• Scoliosis or postural curvature, which would explain a lower shoulder as a spinal problem.
• A mass or volume change in the lung, neck or mediastinum (apical collapse or fibrosis, goitre, a large Pancoast tumour), which would pull or push the trachea.

1. Neuromuscular (most common)

• This is the classic cause. Examination shows a drooping shoulder, trapezius wasting, and inability to shrug against resistance. Abduction above 90° is weak because scapular rotation is impaired, and the scapula flares laterally (S Das, Manual on Clinical Surgery, Accessory Nerve).
• Common causes:
  • Neck dissection, especially radical dissection. Bailey and Love lists weakness and drooping of the shoulder from trapezius paralysis as the main disability after it.
  • Posterior triangle lymph node biopsy.
  • Carotid endarterectomy (Mulholland and Greenfield, Table 87.3, incidence under 1%).
  • Penetrating or blunt trauma, radiotherapy fibrosis, tumours near the jugular foramen, and idiopathic palsy.
• Cummings Otolaryngology describes "eleventh nerve syndrome" as shoulder drooping, aberrant scapular rotation, inability to fully abduct, and a dull ache. The ache comes from trapezius atrophy and adhesive capsulitis.

• Traction injury, birth injury, a Pancoast or apical tumour, radiation plexopathy, and neuralgic amyotrophy (Parsonage-Turner syndrome).
• Look for associated arm weakness and sensory loss.

• Axillary nerve palsy or deltoid paralysis, or a hemiplegic or flail arm. These cause inferior subluxation of the humeral head, so the shoulder looks lowered.
• Weakness of levator scapulae and rhomboids.
• Facioscapulohumeral dystrophy, which is often asymmetric.
• Absent or atrophic pectoralis or chest wall muscles (Poland syndrome, post-mastectomy).

2. Bony and joint causes

• Clavicle fracture or non-union. The distal fragment drops and the clavicle shortens, so the shoulder droops and moves forward.
• Acromioclavicular dislocation. There is a step deformity, with the shoulder dropping relative to the clavicle.
• Shoulder dislocation. In subcoracoid dislocation there is drooping of the shoulder with apparent lengthening of the arm. There is also flattening of the deltoid contour and prominence of the acromion (S Das, Examination of Injuries about the Shoulder).
• Sternoclavicular injury.
• Congenital or developmental causes. Clavicular pseudarthrosis, cleidocranial dysplasia, and humeral or scapular hypoplasia.

3. Postural and other causes

• Habitual postural asymmetry. The dominant side normally sits slightly lower. Heavy carrying on one side or a sagging posture can exaggerate this.
• Costoclavicular or thoracic outlet syndrome. The Thieme Atlas notes that drooping shoulders predispose to costoclavicular compression of the neurovascular bundle. So the droop can be a cause as well as a sign.
• Chronic rotator cuff disease or frozen shoulder. These can lead to muscle wasting and guarding.

Bedside approach

1. Ask the patient to shrug and abduct the arm. Weakness points to CN XI or the brachial plexus.
2. Check for trapezius wasting and a lateral scapular position. If the scapula is winging medially instead, suspect long thoracic nerve (serratus anterior) injury.
3. Take a history of neck surgery, biopsy, trauma, radiotherapy, or a recent viral illness.
4. Palpate the clavicle, AC joint, and humeral head.
5. Look at both sides from behind and from above for fixed deformity, and check for a cervical rib.
6. Investigate as needed:
   • X-ray of the shoulder and clavicle, plus a chest X-ray (apex) and cervical spine film.
   • EMG and nerve conduction studies, or MRI or ultrasound of the nerve or plexus.