The degree of motor dysfunction depends on the extent of the spinal cord lesion. Complete lesions destroy all function below the affected level. Incomplete lesions cause partial weakness, atrophy, and hyporeflexia at the affected level, usually in combination with a distal upper motor neuron lesion, which may predominate and cause weakness, spasticity, and hyperreflexia. A search for subtle signs of a distal upper motor neuron lesion is imperative in any patient with an apparently isolated spinal nerve root lesion, particularly in the cervical region.
In contrast to a complete peripheral nerve lesion, in which motor function is completely lost in the distribution of that nerve, a complete nerve root lesion typically causes weakness (paresis), sometimes severe, but not total paralysis of the various muscles innervated by that nerve root. This is because each muscle is innervated by multiple nerve roots arising from more than one spinal level (see Plate 2-16).
The diaphragm is innervated by the C3, C4 and C5 segments; therefore a lesion high in the cervical spinal cord threatens respiratory function. Shoulder abduction is a good test of C5 function. In the presence of normal function of the deltoid muscle, weak elbow flexors, predominantly the biceps brachii muscles, suggest a C6 lesion. The elbow and wrist extensors, subserved primarily by the triceps brachii and extensor carpi radialis and ulnaris muscles, are innervated by C7. Function of the pronator teres muscle is also helpful in identifying a lesion at C7.
Lesions at C8 predominantly affect the intrinsic muscles of the hand, which are also innervated by T1. The abdominal musculature can be tested clinically for lesions that affect thoracic nerves; a positive Beevor sign indicates weakness below T9 or T10.
The hip flexors and adductors are innervated by L2 and predominantly by L3. The quadriceps femoris muscle is a good marker of L4 function. L5 innervates the ankle dorsiflexors and great toe extensors, while the ankle plantar flexors are innervated by S1 and S2. The lowest segments of the spinal cord (S2, S3, and S4) control the anal sphincter.
Sensory examination often provides the most significant information in localizing a spinal cord lesion. However, if results of the examination are normal, the patient’s symptoms may be the most important clue. The segmental distribution may be most useful in diagnosis when both the nerve root and spinal cord are involved, as seen in a dumbbell tumor, or neurilemmoma (see Plate 2-22).