APPROACH TO THE PATIENT: Muscle Disease
Muscle diseases (myopathies) may be intermittent or persistent and usually present with proximal, symmetric weakness with preserved reflexes and sensation. An associated sensory loss suggests injury to peripheral nerve or the central nervous system rather than myopathy; on occasion, disorders affecting the anterior horn cells, the neuromuscular junction, or peripheral nerves can mimic myopathy. Any disorder causing muscle weakness may be accompanied by fatigue, referring to an inability to maintain or sustain a force; this must be distinguished from asthenia, a type of fatigue caused by excess tiredness or lack of energy. Fatigue without abnormal clinical or laboratory findings almost never indicates a true myopathy.
Muscle disorders are usually painless; however, myalgias, or muscle pains, may occur. Myalgias must be distinguished from muscle cramps, i.e., painful, involuntary muscle contractions, usually due to neurogenic disorders. A muscle contracture due to an inability to relax after an active muscle contraction is associated with energy failure in glycolytic disorders. Myotonia is a condition of prolonged muscle contraction followed by slow muscle relaxation.
CK is the preferred muscle enzyme to measure in the evaluation of suspected myopathies. Electrodiagnostic studies (nerve conduction studies and electromyography, NCS/EMG) are usually necessary to distinguish myopathies from neuropathies and neuromuscular junction disorders. An approach to muscle weakness is presented in Figs. 195-1 and 195-2.
Diagnostic evaluation of intermittent weakness. AChR AB, acetylcholine receptor antibody; CPT, carnitine palmitoyltransferase; EOMs, extraocular muscles; MG, myasthenia gravis; PP, periodic paralysis.
Diagnostic evaluation of persistent weakness. Examination reveals one of seven patterns of weakness. The pattern or weakness in combination with the laboratory evaluation leads to a diagnosis. CK, creatinine kinase; DM, dermatomyositis; FSHD, facioscapulohumeral dystrophy; IBM, inclusion body myositis; MG, myasthenia gravis; OPMD, oculopharyngeal muscular dystrophy; PM, polymyositis.
A varied group of inherited, progressive degenerations of muscle, each with unique phenotypic and genetic features.
DUCHENNE'S MUSCULAR DYSTROPHY
X-linked recessive mutation of the dystrophin gene that affects males almost exclusively. Progressive weakness in hip and shoulder girdle muscles beginning by age 5; by age 12, the majority were nonambulatory in the era prior to the use of glucocorticoids. Survival beyond age 25 is rare. Associated problems include tendon and muscle contractures, progressive kyphoscoliosis, impaired pulmonary function, cardiomyopathy, and intellectual impairment. Palpable enlargement and firmness of some muscles. Becker dystrophy is a less severe form, with a slower course and later age of onset (5–15 years) but similar clinical, laboratory, and genetic features.
Laboratory findings include massive elevations (20–100 × normal) of serum CK, a myopathic pattern on EMG testing, and evidence of groups of necrotic muscle fibers with regeneration, phagocytosis, ...