Muscle weakness can occur for many reasons, including a problem with the muscle, a problem with the nerve that stimulates the muscle, or a problem with the brain. Therefore, to diagnose a congenital myopathy, a neurologist will perform a detailed physical exam as well as tests to determine the cause of weakness. If a myopathy is suspected, possible tests include a blood test for a muscle enzyme called creatine kinase, an electromyogram (EMG) to evaluate the electrical activity of the muscle, a muscle biopsy, and genetic testing.
Respiratory failure due to diaphragmatic weakness can occur at any age and may be independent of the degree of limb weakness. A restrictive pattern on pulmonary function tests (PFTs) may be apparent before the onset of symptoms. Early symptoms of nocturnal hypoxia can include poor sleep, nightmares, morning headache, daytime sleepiness, and weight loss. All patients should have baseline PFTs that are repeated in at least yearly intervals. Treatment options include chest physiotherapy, manually assisted cough, early treatment of respiratory infections, noninvasive ventilation, and tracheostomy combined with permanent ventilation.
Historically the congenital myopathies have been classified on the basis of the major morphological features seen on muscle biopsy – e.g., rods (nemaline myopathy), cores (central core disease and multiminicore disease), central nuclei (centronuclear/myotubular myopathy) and selective hypotrophy of type 1 fibres (congenital fibre type disproportion). Over the past 15 years, the genetic basis of many of the different forms of congenital myopathy has been identified – although it is evident that there are still many additional genes to be discovered.