A group of inherited disorders characterized by developmental abnormalities in 2 or more ectodermal structures.
These include hair, teeth, nails, and sebaceous and sweat glands.
There may be abnormalities in nonectodermal structures and functions.
Distinction is based on clinical features, mode of inheritance, and molecular/genetic findings.
Clinically distinct disorders may be due to different mutations in the same gene (allelic heterogeneity), and clinically similar conditions may be due to mutations in different genes (locus heterogeneity).
The ectodermal dysplasias (EDs) are a complex and heterogenous group of inherited disorders that share developmental defects involving at least 2 of the major structures classically known to derive from the embryonic ectoderm: hair, teeth, nails, and sweat and other eccrine and sebaceous glands. Developmental disorders involving only one type of ectodermal structure, even if associated with other congenital malformations, are not classified as EDs. There are almost 200 conditions classified as EDs, and given the varied clinical presentations and clinical overlap, it can be difficult to diagnose them precisely.1
Various classification schemes have been proposed over the past several decades to help in ordering and thinking about these disorders. Freire-Maia and Pinheiro published an exhaustive review and classification system, with later updates, for these disorders using a numeric system.2 Although this system developed a rational approach to a previously chaotic field, it has little use in clinical practice and does not account for the pathogenesis or genetics of specific conditions. Over the past few decades, many of the genes responsible for these disorders have been identified, leading to several attempts to reclassify the EDs based on molecular and developmental biopsy. In 2008, an international ED classification conference consisting of health care providers, researchers, patient advocate representatives, and administrators convened in an attempt to move toward a more useful and unified system. In 2009, a conference report was released outlining the goal of creating an integrated ED classification system based on the most recent clinical and molecular information available.3 Given the rate of discovery of new molecular information, this has been challenging. Thus, in 2012, a second international conference was held resulting in the development of a multi-axis model approach to EDs.4 This model will be based on a clinical/phenotype axis, a gene-based axis, and a functional/pathways axis and is still in the process of creation.
This chapter will only cover the EDs that are more common and most likely to present to a dermatologist for diagnosis and medical attention. As much as possible, it will be organized in concordance with the ED international classification conference, limited by the complexities discussed above. Given the evolving landscape of EDs, the reader is directed to the following resources for the most current information on EDs: https://omim.org (Online Catalog of Mendelian Inheritance in Man) and http://www.ncbi.nlm.nih.gov/gtr/ (an up-to-date listing of laboratories offering molecular testing).