Malignant hyperthermia (MH) is a heterogeneous pharmacogenetic disorder; it has been reported as autosomal dominant and recessive traits, and with myotonia congenita, Duchenne and Barnes muscular dystrophies, Evans myopathy, central core disease, Kniest, Noonan and King syndromes.
The recognition of Noonan syndrome has helped the differentiation of the Turner-like phenotypes. Variable expressivity and pleiotropism characterized this syndrome and two types may exist. Patients with Noonan-like phenotype, myopathy and MH have been referred to as having King syndrome; 2/5 patients reported by King et al had contractures.
Two unrelated children diagnosed elsewhere as having Noonan syndrome and congenital arthrogryposis appeared to have King syndrome; both had Noonan facies, webbed neck, average intelligence, normal karyotypes and no heart anomaly. The female has had two episodes of MH. She also had hypomimic facies and winged scapules. Muscle biopsy showed evidence of myopathy. The mother of the male was of average stature and intelligence, but had Noonan facies and webbed neck.
Myogenic arthrogryposis, average intelligence and lack of heart defect may allow differentiation between King and Noonan syndromes. The screening platelet bioassay and the caffeine contracture test may be of help in identifying the patients at risk for MH; once identified, these patients should wear Medic alert bracelets.
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Kousseff, B. MALIGNANT HYPERTHERMIA AND NOONAN VERSUS KING SYNDROME. Pediatr Res 18, 306 (1984). https://doi.org/10.1203/00006450-198404001-01279