The brachial plexus is formed from the C5–C8 and T1 nerve roots. It is divided into three trunks: the upper trunk is formed by the junction of the C5 and C6 nerve roots, the middle trunk is a continuation of the C7 nerve root, and the lower trunk is formed by the junction of the C8 and T1 nerve roots. Each trunk gives off an anterior and posterior division. The posterior divisions from each trunk form the posterior cord, which terminates in the axillary and radial nerves. The anterior division from the upper and middle trunk join to form the lateral cord, which terminates in the musculocutaneous nerve and half of the median nerve. The anterior division of the lower trunk continues as the medial cord, which terminates in the ulnar nerve and the other half of the median nerve. Brachial plexus syndromes are an uncommon cause of arm weakness and numbness. As a general rule, it is much more likely that a patient with these symptoms has a cervical radiculopathy (or multiple cervical radiculopathies) or multiple mononeuropathies. Nevertheless, brachial plexus syndromes are important to identify as the likely etiologies and evaluation are different from either radiculopathy or mononeuropathy.
A brachial plexus cord lesion can usually be distinguished from a cervical radiculopathy based on the clinical examination; however, it is usually not possible to distinguish a cervical radiculopathy from a brachial plexus trunk lesion. In the latter scenario, nerve conduction studies and needle electromyography (NCS/EMG) would be indicated to confirm the clinical localization of a brachial plexus cord lesion or to distinguish a brachial plexus trunk lesion from a cervical radiculopathy. NCS should demonstrate normal sensory responses in the setting of radiculopathy. In an upper trunk or lateral cord injury, the lateral antebrachial and median sensory responses should be abnormal. In a middle trunk or posterior cord injury, the radial sensory response should be abnormal. In a lower trunk or medial cord injury, the medial antebrachial and ulnar sensory responses should be abnormal.
Trauma is a frequent cause of brachial plexus injury, particularly motorcycle accidents.
Brachial plexus injury, particularly involving T1 fibers in the lower trunk, is a known complication of coronary artery bypass grafting surgery. It is felt to occur due to stretching or compressive injury during the operation. This injury can result in the unusual NCS findings of a preserved ulnar sensory amplitude with a reduced medial antebrachial cutaneous sensory amplitude.
Radiation plexopathy is a delayed complication of radiation. Patients typically present with insidiously progressive painless weakness. Clinical myokymia or myokymic discharges on EMG can be seen in a subset of patients.
In classic neuralgic amyotrophy (also known as brachial neuritis or Parsonage Turner syndrome), the patient has severe pain that spontaneously resolves. After the pain resolves, arm weakness is noted. Numbness is either absent or minimally present in a patchy distribution. Persistent pain with weakness should prompt reconsideration of the diagnosis with a particular focus on cervical radiculopathy or a mass lesion. The pattern of weakness is unusual in neuralgic amyotrophy in that it does not localize well to a particular portion of the brachial plexus or a single nerve root. There is a predilection for the pronator teres, flexor digitorum profundus, and infraspinatus muscles, in addition to the deltoid, serratus anterior, triceps, and trapezius. While there is no specific treatment, the acute pain in brachial neuritis should be treated symptomatically and may require narcotic pain medications. Weakness typically spontaneously improves by around 3 months after onset.
Neurogenic thoracic outlet syndrome, in which the lower trunk of the brachial plexus is compressed by bone or fibrous tissue, is rare. NCS will show a reduced ulnar sensory amplitude and medial antebrachial sensory amplitude as well as denervation of the ulnar and median-innervated hand muscles on needle EMG. Cervical x-rays can be done to assess for a cervical rib or elongated C7 transverse process, and magnetic resonance imaging of the chest to assess for a fibrous band. Treatment consists of surgical decompression, such as removal of a cervical rib or lysis of a fibrous band.
For most patients, brachial neuritis is a monophasic event. Rarely, patients can have recurrent brachial neuritis or a family history of brachial neuritis. In these patients, a genetic etiology should be considered. The most common genetic mutation in hereditary neuralgic amyotrophy is the SEPT9 mutation. Many but not all patients have associated facial dysmorphisms including hypotelorism or epicanthal folds.
Local mass lesions can compress or invade the brachial plexus causing neurologic dysfunction. Typical causes are breast cancer or lymphoma. Slowly progressive symptoms over weeks-months and persistent pain should raise concern for a mass lesion.