Muscle examination in the evaluation of weakness

INTRODUCTION — The evaluation of the patient presenting with a complaint of "weakness" involves three steps:

●Distinguishing true muscle weakness from motor impairment due to fatigue, pain, or stiffness rather than loss of muscle power

●Localizing the site of the lesion within the neuromuscular system that is producing weakness

●Determining the cause of the lesion

The muscle examination is critical in the diagnostic evaluation of patients complaining of weakness or presenting with motor impairment. Together with the history and general physical examination, demonstrating the presence of true muscle weakness is the first step in this evaluation. Laboratory and electrophysiologic studies of nerves and muscles and muscle biopsy may then be required to complete the evaluation. Serial measurements of muscle strength and function are also necessary in the long-term follow-up of patients with neuromuscular disorders to determine response to treatment.

This topic will describe a practical approach to the muscle examination that can be performed easily by the clinician in the hospital or office setting. The overall approach to patients with muscle weakness that specifically addresses the three issues mentioned above is discussed separately (table 1 and algorithm 1). (See "Approach to the patient with muscle weakness".)

MUSCLE EXAMINATION — When evaluating a patient complaining of muscle weakness, the examiner should follow a systematic approach that includes inspection of muscle, palpation and percussion of muscle, manual muscle strength testing, and assessment of motor function. Joint examination and neurologic examination, including testing of the deep tendon reflexes and sensory testing, are important adjuncts to the muscle examination.

More formalized quantitative and semiquantitative systems for measuring muscle strength and function have been developed for use in clinical trials [1-3]. In addition, more detailed description of testing of specific muscles is available in comprehensive manuals of muscle testing [4-6].

Inspection — Inspection of muscle may provide important clues to the etiology of muscle weakness. The following observations are among the findings that may be seen.

●Localized atrophy may be seen as a result either of localized disuse secondary to joint disease or joint pain (eg, quadriceps atrophy associated with chronic knee pain and disuse) or of lesions of the central or peripheral nervous systems (eg, thenar eminence atrophy due to compressive neuropathy of the median nerve or hemiatrophy and hemiparesis following a stroke).

●Widespread atrophy may occur in longstanding polymyositis unresponsive to therapy and in longstanding muscular dystrophies. The degree of generalized atrophy, as well as a general assessment of the size and musculature of the individual, should be noted as a basis for the subsequent formal testing of muscle strength.

●Enlargement of calf muscles due to pseudohypertrophy is characteristic of the Duchenne and Becker muscular dystrophies. (See "Duchenne and Becker muscular dystrophy: Clinical features and diagnosis".)

●Fasciculations are due to spontaneous firing of motor units and may be observed in normal individuals, particularly with fatigue, or may be a sign of motor neuron disease. As an example, fasciculations of the tongue are seen in the bulbar form of amyotrophic lateral sclerosis. Benign fasciculations tend to be more widespread than those due to motor neuron disease. The lack of accompanying muscle wasting or weakness and longstanding duration also favor the diagnosis of benign fasciculations.

●Ptosis may be a manifestation of myasthenia gravis, and the finding of an exaggerated kyphoscoliosis or lordosis may be a sign of spinal muscular weakness as seen in poliomyelitis.

Palpation and percussion — Helpful findings may be detected by palpation and percussion of muscle.

●In the patient with suspected myopathy, muscle tenderness suggests an infectious etiology or a metabolic myopathy such as McArdle's disease. In comparison, tenderness is an unusual finding in the idiopathic inflammatory myopathies such as polymyositis or dermatomyositis.

●Increased muscle tone or rigidity is present in Parkinson disease, a disorder in which weakness or decreased motor function may be an initial complaint. Increased tone also occurs in upper motor neuron lesions or in demyelinating diseases.

●Myotonia describes abnormal lack of relaxation after muscle contraction and is typical of myotonic dystrophy. (See "Myotonic dystrophy: Etiology, clinical features, and diagnosis".)

●Myoedema is an unusual finding of localized "mounding-up" of the muscle where it has been tapped by a reflex hammer. It is a sign of severe hypothyroidism or cachexia. (See "Hypothyroid myopathy".)

Manual muscle strength testing — The Medical Research Council's grading system for muscle strength is widely used [7]. The examiner assesses the patient's ability to move the muscle against resistance provided by the examiner who, through experience, has developed a sense of the expected range of normal. This will vary from patient to patient depending upon the underlying size and conditioning of the subject. The fully trained athlete obviously can be expected to perform differently from a small, sedentary, or deconditioned individual. The expected strength should also be adjusted for degree of atrophy in patients with wasting illnesses.

The patient's effort is graded on a scale of 0 to 5:

●5 – Muscle contracts normally against full resistance.

●4 – Muscle strength is reduced, but muscle contraction can still move joint against resistance.

●3 – Muscle strength is further reduced, such that the joint can be moved only against gravity with the examiner's resistance completely removed. As an example, the elbow can be moved from full extension to full flexion starting with the arm hanging down at the side.

●2 – Muscle can move only if the resistance of gravity is removed. As an example, the elbow can be fully flexed only if the arm is maintained in a horizontal plane.

●1 – Only a trace or flicker of movement is seen or felt in the muscle, or fasciculations are observed in the muscle.

