Assessment of weakness

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A motor deficit can result as a consequence of a lesion that lies within various places in the neuraxis. Weakness can be secondary to a lesion in the upper motor neuron pool (corticospinal tracts) or within the lower motor neuron pool (anterior horn cell and its peripheral process). Weakness may also be secondary to neuromuscular junction dysfunction. Finally, weakness can be due to muscle disease. Differentiating between these various etiologic factors in the clinical setting can be quite challenging. The EMG study provides an excellent tool for determining the cause of weakness. In a study performed by Nardin et al. (2002), the diagnostic accuracy of electrodiag-nostic testing in the evaluation of patient weakness had an overall accuracy of 91%.

Upper motor lesions

Clinically, patients with lesions of the brain and/or spinal cord present with weakness and most often also have numbness. These patients usually have increased deep tendon reflexes, extensor plantar responses, and increased muscle tone that marks the lesion site. Assessment of a lesion that involves the upper motor neuron pool is done during the EMG portion of the study. These patients have a delayed onset of EMG activity and a decreased ability to recruit motor unit potentials. Several studies have documented that there is a delay of initiation and prolonged termination of muscle contraction in paretic muscles from upper motor impairment (Chae et al., 2002). Following a stroke, changes in descending input may modify interneuronal excitability throughout the cord (Grimby, 1963). The loss of excitability at the level of the spinal interneurons may account for the delayed onset of EMG activity observed in paretic limbs in an upper motor neuron lesion

(Dewald et al., 1999). Additional discussion of upper motor lesions are found in several chapters of this volume (e.g., Volume II, Chapters 17, 18, 36, and 38).

Lower motor lesions

In a patient undergoing rehabilitation, there is the potential for the patient to be immobilized for protracted periods of time. These patients may consequently develop superimposed patterns of weakness, due to deconditioning, development of contractures and joint problems. Additionally, they may develop a lower motor neuron lesion secondary to either a compressive neuropathy or a polyneuropathy (see Volume II, Chapter 40). Being able to distinguish between these varying patterns of weakness is important. NCs are extremely useful in assessing these types of patients. Compressive and entrapment neuropathies usually lead to numbness and weakness in the area of the involved nerve. Although the terms compression and entrapment are often used interchangeably, it is important to recognize the distinction between a nerve entrapped in a fibroosseous tunnel where chronic low pressure and ischemia are the major mechanisms of nerve injury and external compression where, in acute situations, high mechanical pressures can result in a unique structural deformation of the nerve producing a focal demyeli-nation (a neuropraxia). Certain forms of compression, with moderately high external pressure for limited time periods, result in a failure of action potential propagation (conduction block) but not in Wallerian degeneration (Fowler et al., 1972). If the compression is more prolonged, even with relatively low pressures there is a mixed lesion with both action potential blockade and Wallerian degeneration. These histologic findings can be anticipated from careful electrodiagnostic studies.

Electrodiagnostic studies performed on a nerve with a compressive lesion should reveal that the conduction velocity above and below the region of the segmental demyelination are normal (McDonald, 1962). However, across the demyelinated region of the nerve, the velocity of action potential propagation is significantly slowed. Conduction block is defined as

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