Myopathies are a group of disorders characterized by a primary structural or functional impairment of skeletal muscle. They usually affect muscle without involving the nervous system, resulting in muscular weakness. The latter represents one of the most common complaints among patients presenting to family physicians or neurology specialist.
Nevertheless, fatigue, exercise intolerance and myalgias can also occur in a plethora of non-myopathic conditions. Therefore a comprehensive evaluation of each patient should include a thorough examination and combination of adequate laboratory, radiologic, electrodiagnostic and pathologic studies.
Clinical presentation of myopathies
Myopathies can manifest with either “negative” or “positive” symptoms. Negative symptoms include muscle weakness, exercise intolerance and fatigue, while so called positive symptoms usually encompass cramps and myalgias, with occasional contractures, myotonia and myoglobinuria.
Weakness is the most common and most reliable clinical indicator of myopathy. The affected individuals can experience difficulty performing tasks which require the use of proximal muscles, such as getting up from a chair, lifting objects or climbing steps. Distal weakness is less common, although it can be the most prominent symptom in some types of myopathy (e.g. inclusion body myositis, myotonic dystrophy and genetic distal myopathies).
On the other hand, fatigue and exercise intolerance are less reliable negative symptoms as they often reflect the general level of conditioning, emotional disturbance or the impairment of cardiopulmonary status in elderly people. When these symptoms are truly present, additional clinical features are asses in order to exclude certain metabolic myopathies or mitochondrial cytopathies.
Myalgias refer to generalized muscle pain or tenderness. Similar to fatigue, myalgia represents a common complaint which can occur in a myriad of both benign and serious disease processes. Constant muscle pain in a proximal distribution is often characteristic for inflammatory myopathies (such as polymyositis and dermatomyositis), whereas episodic myalgias after exercise arise suspicion that metabolic myopathies can be present.
Cramps are common in metabolic myopathies such as myophosphorylase deficiency (also known as McArdle’s disease) and in hypothyroid myopathy. Joint contractures are uncommon in patients with muscular symptoms, but their presence can facilitate in establishing the diagnosis. Myotonia is often found in sodium or chloride channelopathies, as well as in the myotonic dystrophies.
Various associated symptoms are present in many myopathies and can aid in narrowing the differential diagnosis among rheumatologic, endocrine and inflammatory conditions. For example, dysphagia may appear with weakness in inclusion body myositis and systemic sclerosis, whereas menorrhagia often coincides with the weakness that occurs in hypothyroidism.
Ways to diagnose the condition
The evaluation of patients suspected of having a myopathy always begins with a detailed medical history. The physical examination aims to achieve an objective confirmation of the subjective severity and distribution of muscle symptoms. Further diagnostic tests are ordered to add laboratory evidence in support of or against the suspected diagnosis.
Measurement of serum enzymes is a widely used approach for diagnosing suspected muscle disease. Creatine kinase (CK), also known as creatine phosphokinase, represents preferable screening tool because (unlike lactate dehydrogenase, aldolase and other enzymes) it has relative predominance in skeletal muscle, it is not falsely elevated by hemolysis, and is readily released in cellular injury.
Still, CK elevation is not synonymous to the presence of a myopathy. Many individuals with active motor neuron diseases or severe active axonal neuropathies can exhibit mild to moderate increase in CK levels. Furthermore, muscle trauma, viral infections, strenuous exercises, generalized seizures and even therapy with certain drugs may all be accompanied by transient, albeit severe CK elevation.
Electromiography (EMG) represents the most informative part of the electrodiagnostic study in myopathic disorders. It can confirm the presence of a myopathy, narrow down the differential-diagnostic possibilities, as well as identify an appropriate biopsy site. Still, it must be noted that abnormalities in the EMG can be subtle and very patchy in distribution. Nerve conduction studies and needle myography represent additional diagnostic possibilities.
If the diagnosis is still inconclusive after all the aforementioned approaches, a muscle biopsy is warranted for patients who have a suspected myopathy. Such critical evaluation is necessary as treatments for the myopathies depend on the particular disease and its specific causes.