Neurological Causes of Pain

Plexus Pain

Peripheral pathology may involve the brachial plexus (Fig. 29.1) or individual nerves extending to the hand. Infiltrative or inflammatory lesions of the brachial plexus produce severe brachialgia radiating down the upper limb and also spreading to the shoulder region. Radiation to the ulnar two fingers suggests that the origin is in the lower brachial plexus, and radiation to the upper arm, forearm, and thumb suggests an upper plexopathy. Patients with a thoracic outlet syndrome complain of brachialgia and numbness or tingling in the upper limb or hand when working with objects above the head.

Fig. 29.1 Brachial plexus: schema.

(Netter illustration from www.netterimages.com © Elsevier Inc. All rights reserved.)

Non-Neurological Causes of Neck Pain and Brachialgia

Pain arising in muscle is deep, aching, and boring. In the cervical region, it is localized to the shoulders and sometimes radiates down the arm. If the patient is over 50 years of age, a sedimentation rate should be checked; if it is markedly elevated, the diagnosis of polymyalgia rheumatica should be considered. Patients with fibromyalgia may have pain in the neck, shoulders, and arms, with trigger spots or nodules that are exquisitely tender even to light pressure.

If pain is triggered or aggravated by movement of the upper limb, arthritis or tendonitis is the likely cause. Particular attention should be paid to these characteristics; pain on shoulder joint movement is unlikely to be neuropathic. The pain of epicondylitis may radiate down the forearm in a pseudoneuralgic fashion, but precipitation of pain by movement at the fingers or wrist indicates a rheumatological cause.

Motor Signs

The examination begins with inspection. Particular attention is paid to atrophy of muscles of the shoulders, arms, and the small muscles of the hands. Fasciculations are often due to anterior horn cell disease, but they may be part of the neurology of cervical spondylosis and radiculopathy. Significant sensory signs would argue against anterior horn cell degeneration.

Muscles in the various myotomes must be tested individually. When there is unilateral weakness, the contralateral side can act as a control, but some standard measure of strength is necessary for accurate evaluation when bilateral weakness is present. If one can overcome the action of a patient’s muscle by resisting or opposing its action close to the joint it moves, using an equivalent equipotent muscle of the examiner (fingers test fingers, whole arm tests biceps), then that muscle in the patient is, by definition, weak. The degree of weakness can be graded, and the 5-point (Medical Research Council [MRC]) grading scale is often used. Grade 5 represents normal strength. Grade 4 represents “weakness” somewhere between normal strength and the ability to move the limb only against gravity (grade 3). Grade 4 covers such a wide range of weakness that it is usually expanded. One simple expansion is into “mild, moderate, or severe.” When the muscle can move the joint with the effect of gravity eliminated, it is graded at 2, and grade 1 is just a flicker of contraction.

The lower limbs must always be examined, even when the patient complains of symptoms only in the upper limbs. Evidence of a myelopathy as evidenced by the finding of sensory or motor dysfunction in the lower limbs, when combined with the presence of radicular signs in the upper limbs, indicates a spinal cord lesion in the neck.

The distribution of weakness is all important in localizing the problem to nerve root, plexus, peripheral nerve, muscle, or even upper motor neuron (central weakness). It is useful to use a simplified schema of radicular anatomical localization when evaluating nerve root weakness because overlap of segmental innervation of muscles can complicate the analysis (Table 29.1).

Table 29.1 Segmental Innervation Scheme for Anatomical Localization of Nerve Root Lesions

The thoracic outlet syndrome, or brachial plexus entrapment, is an overdiagnosed condition. Maneuvers designed to test for compromise of the neurovascular structures passing through the thoracic outlet are often difficult to interpret. In these maneuvers, the arm is extended at the elbow, abducted at the shoulder, and then rotated posteriorly. The examiner palpates the radial pulse while listening with a stethoscope over the brachial plexus in the supraclavicular fossa. The patient takes a deep inspiration and turns the head to one or the other side. Many normal individuals lose their radial pulse, but the emergence of a bruit does suggest at the least vascular entrapment (Adson test). The patient then exercises the hands held above the head with extended elbows—numbness, pain, or paresthesias, often with pallor of the hand, support the diagnosis (Roos test).

A distribution of weakness that does not conform to a clearly defined anatomical distribution of a single peripheral nerve in the upper limb suggests plexopathy. Upper plexus lesions cause mainly shoulder abduction weakness, and lower plexus lesions will affect the small muscles of the hand.

