Q&A: Rating Brachial Plexus Injury

Abstract

QUESTION: How do I assess impairment for a patient with a brachial plexus injury involving the middle and lower trunks but not the upper trunk?ANSWER: Rating impairment for a brachial plexus injury using the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Sixth Edition, is straightforward. Table 15-20, Brachial Plexus Impairment: Upper Extremity Impairments (6th ed, 434-435), lists impairments for the entire brachial plexus, as well as upper, middle, and lower trunks individually, each in different rows. If both middle and lower trunks were injured, each would be rated, and the upper limb impairments combined per instructions number 8 and 13 on page 481.The classic anatomic description of the brachial plexus describes the C5 through T1 nerve roots as contributing to the plexus, with the middle trunk composed of just the C7 nerve root, while the lower trunk is formed by the fusion of the C8 and T1 nerve roots. However, this “classic plexus” is found in only 37% to 77% of cadaver dissections.1 The two most common anatomic anomalies are “prefixed” and “postfixed” plexi. In a prefixed plexus, the C4 nerve root has a substantial contribution, while the T1 nerve root has minimal or no contribution to the plexus. In a postfixed plexus, there is minimal or no C5 contribution to the plexus and a substantial T2 nerve root contribution. Thus, standard charts of the nerve root contributing to individual peripheral nerves or trunks of the plexus may not correspond to an individual patient's anatomy.Assuming C7 supplies sensation to the middle finger and C8 and T1 to the little finger and ulnar forearm, rating sensory deficit without obvious duplication is more straightforward.Rating the motor deficit is more complex. No nerve to a single muscle leaves either the middle or lower trunk of the classic plexus. Hence, one cannot determine whether the injury does or does not involve the trunk by involvement or sparing that nerve, respectively. Normal paraspinal muscles on needle electromyography (EMG) with clear denervation in limb muscles helps localize the lesion as distal to the nerve root division into anterior and posterior primary rami. Stimulating proximal to the plexus at the level of individual nerve root during nerve conduction testing is challenging and rarely done. Magnetic resonance imaging (MRI) may not clarify the level of injury. However, in cases of open trauma with immediate surgery to repair vascular damage, the operative report may accurately describe the extent of any concomitant brachial plexus injury.Because most muscles are innervated by more than one nerve root—a fail-safe redundancy in the case of injury—residual weakness in any one muscle may be from injury to the lower trunk, middle trunk, or both. Hence, it is wise to rate the severity of the motor loss on physical examination by assuming any one muscle is innervated by nerves derived from only one trunk.For example, the triceps is classically described as innervated by the C7, C8, and partially by the T1 nerve roots. If the triceps has grade 3/5 weakness on examination, and if that is considered middle trunk weakness when rating the middle trunk, and lower trunk weakness when rating the lower trunk, the motor deficit rating will be duplicative (rating the same weakness twice).Similarly, classic innervation charts show C7 and C8 contributing to the flexor carpi ulnaris, flexor digitorum profundus, abductor digiti minimi, first dorsal interosseous, extensor digitorum communis, and extensor carpi ulnaris. Logically weakness in these muscles should be attributed to and rated as either middle or lower trunk motor deficits, but not both.Brachial plexus injuries are uncommon, and it takes considerable time and thought to thoroughly examine and correctly rate the residuals.