Neuromuscular Disorders

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Chapter 108

Neuromuscular Disorders

Principles of Disease

The neuromuscular unit has four components: the anterior horn cells of the spinal cord, the peripheral nerve, the neuromuscular junction, and the muscle innervated. The level of the pathologic process determines associated signs and symptoms (Table 108-1). Myelopathies involve the spinal cord; radiculopathies involve the nerve roots as they leave the spinal cord; neuropathies involve the peripheral nerves; and myopathies involve the muscle. The use of physical signs to differentiate these disorders is discussed in Chapter 13.

Neuropathies involve the axon or the myelin sheath of the nerve. Nerve conduction studies can differentiate the locations of involvement. As the conduction along the axon is disrupted, the subsequent delay in transmission first causes symptoms in the muscles controlled by longer nerve axons, resulting in a history of ascending weakness. As the myelin destruction or axonal degeneration progresses, patients usually note a slowly progressive course of symptoms.

The neuromuscular junction is composed of the presynaptic membrane, the postsynaptic membrane, and the synaptic cleft. The neurotransmitter is acetylcholine (ACh). The motor synapse is a nicotinic receptor, whereas muscarinic synapses link the central nervous system with the autonomic nervous system. Disorders of the postsynaptic nicotinic receptors produce weakness. Postsynaptic ACh receptors (AChRs) are continually turned over at a rate that is related to the amount of stimulation. A disorder of transmission often leads to increased production of AChRs. Myasthenia gravis (MG) is the prototype of neuromuscular junction diseases.

Clinical Findings

History

The history of patients with complaints of weakness focuses on the acuity and progression of onset and the potential for airway compromise. Any complaint of difficulty in breathing or swallowing raises suspicion of bulbar involvement and concern for life-threatening deterioration. The history must elicit whether the weakness is muscle weakness or nonspecific generalized fatigue. Weakness is the inability to exert normal force, whereas fatigue implies a decrease in force with repetitive use. When muscle weakness exists, the clinician should determine whether it is focal or generalized, proximal or distal. The history of present illness should include the duration of symptoms, exacerbating and mitigating factors, and presence of associated symptoms such as fever, weight loss, and bowel or bladder changes.

Historical elements might explain the presenting complaint: a preexisting neuromuscular disorder that could lead to deterioration; prior episodes or a family history of weakness suggesting periodic paralysis; a recent respiratory or diarrhea illness suggesting a postinfectious, autoimmune process, such as transverse myelitis or Guillain-Barré syndrome (GBS); a cancer history suggesting a metastatic tumor as the cause of a compressive myelopathy; and a food or travel history suggesting botulism or tick exposure.

Physical Examination

The examination should first assess the patient’s ability to breathe and ventilate and then evaluate the degree of weakness and the location of the lesion. The presence of swallowing and a strong cough suggest that the patient has sufficient protective and ventilatory reserve. The muscles used to lift the head off the bed may weaken before those of respiration and should be assessed. A patient who is not yet intubated but is complaining of shortness of breath or difficulty in breathing should have frequent measurements of forced vital capacity (FVC). Normal FVC ranges from 60 to 70 mL/kg; when the FVC reaches 15 mL/kg, ventilatory support is necessary. If vital capacity cannot be measured, a maximal negative inspiratory force (NIF) is easily determined. NIF of less than 15 mm Hg suggests the need for intubation. Blood gas analysis is not helpful because functional reserve can be severely diminished by the time a patient has either hypercarbia or hypoxia.

The assessment of vital signs is important because some causes of weakness may result in dysregulation of the autonomic system. A systematic neurologic examination assesses the patient’s mental status, cranial nerves, motor function, sensory function, deep tendon reflexes, and coordination, including cerebellar function. The motor examination begins by determining whether the weakness is unilateral or bilateral and which muscle groups are involved. Key components of the examination include motor strength, muscle bulk, and presence of fasciculations. Box 108-1 provides the grading system used in motor strength assessment. Table 108-2 provides the findings used to distinguish upper motor neuron from lower motor neuron processes.

BOX 108-1   Grading Score for Motor Strength

Differential Consideration

Motor Neuron Disease

The characteristic findings of motor neuron disease combine signs of both upper and lower motor neuron dysfunction, including hyperreflexia, muscle wasting, and fasciculations. Pain is not a component of the clinical picture. Amyotrophic lateral sclerosis, the prototypical motor neuron disease, is a progressive, incurable, neurodegenerative disorder of unclear etiology. It is most common in elders but is seen in people as young as 20 years. Patients may present with upper extremity, lower extremity, or bulbar weakness, which is often initially asymmetrical. The median survival time is 3 to 5 years; most patients ultimately succumb to respiratory failure.

Poliomyelitis affects the anterior horn cells and results in lower motor neuron disease without sensory involvement. The weakness can be symmetrical or more often asymmetrical. Patients initially have a clinical picture similar to that of viral meningitis, with fever and neck stiffness. Currently, most cases follow exposure of an immunocompromised host to the oral polio vaccine, and this should be sought in the history. The cerebrospinal fluid analysis resembles that of viral meningitis.

Diseases of the Neuromuscular Junction

Disorders of the neuromuscular junction cause motor fatigability. The initial depolarization at the nerve endplate stimulates a maximum number of AChRs on the muscle cell, producing a normal or nearly normal strength response. Repeated stimulation leads to diminishing motor strength, which is caused by one of three mechanisms: blockage of the receptors, as in MG; decrease in the amount of ACh released, as in botulism; or inactivation of ACh by irreversible binding, as in organophosphate poisoning.

A decrease in the release of ACh can cause a combination of nicotinic and muscarinic effects. The clinical manifestations of this are anticholinergic findings, such as decreased visual acuity, confusion, urinary retention, tachycardia, low-grade fever, and dry, flushed skin. In the case of Lambert-Eaton myasthenic syndrome, weakness is more pronounced at the beginning of muscle use and improves with repeated use as more ACh builds up in the synaptic cleft with each stimulation. Diseases of the neuromuscular junction are considered in patients who present with generalized weakness in association with an acute cranial nerve deficit. Muscle tone is generally diminished and sensation is preserved in diseases of the neuromuscular junction.

Diagnostic Strategies

Specific Disorders

Disorders of the Neuromuscular Junction

Myasthenia Gravis

Perspective.: MG affects approximately 60,000 Americans.1 The age at onset is bimodal; women are most commonly affected between the ages of 20 and 40 years and men between 50 and 70 years. Whereas new cases of MG are occasionally diagnosed in the emergency department (ED), it is much more common for patients with established disease to present with exacerbations of their disorder, often caused by precipitating factors.

Clinical Features.:

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