The evaluation of the patient presenting with a complaint of “weakness” involves three steps:
- Distinguishing true muscle weakness from asthenia or motor impairment not due to loss of muscle power
- Localizing, within the neuromuscular system, the site of the lesion that is producing weakness
- Determining the cause of the lesion
This topic will review the approach to the patient with muscle weakness by addressing these three issues. The specific tests used to assess muscle weakness are discussed separately, as are the clinical manifestations and evaluation of respiratory muscle weakness. (See "Muscle examination in the evaluation of weakness" and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation".)
DISTINGUISHING TRUE MUSCLE WEAKNESS FROM ASTHENIA
Many patients who complain of weakness are not objectively weak when muscle strength is formally tested. A careful history and physical examination will permit the distinction between asthenia, motor impairment due to pain or joint dysfunction, and true weakness.
History — Patients with a variety of systemic disorders may interpret difficulties performing certain tasks as weakness. Included in this group are cardiopulmonary disease, joint disease, anemia, cachexia from malignancy or chronic infectious or inflammatory disease, and/or depression. Patients with any of these conditions may be functionally limited but not truly weak. Furthermore, careful questioning will reveal that the patient is limited by shortness of breath, chest pain, joint pain, fatigue, poor exercise tolerance, paresthesias, or spasticity rather than a true decrease in muscle power.