Pain at the back of the shoulder has many causes, including rotator cuff tendonitis, shoulder arthritis, and referred pain from the neck. A rare cause is quadrilateral space syndrome. So what is it, how do we diagnose it, and what do we do about it?
What is the quadrilateral space?
The quadrilateral space is a small gap at the back and below the shoulder joint, between two muscles — teres minor and teres major. It contains the axillary nerve and an artery, which supply the shoulder joint and muscles. Pressure in this space can trap the nerve, leading to axillary nerve entrapment symptoms.
What causes quadrilateral space syndrome?
Causes of trapping of the axillary nerve include:
- Fibrous bands or muscle swelling within the space
- Paralabral cysts arising from the lower part of the shoulder labrum
- A growth, such as a tumour
- Repetitive overhead or throwing sport — we think repeatedly moving the shoulder into the “cocking” position narrows the space
Symptoms of quadrilateral space syndrome
This syndrome typically produces pain at the back of the shoulder, often when moving the arm into the cocking position during a throw. Sometimes the muscles at the back of the shoulder waste away (atrophy), and some people report pins and needles or numbness in the upper arm. These symptoms are related to axillary nerve entrapment.
On examination, there’s often point tenderness in the quadrilateral space, with weakness and wasting of the deltoid and the muscles behind the shoulder – supplied by the axillary nerve.
Other causes of shoulder muscle wasting
Not all shoulder muscle wasting is quadrilateral space syndrome. Other causes include brachial neuritis (Parsonage-Turner syndrome), suprascapular nerve compression at the back of the shoulder, thoracic outlet syndrome, and nerve compression from the neck.
How is quadrilateral space syndrome diagnosed?
An MRI of the shoulder is usually the most helpful investigation. It often shows swelling and wasting of the teres minor and deltoid muscles, and sometimes reveals a paralabral cyst compressing the space. In complex cases where the diagnosis is uncertain, a diagnostic local anaesthetic block can help confirm it.
How is quadrilateral space syndrome treated?
Treatment depends on the cause.
In mild cases with a normal MRI, physiotherapy improves pain and function over a few months. Sometimes an ultrasound-guided cortisone injection can be directed next to the axillary nerve, and a paralabral cyst can be aspirated to reduce pressure on the nerve.
For more severe symptoms, or where there’s clear compression such as a paralabral cyst, surgical decompression and labral repair are needed.
Frequently asked questions about quadrilateral space syndrome
What does quadrilateral space syndrome feel like?
Usually a deep ache at the back of the shoulder, worse when the arm is raised and rotated into the throwing (cocking) position, sometimes with pins and needles or numbness in the upper arm. Over time, the muscles at the back of the shoulder can visibly waste. These clinical signs and symptoms are related to axillary nerve entrapment in the quadrilateral space.
Who gets quadrilateral space syndrome?
It’s most common in overhead and throwing athletes — the repeated cocking position is thought to narrow the space and irritate the axillary nerve. It can also arise from a paralabral cyst, fibrous bands, or, rarely, a growth.
Is quadrilateral space syndrome serious?
It’s usually not dangerous, but it can be persistent and cause muscle wasting if left untreated. Because several other nerve conditions look similar — and a few point to conditions that require specific treatment (such as a cyst or, rarely, a tumour) — an accurate diagnosis with MRI matters.
Can it be treated without surgery?
Yes, in mild cases with a normal MRI — physiotherapy over a few months, sometimes with a guided injection. Surgery is reserved for severe cases or for cases with a clear structural cause, such as a paralabral cyst compressing the nerve.
Final word from Sport Doctor London about quadrilateral space syndrome
Quadrilateral space syndrome is a rare but recognised cause of back-of-shoulder pain, particularly in throwing athletes, due to entrapment of the axillary nerve. Most mild cases improve with physiotherapy, whereas a clear structural cause, such as a cyst, may require surgery. Getting the diagnosis right — and excluding the look-alikes — is the key.
If you have persistent back-of-shoulder pain, Dr Masci can assess you in London, including ultrasound in clinic. Contact the team here or call +44 (0) 203 488 0350.
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