Sensory Signs

Skin sensation is tested in a standardized manner starting with pinprick appreciation at the back of the head (C2), followed by sequentially testing sensation in the cervical dermatomes, down the shoulder, over the deltoid, down the lateral aspect of the arm to the lateral fingers, and then proceeding to the medial fingers and up the medial aspect of the arm (Fig. 29.2). The procedure is repeated with a wisp of cotton to test touch sensation and test tubes filled with cold and warm water to test temperature sensation. Vibration sense is rarely abnormal in the fingers. Position sense in the distal phalanx of a finger is tested by immobilizing the proximal joint and supporting the distal phalanx on its medial and lateral sides and then moving it up or down; the patient, with closed eyes, reports movement and its direction. Loss of position sense in the fingers usually indicates a very high cervical cord lesion.

Fig. 29.2 Diagram of the dermatomes in the upper limbs. A, Anterior aspect. Although variability and overlap across the interrupted lines are evident, little or no overlap occurs across the continuous lines (i.e., dorsal and ventral axial lines). The examiner should routinely choose one spot in the “middle” of a dermatome and test at that point in all patients. C4 usually terminates at the point of the shoulder, T3 is almost always in the axilla, and T4 spreads across the chest so that C4 abuts T4 approximately at the nipple line. B, Posterior aspect.

Tendon Reflexes

Examination of the tendon reflexes helps localize segmental nerve root levels, but in cervical spondylosis, which is by far the most common underlying pathology, the reflexes are often preserved or even increased despite radiculopathy because of an associated myelopathy. An absent or decreased biceps reflex localizes the root level to C5, and an absent triceps reflex localizes the level to C6 or C7. An absent biceps reflex but with spread so that triceps or finger flexors contract is called an inverted biceps jerk and is strong evidence for C5 radiculopathy.

Non-Neurological Signs

The arm is passively abducted and internally and externally rotated at the shoulder. The complaint of pain on movement or at a point in the abductor arc indicates local shoulder joint pathology, usually shoulder tendonitis or pericapsulitis. The tendons anteriorly and at the lateral point of the shoulder may be tender to pressure. More diffuse tenderness anterior to the shoulder joint indicates bursitis. Tenderness over the medial or lateral epicondyle at the elbow indicates local inflammation, and pain on active or passive wrist or finger joint movement suggests tendonitis or arthritis.

Intramedullary Lesions

Primary intramedullary lesions may be neoplastic, inflammatory, or developmental. The most common presenting symptom of spinal cord tumor is pain, which is present in about two-thirds of patients, usually radicular in distribution, often aggravated by coughing or straining, and worse at night. A minority of patients show dissociated sensory signs (segmental loss of pin prick and temperature sensation with preserved light touch, vibration, and position sense) in the upper limbs to suggest central cord pathology. Long-tract signs in the lower limbs will ultimately develop. Magnetic resonance imaging (MRI) reveals swelling of the spinal cord. The most common tumors are glioma, lymphoma, and ependymoma.

Cervical myelitis presents with rapid onset of radicular and long-tract symptoms and signs and may be due to MS, postinfectious encephalomyelitis, or may be without an obvious cause (idiopathic).

Syringomyelia, a cystic intramedullary lesion of variable and unpredictable progression, may present with deep aching or boring pain in the upper limb, often characteristically referred to the ear. Asymmetrical lower motor neuron signs in the upper limbs, with dissociated suspended sensory loss (i.e., has an upper and lower border to the impairment of pin prick and temperature sensation), is very suggestive of a syrinx. However, the most common cause of intramedullary cord dysfunction is extrinsic spinal cord compression.

Extramedullary Lesions

Extramedullary lesions may result in any combination of root, central cord, and long-tract signs and symptoms. The most common cause of extrinsic nerve root and spinal cord compression is cervical spondylosis. This is a degenerative disorder of the cervical spine characterized by disc degeneration with disc space narrowing, bone overgrowth producing spurs and ridges, and hypertrophy of the facet joints, all of which can compress the cord or nerve roots. Hypertrophy of the spinal ligaments, with or without calcification, may contribute to compression. Hypertrophic osteophytes are present in approximately 30% of the population, and the incidence increases with age. The presence of such degenerative changes does not indicate that the patient has symptoms due to these changes; astuteness in diagnosis is necessary. Furthermore, the degree of bony change does not always correlate with the severity of the signs and symptoms it produces. This degenerative process is sometimes referred to as hard disc