The brachial, cervical, and lumbosacral plexuses are the anatomic crossroads where spinal nerve roots reorganize into the peripheral nerves of the limbs. A lesion of the plexus produces a pattern that is broader than a single peripheral nerve and broader than a single nerve root — the deficit involves multiple nerves derived from the affected part of the plexus, with sensory and motor signs that do not fit a clean radicular or single-nerve distribution. Plexopathies are clinically important because they are often missed (mistaken for radiculopathy or peripheral neuropathy), they are often painful, and their causes include the serious (tumor, radiation, vasculitis, infection) and the recoverable (idiopathic Parsonage-Turner, postpartum lumbosacral plexopathy). This page covers the anatomy of the three plexuses and the syndromes that arise from their lesions.

The Brachial Plexus

Anatomy

The brachial plexus arises from the ventral rami of C5-T1 (with variable contributions from C4 [prefixed] or T2 [postfixed]). The plexus is organized into:

• Roots: C5, C6, C7, C8, T1 — the five ventral rami.
• Trunks: Upper trunk (C5-C6), Middle trunk (C7), Lower trunk (C8-T1).
• Divisions: Each trunk divides into an anterior and posterior division.
• Cords: Lateral cord (anterior divisions of upper and middle trunks), Posterior cord (posterior divisions of all three trunks), Medial cord (anterior division of lower trunk).
• Terminal branches: Musculocutaneous (from lateral cord), Median (from lateral and medial cords), Ulnar (from medial cord), Axillary (from posterior cord), Radial (from posterior cord).

Some branches arise from intermediate points:

• From roots: Long thoracic nerve (C5-C7) to serratus anterior, dorsal scapular nerve (C5) to rhomboids.
• From upper trunk: Suprascapular nerve (C5-C6) to supraspinatus and infraspinatus.
• From lateral cord: Lateral pectoral nerve.
• From medial cord: Medial pectoral, medial brachial cutaneous, medial antebrachial cutaneous.

Upper Trunk Plexopathy (C5-C6, “Erb Palsy”)

Damage to the upper trunk produces weakness in muscles innervated by C5 and C6:

• Shoulder abduction (deltoid, supraspinatus) — arm hangs at side.
• External rotation at shoulder (infraspinatus) — arm internally rotated.
• Elbow flexion (biceps, brachialis, brachioradialis) — elbow extended.
• Forearm supination (biceps) — forearm pronated.
• Wrist extension may be weak (extensor carpi radialis from C6).

The classical posture is the “waiter’s tip” — arm adducted, internally rotated, elbow extended, forearm pronated, wrist flexed. Sensory loss involves lateral arm and forearm.

Causes: birth trauma (Erb-Duchenne palsy), motorcycle accident with shoulder depression, falls, traction injuries.

Lower Trunk Plexopathy (C8-T1, “Klumpke Palsy”)

Damage to the lower trunk produces weakness in muscles innervated by C8 and T1:

• Intrinsic hand muscles (ulnar and median innervated): interossei, lumbricals, thenar and hypothenar muscles.
• Long finger flexors (deep flexor muscles of fingers).
• The hand becomes weak with a “claw hand” deformity from intrinsic muscle weakness and unopposed long extensor and flexor pull.

Sensory loss involves the medial arm, medial forearm, and ulnar two fingers. Horner syndrome is often present because the sympathetic fibers traveling with T1 are involved.

Causes: birth trauma with arm extension (rare), Pancoast tumor (lung apex), traction injuries (rare).

Middle Trunk Plexopathy (C7)

Often involved with adjacent trunks; rarely isolated. Weakness of triceps, wrist extensors, finger extensors. Sensory loss in dorsum of hand and posterior arm.

Cord-Specific Patterns

• Lateral cord: Median and musculocutaneous distribution. Weakness of biceps, pronator teres, forearm flexors, thumb opposition. Sensory loss thumb to ring finger half.
• Medial cord: Ulnar plus medial portion of median. Weakness of ulnar-innervated muscles plus the long flexors. Often with Horner syndrome.
• Posterior cord: Axillary plus radial. Weakness of deltoid, triceps, wrist extensors, finger extensors. Sensory loss over deltoid, posterior arm, and dorsum of hand.

Pan-Plexus Injury

Severe traction injury (motorcycle accident, fall from height) can damage the entire plexus, producing a flail arm with complete sensory and motor loss. Often with Horner syndrome (T1 involvement). Avulsion at the cord level produces a pseudomeningocele on MR imaging — a critical finding distinguishing avulsion from rupture.

