Q&A: Diagnosis of Progressive Weakness

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 lassitude, motor impairment due to pain or joint dysfunction, and true weakness.

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In this article:
Evaluating muscle weakness
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

Evaluating muscle weakness

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

• Distinguishing true muscle weakness from lassitude 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.

1). Distinguishing true muscle weakness from lassitude or motor impairment not due to loss of muscle power. 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 lassitude, motor impairment due to pain or joint dysfunction, and true weakness.

2). Localizing, within the neuromuscular system, the site of the lesion that is producing weakness. One useful systematic approach to the myriad lesions of the neuromuscular system that can cause true muscle weakness is to place the various lesions into categories determined by the organization of the neuromuscular system. This begins with the motor cortex and proceeds through the corticospinal tracts, anterior horn cells, spinal nerve roots, peripheral nerves, neuromuscular junction, and finally muscle. Lesions of the central and peripheral nervous systems should be identifiable from a detailed neurologic examination. The distribution of weakness, the presence or absence of deep tendon reflexes, the Babinski sign, and related sensory defects are all important in localizing the lesion in the nervous system. Confirmation of findings derived from the physical examination can usually be obtained from electromyographic (EMG) testing in a cooperative patient

3). Determining the cause of the lesion. Once the neuromuscular site of the lesion causing weakness has been identified, the disorders at each site can be categorized as genetic, inflammatory/immunologic, infectious, neoplastic, toxic, or metabolic in origin

MCQ exam: clinical scenario

A 55 year old woman complains to her general practitioner of a progressive difficulty of holding objects with her left hand. She also admitted to swallowing difficulties over the past 6 months. Examination demonstrates widespread lower motor neuron involvement.

The most appropriate diagnostic test is (please choose one):

a). Serum complement levels
b). Muscle biopsy
c). Radiograph of the wrist
d). Antinuclear antibody titre
e). Electromyography

MCQ exam: answer

The correct answer is E.

MCQ exam: explanation

This is a case of Motor Neurone Disease.

In adults, the finding of widespread lower motor neuron signs is virtually diagnostic of motor neuron disease, especially if Babinski signs or clonus appear. Even if these definite upper motor neuron signs are lacking the diagnosis is similarly secure if inappropriately active tendon reflexes or Hoffmann signs are found in arms with weak, wasted, and twitching muscles.

There is no pathognomonic laboratory abnormality, but the clinical diagnosis should be confirmed by electromyographic (EMG ) evidence of active denervation in at least three limbs. Nerve conduction velocities should be normal or nearly so; conduction block is rare in patients with frank upper motor neuron signs. CSF protein content is increased above 50 mg/dl in about 30% of patients and above 75 mg/dl in about 10%; the higher values seem more likely to occur in the presence of monoclonal gammopathy or lymphoma.

Reference(s)
1). UpToDate: Approach to the patient with muscle weakness. Available online: https://www.uptodate.com/contents/approach-to-the-patient-with-muscle-weakness