Incidence and Prevalence
- Rare inherited disorders of varying incidence: The bone dysplasias are a disparate group of rare inherited disorders. Their nomenclature and classification are complex, and diagnosis usually requires expert genetic and radiological opinion (41). The incidence and mode of inheritance of some of the more frequently encountered conditions are shown in the table.
- Metabolic condition present at birth: The diagnosis of the bone dysplasias is rarely the province of the pediatric neurosurgeon. In most instances, the diagnosis will have been made by the time neurosurgical opinion is sought. The suspicion of a bone dysplasia may arise at birth or in infancy as a result of dysmorphic features, growth retardation, joint deformity, or systemic illness such as respiratory insufficiency.
- Age for onset of symptoms variable: The neurological consequences of the bone dysplasias (at least those that are of relevance to the neurosurgeon) for the most part arise as a consequence of bony or ligamentous involvement. This may comprise bone deformity, weakness, ligamentous thickening, or ligamentous laxity. The age by which these changes result in neurological deterioration is variable.
- Hunter syndrome only X-linked: Hunter syndrome (mucopolysaccharidosis type II) is the only X-linker disorder among the more common metabolic bone disorders.
- Advanced parental age: An affected parent increases the risk according to the usual rules of inheritance. However, children with many of these conditions may be born to unaffected parents as a result of a spontaneous mutation. Advanced paternal age has been associated with increased risk of new mutations in cases of achondroplasia, Marfan syndrome, thanatophoric dysplasia, and osteogenesis imperfecta.
Relationships to Other Disease States and Syndromes
- Comorbidities common: Children with metabolic bone disease commonly have significant comorbidities (for example, respiratory and cardiac involvement). These conditions may be life limiting and frequently have implications for surgery and anesthesia.