Musculoskeletal System
Trapezius and spinal accessory nerve injury
The trapezius, which lifts the scapula in a shrug, is supplied by the spinal accessory nerve, so injury to that nerve weakens the shrug and drops the shoulder.
Remember
A weak shrug and a drooping shoulder after an operation at the side of the neck point to damage to the spinal accessory nerve (cranial nerve XI) and a weak trapezius, while skin sensation is preserved.
The trapezius is a broad, triangular muscle of the upper back. Its upper fibres elevate the scapula (lift the shoulder blade), and that movement is the shrug. It also rotates the scapula upwards, which lets the arm be abducted (lifted out to the side) all the way overhead.
Its motor supply (the nerve fibres that make it contract) comes from the spinal accessory nerve, which is cranial nerve XI. This nerve is purely motor, so it carries no skin sensation. An injury to it therefore weakens the shrug while sensation over the shoulder is preserved.
The nerve runs relatively close to the surface as it crosses the posterior triangle, the area on the side of the neck that has sternocleidomastoid (the other muscle this nerve supplies) in front, trapezius behind and the clavicle (collar bone) below. A lymph node biopsy or other operation there can injure it, and it is the nerve most often injured by surgery in that region.
Without its nerve, trapezius is weak. The scapula slips outwards and rotates downwards, so the shoulder droops and shrugging is impaired. Overhead abduction is limited because trapezius, which normally rotates the scapula upwards, is weak. The first part of abduction is spared, because it does not depend on the trapezius.
To test the nerve, have the patient shrug against resistance. Do not confuse this picture with a long thoracic nerve injury, which affects serratus anterior (the muscle that holds the scapula against the chest wall) and causes winging of the scapula (the medial border lifts away from the chest wall) rather than a drooping shoulder. A spinal accessory nerve injury can sometimes cause winging of the scapula too, but it also gives a weak shrug and a drooping shoulder.
Key facts
- Nerve
- the spinal accessory nerve (cranial nerve XI) gives trapezius its motor supply.
- Action
- the upper fibres of trapezius elevate the scapula, which produces the shrug.
- Site of injury
- the nerve is relatively superficial in the posterior triangle, so neck operations can injure it.
- Typical result
- a drooping shoulder, often pain, and limited overhead abduction (the first part of abduction is spared).
- Sensation
- skin sensation over the shoulder is preserved, because the nerve is purely motor.
- Bedside test
- ask the patient to shrug the shoulders against resistance.
Exam traps
- Levator scapulae also elevates the scapula, but it is a deep muscle supplied by the dorsal scapular nerve, so a spinal accessory nerve injury spares it and it does not account for the droop.
- Serratus anterior holds the scapula against the chest wall, and injury to its long thoracic nerve causes medial winging of the scapula rather than a drooping shoulder with a weak shrug.
- The spinal accessory nerve is the right nerve for a shrug, but it supplies trapezius, not latissimus dorsi (a large superficial back muscle), which is supplied by the thoracodorsal nerve.
- Supraspinatus performs the first 15 degrees of abduction and is supplied by the suprascapular nerve, so it is working when early abduction is preserved but overhead reach is poor.
Read the full notes
Tested in 10 SimpleMed+ questions
Practise it in Musculoskeletal System, with the answer and a full explanation after every question.
Practise these questions