●0 – No movement is observed.

This system was initially devised to evaluate trauma victims with rhabdomyolysis and is weighted toward the lower end of the range of strength. It is relatively less sensitive for the evaluation of mild weakness which is the usual range for patients with myopathies. To compensate for this, the addition of 4+ and 4- categories are added to the scale.

This semiquantitative analysis of muscle strength relies on the full effort and cooperation of the patient. Thus, patients who are unable to follow instructions due to cognitive deficits or decreased level of consciousness cannot be tested. Decreased effort because of pain, fatigue, or malingering may be identified by "give" weakness, in which there is initially a full effort followed by the abrupt loss of strength against resistance; this type of response does not occur in the truly weak individual. If intentionally decreased cooperation is suspected, observing the patient performing simple functional tasks such as sitting up from a supine position or toe or heel walking may reveal a discrepancy compared with the patient's performance on directed muscle strength testing.

Distribution of muscle weakness — The distribution of weakness (localized [asymmetric] or generalized [symmetric]) is an important clue to diagnosis (algorithm 1). (See "Approach to the patient with muscle weakness".)

Localized weakness — Localized weakness is usually due to disease of the central or peripheral nervous system. Lesions of the motor cortex, spinal cord, spinal nerve root, and peripheral nerve have distinct distribution patterns. Common examples include isolated weakness of ankle dorsiflexion or "foot drop" resulting from mononeuritis multiplex due to vasculitis or a herniated disc that causes a lumbar radiculopathy; atrophy of the thenar eminence and weakness of hand grip due to median nerve compression or carpal tunnel syndrome; and quadriceps weakness due to a femoral neuropathy. (See "Overview of lower extremity peripheral nerve syndromes" and "Overview of upper extremity peripheral nerve syndromes" and "Acute lumbosacral radiculopathy: Etiology, clinical features, and diagnosis" and "Clinical manifestations and diagnosis of vasculitic neuropathies".)

Focal weakness may also result from joint disease. Patients with joint damage from rheumatoid arthritis will develop weakness of the adjacent muscles due to disuse and atrophy or to joint deformity limiting full muscle function. Patients who have undergone hip replacement frequently develop focal weakness of the hip abductors that causes a characteristic lurching gait. It may be difficult to distinguish true muscle weakness from loss of function due to pain in individuals with joint disease.

Generalized weakness — Generalized or symmetric muscle weakness may be proximal or distal. Proximal weakness involves the axial musculature, as well as the large muscles of the arms and thighs; it is the hallmark of most myopathies.

●Deltoid strength can be tested by having the patient abduct his or her arms to 90 degrees with elbows flexed and having the patient resist the downward pressure exerted by the examiner.

●Iliopsoas strength can be assessed by observing the patient sitting up from the supine position, which should be easily accomplished without the use of the hands.

●Proximal lower extremity weakness can be detected by having the patient straight leg raise against resistance.

Distal strength is assessed by measuring grip strength and by testing wrist and ankle dorsiflexion. Neuropathic causes of weakness should be excluded, as mononeuritis multiplex due to necrotizing vasculitis is often characterized by wrist or foot drop. The distribution of weakness in patients with an inflammatory myopathy is typically symmetric and proximal. Distal muscle weakness, if present, tends to be very mild and usually does not cause significant functional impairment. Muscle atrophy is generally not present in early cases, even in patients with severe weakness; however, it may be seen with advanced disease. This pattern is key to distinguishing polymyositis or dermatomyositis from other causes of diffuse muscle weakness, such as inclusion body myositis, in which both proximal and distal weakness occur and in which weakness is more likely to be asymmetric and is more likely to be accompanied by atrophy; muscular dystrophy, which has specific patterns of involvement depending upon the type; or motor neuron disease, which typically causes distal weakness. (See "Clinical manifestations of dermatomyositis and polymyositis in adults".)

Assessment of motor function — Several simple functional tests can be performed in the office and may be tailored to the individual patient's clinical setting. These tests include observing the patient rising from a seated position without the use of the hands or sitting up from the supine position on the examination table, climbing on and off an examination table, performing serial deep knee bends, elevating arms above the head, or lifting a weight with an extended arm to eye level. Observation of the patient during toe and heel walking is an easy way to assess dorsiflexion and plantar flexion ankle strength. The timed 50-foot walk can be easily repeated and followed at serial visits.

SUMMARY

●Formal measurement of muscle strength using the Medical Research Council scale will demonstrate the presence of true muscle weakness. The muscle should be inspected and palpated to determine if atrophy, enlargement, tenderness, fasciculations, or other changes are present. (See 'Manual muscle strength testing' above and 'Muscle examination' above and 'Inspection' above and 'Palpation and percussion' above.)

●Assessment of the distribution of muscle weakness will help distinguish between neurologic and myopathic disorders. It is important to determine whether weakness is localized or generalized and whether it is largely proximal, distal, or both. (See 'Distribution of muscle weakness' above.)

●Confounding variables such as patient effort, degree of muscle atrophy, and presence of pain need to be considered. (See 'Muscle examination' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Marc L Miller, MD, who contributed to an earlier version of this topic review.