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As outlined in Chapter 4, the important question, "Where is the lesion?" (or "What is the precise location of the deficit?"), must be followed by the equally important question, "What is the lesion?" (or "What is the nature of the disease?"). The answers should lead to the differential diagnosis and correct diagnosis and should guide therapy.
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THE LOCATION OF LESIONS
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Where is the lesion? In thinking about the location of the lesion, it is important to systematically survey the nervous system. Lesions can be located in one or more of the following anatomic sites:
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Muscles. In muscle diseases, one sees weakness, sometimes with muscle atrophy. Deep tendon reflexes are usually depressed. Diseases of muscle include the dystrophies, which have specific genetic patterns and stages of onset and may preferentially involve certain muscle groups; and inflammatory disorders of muscle such as polymyositis. Diagnosis may be aided by measuring the level of enzymes (such as creatine phosphokinase) in the serum because damage to muscle fibers may lead to their release. Electromyography and muscle biopsy may help with diagnosis.
Motor end-plates. Disorders of the motor end-plate include myasthenia gravis and the Lambert–Eaton myasthenic syndrome. In these disorders, there is weakness, sometimes accompanied by abnormal fatigability resulting from abnormal function (eg, decreased effect of acetylcholine [ACh] on the postjunctional muscle or decreased release of ACh) at the neuromuscular junction. Weakness may involve the limbs or trunk or muscles involved in chewing, swallowing, or eye movements. In addition to the characteristic clinical pattern, electromyography may be helpful in diagnosis.
Peripheral nerves. Peripheral nerve lesions may be differentiated from lesions of muscle or motor end-plate by clinical criteria, electrical tests, or biopsy. In many disorders of peripheral nerves, both motor (lower motor neuron) and sensory deficits are present, although in some cases motor or sensory function is impaired in a relatively pure way. In most peripheral neuropathies, functions subserved by the longest axons are impaired first, so that there is a "stocking-and-glove" pattern of sensory loss, together with loss of distal reflexes (such as the ankle jerks) and weakness of distal musculature (ie, intrinsic muscles of the feet), which is in severe cases accompanied by muscle atrophy.
Roots. A motor root lesion results in a precise segmental motor deficit, which in some cases (eg, plexus lesions) is mediated through several nerves. A single sensory deficit may be difficult to diagnose because of the adjacent overlapping dermatomes (see Fig 5–9). When a nerve root carrying axons mediating a deep tendon reflex is affected, the reflex may be depressed (see Table 5–5). Sensory root symptoms may include pain that is worsened with the Valsalva maneuver, the forced expiratory effort caused by laughing, sneezing, or coughing.
Spinal cord. The staggered pattern of decussation of the lateral corticospinal tract, dorsal column–medial lemniscal system, and spinothalamic tracts often permits localization of lesions within the spinal cord. ...