Lower Motor Neuron Syndromes

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Diseases of the lower motor neuron may affect the cell body itself in the anterior spinal gray or its axon as it leaves the spinal cord in the spinal root and becomes a peripheral nerve. Signs of disease of the lower motor neuron include muscular weakness, atrophy, fasciculations, and loss of tendon reflexes.

PROGRESSIVE SPINAL MUSCULAR ATROPHY SYNDROMES

There may be selective degeneration of the anterior horn cells of the spinal cord or brain stem or both. This results in a progressive weakness and wasting of muscle and loss of tendon reflexes. The upper motor neuron is not affected. The spinal muscular atrophies (SMA) are genetic diseases of childhood and adolescence and, less commonly, adulthood. Regardless of whether they are recessive or dominant, the defect is linked to the same region on chromosome 5. In addition to patterns of heredity, clinical classifications are based on age of onset, rate of progression, and the muscles affected. y The infantile-onset form (Werdnig-Hoffman disease) is the most severe (lethal) form and is recessive. The later-onset, more benign forms are more likely to be dominant. The variety that affects the proximal musculature and simulates muscular dystrophy is known as the Wohlfart-Kugelberg-Welander syndrome. The distal musculature is preferentially affected in various combinations with the ocular and bulbar muscles in Kennedy's syndrome.

Lower motor neuron disorders that are restricted to certain cranial nerve nuclei or combinations are much less common. The Moobius syndrome, which affects the sixth and seventh cranial nerves, appears to be a failure of development of the motor neurons and presents as a congenital disorder. The progressive bulbar palsy of Fazio-Londe is a rare fatal disease that affects all motor nuclei of the brain stem.

RADICULOPATHY SYNDROMES

Lesions of the spinal nerves and roots produce weakness, atrophy, and sometimes fasciculations in the muscles innervated by the affected root or, in some cases, multiple roots. Localization of the specific nerve root affected by the lesion requires a knowledge of each group of muscles supplied by a single anterior spinal root (myotome) and each cutaneous area supplied by the posterior spinal root ( dermatome). Differentiation of these syndromes from peripheral nerve or plexus lesions thus depends on the distribution of the motor and sensory signs and whether the signs conform to those produced by a particular myotome or dermatome. Root lesions may interrupt the afferent or efferent arc of the phasic stretch reflex and thus affect tendon jerks. Thus, a reduction or loss of a reflex is very helpful in localizing the root involved. Table 15-4 correlates the reflex in the cervical or lumbosacral root subserving that reflex.

The spinal nerve roots may be injured directly by trauma or compressed by lesions such as tumors or herniated discs. The most common radiculopathies affect the fifth and sixth cervical roots and the fifth lumbar and first sacral roots, since those are the roots most often compressed by herniated discs. Because muscles are innervated by more than one spinal nerve root, it may be difficult to differentiate a C5 from a C6 or a C6 from a C7 root injury. For example, a C6 root lesion may depress either the biceps or triceps reflex. Differentiating a C5 from a C7 root lesion is easier because the C5 lesion reduces the biceps jerk, whereas the C7 reduces the triceps. The same muscles are also the weakest. Therefore, one must also use the distribution of pain and sensory loss as a guide to supplement the motor signs and provide a pattern of motor and sensory change that can be assigned to one spinal segment.

In the lower extremity, the differentiation of an L5 from an S1 root lesion is most readily made by watching the patient walk on the heels and toes. Weakness of dorsiflexion (difficulty in walking on the heels) is usually due to an L5 root lesion, reflecting the fact that the anterior tibial muscle receives its major innervation from that segment. Difficulty in walking on the toes due to weakness of the gastrocnemius muscles reflects the fact that its major innervation is provided by the S1 root. The ankle jerk may be affected by either lesion but is more often abolished by the S1 root lesion.

PLEXOPATHY SYNDROMES

Plexopathies are more difficult to recognize and localize than lesions of the spinal roots or peripheral nerves because the anatomy of the plexuses is so complex (see

Figs 15-6 and 15-7 ). Brachial plexopathies present with a variety of clinical syndromes that depend on the part of the plexus involved. Trauma is the most frequent cause of damage, which usually results from a traction, compression, or stretch injury of the plexus. The most common syndromes are summarized in Tablie...15-11i . The brachial plexus is also subject to injury from radiation or malignancies, compression by tumors in lymph nodes or the apex of the lung, or immunologic attack ("brachial neuritis'' or Parsonage- Turner syndrome). The best clue to the presence of a brachial plexopathy is a motor and sensory deficit that involves more than one spinal or peripheral nerve. The motor signs (weakness, atrophy, and loss of tendon jerk) are much more prominent than the sensory changes, which are often patchy and incomplete. Diffuse aching pain, sometimes quite severe, is often present.