Major Causes of Brachial Plexopathy

• Traumatic: Erb (upper trunk), Klumpke (lower trunk), pan-plexus from severe stretch.
• Neoplastic: Pancoast tumor (apical lung cancer) classically affects lower trunk → C8-T1 distribution + Horner. Lymphoma can also affect.
• Radiation: After radiation for breast cancer or lymphoma. Often delayed (months to years), usually painless, slowly progressive, affects upper trunk preferentially.
• Inflammatory (Parsonage-Turner / neuralgic amyotrophy): Acute, severe shoulder pain followed by weakness. Often after vaccination, viral illness, or surgery. Patchy involvement, often upper plexus. Most recover but slowly (months).
• Postoperative: After cardiac, thoracic, or sometimes shoulder surgery — positioning, retraction, or median sternotomy can stretch the plexus.
• Idiopathic / postpartum: Postpartum brachial plexopathy from positioning during epidural anesthesia or pushing.
• Thoracic outlet syndrome: True neurogenic TOS is rare and involves the lower trunk, classically with a cervical rib.
• Hereditary: Hereditary neuralgic amyotrophy — recurrent painful plexopathy. Hereditary neuropathy with liability to pressure palsies (HNPP).
• Vasculitic: Polyarteritis nodosa, microscopic polyangiitis, lupus.

The Cervical Plexus

Anatomy

Arises from C1-C4 ventral rami. Less clinically prominent than brachial. Major branches:

• Sensory branches: Lesser occipital, great auricular, transverse cervical, supraclavicular nerves — supply skin of neck, lower face, upper chest and shoulder.
• Motor branches: Ansa cervicalis (to strap muscles), phrenic nerve (C3-C5) to diaphragm.

Phrenic Nerve and Diaphragm

The phrenic nerve is the most clinically important branch. Lesions cause unilateral or bilateral diaphragmatic paralysis — dyspnea on lying flat (orthopnea), elevated hemidiaphragm on chest X-ray, paradoxical abdominal motion. Causes include neuralgic amyotrophy, surgical injury (especially cardiac), tumor, idiopathic.

Cervical Plexus Block

Performed for carotid endarterectomy. Lesions from blocks are uncommon.

The Lumbosacral Plexus

Anatomy

Two interconnected plexuses:

• Lumbar plexus: L1-L4 ventral rami. Major nerves: ilioinguinal, iliohypogastric, lateral femoral cutaneous, femoral, obturator.
• Sacral plexus: L4-S4 ventral rami. Major nerves: sciatic (tibial + common peroneal), superior and inferior gluteal, pudendal.
• Lumbosacral trunk (L4-L5): connects the two plexuses, vulnerable to compression at the sacral ala (obstetric injury).

Lumbar Plexopathy

Affects nerves derived from L1-L4. Clinical picture:

• Weakness of hip flexion (iliopsoas, L2-L3).
• Weakness of knee extension (quadriceps, L2-L4 via femoral nerve).
• Weakness of hip adduction (adductors, L2-L4 via obturator nerve).
• Sensory loss anteromedial thigh.
• Reduced or absent patellar reflex.
• Often painful at onset.

Causes: Diabetic amyotrophy (diabetic lumbosacral radiculoplexus neuropathy, Bruns-Garland syndrome) is the classic — acute or subacute painful proximal leg weakness with associated weight loss in older diabetic patients, often unilateral but can become bilateral. Most improve but slowly (months to year). Other causes: retroperitoneal hematoma (especially with anticoagulation), psoas abscess, tumor (lymphoma, sarcoma), radiation, vasculitis, idiopathic lumbosacral plexitis.

Sacral Plexopathy

Affects nerves derived from L4-S4. Clinical picture:

• Weakness of hip extension (gluteus maximus, L5-S1 via inferior gluteal nerve).
• Weakness of hip abduction (gluteus medius/minimus, L4-S1 via superior gluteal nerve).
• Weakness of knee flexion (hamstrings, L5-S2 via sciatic).
• Weakness of all distal leg muscles (sciatic → tibial + peroneal).
• Sensory loss posterior thigh, lateral and posterior leg, dorsum and sole of foot.
• Reduced or absent Achilles reflex.

Causes: pelvic tumor (cervical, rectal, prostate, bladder), retroperitoneal mass, pelvic surgery, radiation, obstetric trauma, pelvic fracture, vasculitis.

Lumbosacral Trunk and Postpartum Plexopathy

The lumbosacral trunk (L4-L5) crosses the sacral ala in a vulnerable location. Compression during prolonged labor — particularly if the fetal head sits at the brim for long — can cause a foot drop on one side (peroneal-predominant sacral plexopathy). The picture often appears in the postpartum period and improves over months.

