep and additional whorls, eyebrow abnormalities comprising broad, arched, and sparse or thick eyebrows, underdeveloped alae nasi, smooth philtrum, and low-set ears with overfolded helices facilitate a gestalt diagnosis. (Guran et al., 2019; Altunoglu et al., 2022).

Intermediate junctional epidermolysis bullosa 5A (JEB5A) is an autosomal recessive blistering disease of skin and mucous membranes. Blistering is less severe than in severe JEB (see 226700). The plane of skin cleavage is through the lamina lucida of the cutaneous basement membrane zone. Nails may be dystrophic and dental enamel defects are present. Blistering does not affect the life span of affected individuals (summary by Has et al., 2020). For a discussion of genetic heterogeneity of the subtypes of JEB, see JEB1A (226650). Reviews Has et al. (2020) reviewed the clinical and genetic aspects, genotype-phenotype correlations, disease-modifying factors, and natural history of epidermolysis bullosa.