ABSTRACT
The phenotype of Smith-Magenis syndrome (SMS) is characterized by a distinct pattern of features that include infantile hypotonia, minor craniofacial and skeletal anomalies, middle ear/laryngeal anomalies and ocular abnormalities, marked early expressive speech/language delays, psychomotor and growth retardation, and, to a lesser extent, genitourinary and cardiac anomalies. This chapter seeks to increase clinical awareness about SMS and assists the clinician in recognizing the salient and sometimes subtle constellation of features in SMS at different ages. Smith-Magenis syndrome is classified as a contiguous gene deletion syndrome in which the phenotype results from physically linked, unrelated dosage-sensitive genes. Smith-Magenis syndrome occurs de novo with few exceptions, suggesting a low recurrence risk. Originally identified in SMS, the genomic mechanism of nonallelic homologous recombination or unequal crossing over leads to other contiguous gene syndromes, including Prader–Willi syndrome, Angelman syndrome, Williams syndrome and DiGeorge/Velocardiofacial syndrome.