Distinguishing Plexopathy from Radiculopathy and Mononeuropathy

The paraspinal muscles are key: they are innervated by the posterior ramus of the spinal nerve, which branches off before the plexus. A radiculopathy affects the paraspinals (denervation on EMG); a plexopathy does not. This is one of the most useful EMG findings for distinguishing.

Diagnostic Workup

1. Careful neurologic examination to define the distribution of weakness, sensory loss, and reflex change.
2. EMG/nerve conduction studies to confirm plexus involvement, define severity, and distinguish from radiculopathy or mononeuropathy.
3. MRI of the plexus (specialized neurography sequences are sometimes needed) to look for mass, inflammation, or root avulsion.
4. Targeted laboratory testing based on suspected cause: glucose, HbA1c (diabetes), ANCA (vasculitis), ANA (lupus), inflammatory markers, antiganglioside antibodies.
5. Specialized imaging: chest CT or PET for Pancoast tumor or lymphoma; pelvic CT/MRI for sacral plexus tumors.
6. CSF analysis: may be useful if inflammatory or infectious plexopathy suspected.

🔍 Did You Know?

Parsonage-Turner syndrome (neuralgic amyotrophy, idiopathic brachial plexitis) has a remarkably stereotyped presentation that is often missed because clinicians don’t think of it. The patient — often a young to middle-aged adult — develops severe, deep, aching shoulder pain of relatively sudden onset, sometimes after an upper respiratory infection, vaccination, surgery, or unusual physical exertion. The pain is intense, often requiring opioids, and typically lasts days to weeks. As the pain subsides, the patient notices weakness — often in the shoulder girdle, with patchy involvement of suprascapular, axillary, long thoracic, or anterior interosseous nerves. Scapular winging from serratus anterior weakness is a clue. Recovery happens but slowly: most patients regain useful function over six months to two years; some have permanent weakness. The condition is autoimmune and recurrent in some patients (hereditary neuralgic amyotrophy, linked to SEPT9 mutations). Recognition matters because the natural history is recoverable and the workup can stop at clinical diagnosis, avoiding extensive testing for stroke, motor neuron disease, or radiculopathy. Steroids during the painful phase may shorten the painful phase but do not seem to affect long-term recovery. Patience and physical therapy are the mainstays.

Pitfalls and Pearls

• Plexopathy involves multiple nerves from one plexus region — broader than mononeuropathy, broader than radiculopathy.
• Paraspinal EMG is normal in plexopathy, abnormal in radiculopathy. This is the most useful EMG distinction.
• Pancoast tumor presents with lower trunk brachial plexopathy + Horner syndrome + shoulder/arm pain. Get a chest CT.
• Radiation plexopathy is typically painless, slowly progressive, and affects upper trunk preferentially. Distinguishes from tumor recurrence (painful, faster, often lower trunk).
• Parsonage-Turner = severe pain followed by patchy upper plexus weakness. Recovery is slow but usually substantial.
• Diabetic amyotrophy (lumbosacral radiculoplexus neuropathy) is a painful, subacute, proximal leg weakness in older diabetics, often with weight loss.
• Postpartum foot drop from lumbosacral trunk compression at the sacral ala. Improves over months.
• Erb palsy = upper trunk = waiter’s tip posture. Klumpke palsy = lower trunk = claw hand + Horner.
• Pan-plexus avulsion produces a flail arm with Horner; pseudomeningocele on MRI distinguishes avulsion from rupture and changes surgical planning.
• Phrenic nerve injury presents with orthopnea, elevated hemidiaphragm, paradoxical breathing.
• Lumbar plexopathy from retroperitoneal hematoma in anticoagulated patients — psoas hemorrhage. Reverse anticoagulation, image with CT.
• True neurogenic thoracic outlet syndrome is rare — affects lower trunk, often with cervical rib. Most “thoracic outlet syndrome” diagnoses are actually vascular or non-organic.

References

1. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.
2. Wilbourn AJ. Plexopathies. Neurol Clin. 2007;25(1):139-171.
3. van Alfen N. Clinical and pathophysiological concepts of neuralgic amyotrophy. Nat Rev Neurol. 2011;7(6):315-322.
4. Dyck PJ, Norell JE, Dyck PJ. Microvasculitis and ischemia in diabetic lumbosacral radiculoplexus neuropathy. Neurology. 1999;53(9):2113-2121.
5. Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 11th ed. McGraw-Hill; 2019.