Lesions of the brachial plexus most commonly affect the upper and lower trunks. The upper trunk of the plexus is affected most commonly, giving rise to paralysis and atrophy in muscles supplied by the C5-C6 roots that comprise the upper trunk (Erb-Duchenne type). This upper trunk palsy is most often due to traumatic hyperextension of the head and shoulder but can be due to direct pressure caused by carrying heavy objects (knapsack palsy). The upper trunk is the part of the plexus most commonly affected by an idiopathic inflammatory neuritis (Parsonage-Turner syndrome). The lower brachial plexopathy (Dejerine-Klumpke type) is usually the result of trauma to the lower trunk of the plexus, which contains the eighth cervical and first thoracic roots. It is the plexus lesion most often associated with an infiltrating tumor from the apex of the lung (Pancoast tumor). Because the sympathetic fibers destined for the superior cervical ganglion exit the spinal cord with the first thoracic root, a Horner's syndrome (ptosis, miosis, and anhidrosis) may result; if present, it can be a helpful confirmatory sign.

Like lesions of the brachial plexus, a lumbosacral plexopathy is recognized by deficits in the distribution of multiple spinal and peripheral nerves in the lower extremity (see Table 15-11 ). Also like the brachial plexus, the lumbosacral plexus is affected by idiopathic inflammatory neuritis, radiation,

TABLE 15-11 -- EXAMPLES OF PLEXOPATHIES CAUSING WEAKNESS

Plexopathy

Pattern of Weakness

Causes

BRACHIAL

Upper trunk (C5-C6)

"Porter's tip position"; biceps, brachioradialis jerks reduced or absent

Pressure on shoulder ("knapsack palsy'); brachial neuritis

Lower trunk (C8-T1)

Wrist and finger flexion, intrinsic hand muscles

Trauma, Pancoast's tumor of lung

LUMBOSACRAL

Lumbar segments (T12-L4)

Flexion, abduction, and eversion of thigh; absent knee jerk

Tumor, surgical trauma

Sacral segments (L5-S3)

Hip extension, abduction and internal rotation of thigh, flexion of leg and all movements of foot ("flail foot"); absent ankle jerk

Tumor, surgical trauma

ENTIRE PLEXUS

Incomplete weakness of various muscles

Diabetes mellitus, idiopathic plexitis

and infiltrating neoplasms. It is less likely to be injured by trauma than the brachial plexus, but pelvic hematomas in the psoas muscle resulting from anticoagulation and surgical trauma are more common. Lesions of the lumbar segments produce weakness of all movements of the thigh with reduction or loss of the patellar tendon jerk. Lesions of the sacral portion of the plexus result in a weakness of the foot and flexion of the knee with reduction or loss of the ankle jerk. Because most of the motor output of the lumbar portion of the plexus is contained in the femoral nerve and the output of the sacral portion is found in the sciatic nerve, it may be difficult to distinguish lumbosacral plexus lesions from lesions of their respective major peripheral nerves. To distinguish a lumbar plexus lesion from a femoral neuropathy, a diligent search should be made for weakness of the adductor muscles innervated by the obturator nerve or of sensory loss in the inguinal region or over the genitalia, which are outside the distribution of the femoral nerve. Weakness of abduction and internal rotation of the thigh and of hip extension, or sensory loss on the posterior thigh in the distribution of the posterior femoral cutaneous nerve, helps to distinguish a lesion of the sacral plexus from a sciatic palsy, which lacks these signs.

PERIPHERAL NEUROPATHY AND MONONEUROPATHY SYNDROMES

Interruption of the motor fibers in a motor or mixed nerve leads to weakness or paralysis and atrophy of the muscles innervated by that nerve. Fasciculations are less likely to occur in peripheral nerve lesions than in root or anterior horn cell lesions. Table.^zli? lists the peripheral nerves most often affected by lesions, the muscles they supply, and any stretch reflex subserved by that nerve. Because a muscle is generally served by only one peripheral nerve, the weakness and wasting in that muscle are generally easier to detect than in a root lesion because the same muscle is generally served by two or more myotomes. The localization is confirmed by the pattern of sensory loss, which in most cases differs from the segmental dermatomal loss secondary to a root lesion. Lesions of a single peripheral nerve are most commonly due to entrapment, trauma, or infarction of the nerve secondary to diabetes or an inflammatory vasculitis. When a single peripheral nerve is damaged, it is termed a mononeuropathy. Mononeuropathy multiplex refers to involvement of several major named nerves in more than one limb. This condition most often occurs in systemic diseases such as autoimmune vasculitis, especially polyarteritis nodosa and diabetes mellitus. y

In polyneuropathy, the essential feature is impairment of function of many peripheral nerves simultaneously, causing symmetrical loss of function starting in the distal extremities. The legs are almost always affected before the arms. Hyporeflexia or areflexia often precedes any overt motor or sensory symptoms. The ankle jerks are affected first and then the knee jerks before any change occurs in the reflexes of the arms.

Polyneuropathies may be mainly sensory, mixed motor and sensory, or primarily motor. The motor component begins as weakness and atrophy in the intrinsic muscles of the feet; atrophy of the extensor digitorum brevis is often the first helpful clue. If the motor impairment progresses, the leg muscles are affected next, producing weakness of dorsiflexion and plantar flexion of the foot. Only then may the intrinsic muscles of the hand be affected. Generally, the more proximal muscle groups are not affected unless a rapidly progressive disorder is present such as an acute inflammatory demyelinating polyneuropathy (AIDP, Guillain-Barre syndrome) or another neuropathy mimicking AIDP such as porphyria and acquired immune deficiency syndrome (AIDS). The common distal symmetrical and slowly progressive polyneuropathies are most often due to systemic disorders, especially diabetes mellitus, alcoholism, and uremia.

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