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Hypomelanosis of Ito (HI) is a multisystemic neurocutaneous condition with hypopigmented skin lesions along the Blaschko lines. Prevalence is unknown but incidence has been estimated at between 1/10,000 and 1/8,500. HI is causally heterogeneous and, with the exception of the pigmentary skin anomalies that are by definition mandatory for recognition of the condition, the phenotypic presentation is very variable. The skin phenotype is characterized by unusual unilateral or bilateral cutaneous macular hypopigmented whorls, streaks and patches, corresponding to the lines of Blaschko, that usually develop within the first two years of life. Neurological, skeletal and ocular symptoms have also been reported. Anomalies of the central nervous system may include intellectual deficit, motor retardation, seizures, microcephaly, macrocephaly and hypotonia. Ophthalmological abnormalities consist of strabismus, cataracts, nystagmus and retinal degeneration. Skeletal defects include short stature, facial and limb asymmetry, tooth abnormalities, pectus carinatum or excavatum, scoliosis, and finger anomalies. Cystic renal changes have been described in rare cases. Various chromosomal anomalies have been identified in some patients and several authors have suggested that the phenotype recognized as HI is the result of cutaneous mosaicism, either for a monogenic or a chromosomal disorder, rather than being a distinct disease. HI is generally sporadic, but autosomal dominant, recessive and X-linked modes of inheritance have occasionally been reported. In a minority of cases, the mosaicism can be detected in cultured fibroblasts, which show cells with different karyotypes; the repartition of cell lines correlates to some extent with the hypopigmented patches, but usually both cell lines are found in normal and in hypopigmented skin. In females with HI, the possibility of an inheritable X-linked disorder must be considered, for which the ``HI pattern`` in fact reflects clonal X inactivation of the normal or mutated X chromosome. Cutaneous ultrastructural analysis shows a decreased number of melanocytes, containing fewer melanosomes. Differential diagnosis should include incontinentia pigmenti (Bloch-Sulzberger syndrome) characterized by a dynamic course of more inflammatory cutaneous lesions; tuberous sclerosis marked by multiple cutaneous lesions with irregular borders and frequently with an `ash leaf` configuration, and Pallister-Killian syndrome resulting from a mosaic state for chromosome 12p tetrasomy (see these terms). When associated with hyperpigmented macules, neurofibromatosis types 1 or 2 (see these terms), and a homozygous state for mismatch repair genes should also be considered in the differential diagnosis. Treatment of the cutaneous lesions is not required. Care should be multidisciplinary with periodic consultations with a pediatric ophthalmologist, neurologist and orthopedic specialist, depending of the associated anomalies. Seizures should be treated with anticonvulsivants. The prognosis is determined by the associated abnormalities. Cutaneous lesions have a good prognosis. |