Approach to the patient with muscle weakness

From Iusmphysiology

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Summary

• "A patient who presents with a complaint of muscle weakness is appropriately assessed for respiratory muscle weakness. If ventilatory compromise is suspected (eg, due to the presence of tachypnea) then testing of vital capacity and of maximal inspiratory and expiratory pressures is recommended (see 'Respiratory muscle weakness' above."
• "The medical history and physical examination generally permit the distinction between whose weakness is the result of a non-neuromuscular disorder (eg, those who are easily fatigued due to cardiac or pulmonary disease, limited by joint pain or stiffness, etc) from those with weakness that is due to reduced muscle strength (see 'Distinguishing true muscle weakness from asthenia' above."
• "If formal muscle testing (see 'Physical examination' above) confirms the presence of muscle weakness, a neurologic examination is performed to localize the site of the lesion (ie, to the central nervous system, peripheral nervous system, neuromuscular junction, or muscle). The distribution of muscle weakness (ie, generalized, distal, proximal, or localized) may help to narrow the number of possible causes (see 'Distribution of weakness as a guide to localizing the lesion' above."
• "Laboratory tests that may be useful in the evaluation of weakness include: serum electrolytes, calcium, magnesium, and phosphate; creatine kinase, aldolase, lactate dehydrogenase, serum aminotransferases, and thyroid stimulating hormone. If there are clinical features that suggest a systemic rheumatic or collagen disease then serologic studies are warranted, including: antinuclear antibodies (ANA). A positive urine test for blood in the absence of red cells in the sediment suggests myoglobinuria. (See 'Laboratory studies' above.)"
• "Electrophysiologic nerve and muscle testing (ie, measurement of nerve conduction velocity, sensory action potentials, and electromyography) may be of value in localizing the site of peripheral neuromuscular disorders (see 'Electrophysiologic studies' above."
• "If the evaluation suggests that myopathy is responsible for weakness, and no drug, toxin, metabolic, or endocrine disorder can be identified as the cause, then we recommend a muscle biopsy as the next step in attempting to arrive at a specific diagnosis (see 'Muscle biopsy' above."