Autosomal Dominant Disorders

CLINICAL CHARACTERISTICS

The autosomal dominant disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

DIAGNOSIS/TESTING: The diagnosis of an autosomal dominant disorder is established in a proband with characteristic clinical and neurophysiologic findings, radiographic findings in the skeletal dysplasias, and a heterozygous pathogenic variant identified on molecular genetic testing.

MANAGEMENT

Treatment is focused on symptom management. Affected individuals are often evaluated and managed by a multidisciplinary team that may include neurologists, physiatrists, orthopedic surgeons, and physical and occupational therapists. SNHL is managed by specialists to determine the best management options. For neuromuscular disorders, neuropathy and respiratory dysfunction are managed in a routine manner; individuals with laryngeal dysfunction require ENT evaluation that should include speech therapy, laryngoscopy, and, in some instances, surgery. For skeletal dysplasias, physical therapy/exercise and heel-cord stretching to maintain function; surgical intervention when kyphoscoliosis compromises pulmonary function and/or causes pain and/or when upper cervical spine instability and/or cervical myelopathy are present. For neuromuscular disorders, annual neurologic examinations, physical therapy assessments, ENT monitoring of laryngeal function, dynamic breathing chest x-ray, and hearing assessment. For skeletal dysplasias, annual evaluation for joint pain and scoliosis; assessment for odontoid hypoplasia before a child reaches school age and before surgical procedures involving general anesthesia; annual hearing assessment. For neuromuscular disorders, obesity, as it makes walking more difficult; diabetes; medications that are toxic or potentially toxic to persons with a peripheral neuropathy. For skeletal dysplasias, extreme neck flexion and extension (in those with odontoid hypoplasia); activities that place undue stress on the spine and weight-bearing joints. Ideally a woman with disorder would seek consultation from a high-risk OB-GYN or maternal-fetal medicine specialist to evaluate risk associated with pregnancy and delivery.

GENETIC COUNSELING

disorders are inherited in an autosomal dominant manner. Most individuals diagnosed with an autosomal dominant disorder have an affected parent. However, since the most severe skeletal phenotypes can be lethal in childhood (or in utero), children with these phenotypes likely have a pathogenic variant and unaffected parents. Each child of an individual with an autosomal dominant disorder has a 50% chance of inheriting the pathogenic variant. Specific phenotype, age of onset, and disease severity cannot be predicted accurately because of reduced penetrance and variable expressivity. However, in general, a child who inherits a pathogenic variant associated with neuromuscular disease or skeletal dysplasia from an affected parent is likely to have the same phenotype as the parent. Prenatal and preimplantation genetic testing are possible if the pathogenic variant has been identified in an affected family member.