Chapter 30: Diseases of Muscle

CAUSES OF MUSCLE DISEASE

Muscle disease can be caused by:

• Inflammatory diseases: polymyositis, dermatomyositis, inclusion body myositis, immune-mediated necrotizing myopathy

• Medications: most commonly statins and corticosteroids, but other immunosuppressive agents in addition to steroids (e.g., cyclosporine, tacrolimus), zidovudine, and amiodarone can also cause myopathy

• Systemic diseases: HIV, endocrine disease, rheumatologic disease, critical illness

• Genetic disorders, which can be further divided into genetic defects in:

  • Structural muscle proteins: muscular dystrophies, congenital myopathies

  • Metabolic pathways: glycogen storage diseases, disorders of lipid metabolism

  • Mitochondria: mitochondrial myopathies such as myoclonic epilepsy with ragged red fibers (MERRF)

  • Ion channels: periodic paralyses

CLINICAL FEATURES OF MUSCLE DISEASE

The hallmark of muscle disease is weakness. Diseases of muscle are distinguished from central nervous system or peripheral nervous system causes of weakness by lack of upper or lower motor neuron signs (although severe weakness can cause decreased reflexes), lack of sensory changes, and by the pattern and distribution of weakness. The most common pattern of weakness in diseases of muscle is symmetric proximal weakness, although there are myopathies that specifically affect particular distal muscle groups in isolation (e.g., distal myopathies) or in addition to proximal muscles (e.g., inclusion body myositis), and some muscle diseases can begin (and/or remain) asymmetric (e.g., inclusion body myositis, fascioscapulohumeral muscular dystrophy). Weakness in some muscle diseases develops insidiously (e.g., dermatomyositis, polymyositis, muscular dystrophies), but in others it can be rapidly progressive (e.g., immune-mediated necrotizing myopathy). In some muscle diseases weakness occurs only with particular precipitants (e.g., exercise in metabolic myopathies, potassium level in periodic paralyses). When weakness is exercise–induced, diseases of the neuromuscular junction must also be considered (see Ch. 29).

LABORATORY TESTING IN MUSCLE DISEASE

An elevated serum creatine kinase (CK) is seen in many muscle diseases, but may not necessarily be elevated in all muscle diseases. CK elevation can also be caused by muscle injury, exercise, and generalized tonic-clonic seizures. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) (“liver enzymes”) do not just rise in liver disease, but may also be elevated due to muscle breakdown. Aldolase elevation can also be caused by either liver or muscle disease. An increase in gamma-glutamyl transferase (GGT) with elevated AST/ALT can help to distinguish liver injury as the cause of the rise in AST/ALT because GGT is present only in liver and not in muscle. Myoglobinuria suggests muscle breakdown, which can occur due to primary muscle disease or in the setting of muscle injury (e.g., extreme exercise, toxins, trauma).

In nerve conduction studies in muscle diseases, compound motor action potential (CMAP) amplitudes may be reduced due to loss of muscle fibers, but sensory nerve action potentials (SNAPs) and conduction velocities should be normal (unless there is concurrent neuropathy). Electromyography (EMG) in muscle diseases can reveal increased insertional activity, fibrillation potentials, decreased motor unit action potential (MUAP) ...