The quadrilateral space is located behind (posteriorly) and below (inferiorly) the ball-and-socket joint (glenohumeral joint) of the shoulder. Its boundary is formed by the teres minor muscle above, teres major below, long head of the triceps towards the midline (medially), and the shaft of the upper arm (humerus) towards the outside (laterally). The quadrilateral space contains the axillary nerve that supplies the teres minor and deltoid muscles, and the posterior humeral circumflex artery (PHCA).
Quadrilateral space syndrome (QSS) is a rare condition where the axillary nerve of PHCA are compressed within the quadrilateral space. This compression is most commonly due to the development of fibrous bands or increased muscle size and bulk (hypertrophy) of the muscles that form the boundaries of the quadrilateral space.
QSS can also develop as a result of trauma to the shoulder or from the development of cysts at the glenoid labrum. Compression of the axillary nerve can also result from an aneurysm of the PHCA where the arterial wall becomes distended and compresses the nerve. Most aneurysms do not show symptoms and are not dangerous. However, a rupture of an aneurysm is more serious and would require urgent medical attention.
Generally, QSS can present in young athletes, between twenty five and thirty five, who participate in overhead sports such as baseball, basketball and swimming.
As the QSS contains both nerves and arteries, the symptoms can be described as neurogenic (nerve related) or vascular (artery related).
Neurogenic QSS can present with a non-specific and poorly localised pain around the shoulder. There may also be changes in sensation or pins and needles (paraesthesia) that do not follow any specific anatomical pattern that would be associated with spinal nerves. Weakness and muscle wasting (atrophy) of the teres minor and deltoid muscles would be expected with neurogenic compression.
Weakness would be most obvious with the arm and shoulder in a typical throwing position of external rotation and abduction. Active and resisted testing of the muscles in this position could lead to an exacerbation of symptoms.
Vascular QSS can present with dull pain, colour changes at the shoulder and hand, and a lack of pulses, which would indicate ischaemia as the blood and oxygen supply to the tissues around the shoulder is compromised.
Palpation of the quadrilateral space will often be tender when QSS is present.
QSS is a rare condition. As such, diagnosis can be difficult and there are many other differential diagnoses that may be considered prior to considering QSS. This may result in a delay in providing the definite diagnosis and starting treatment.
An initial history taking and physical assessment by a suitable GP, physiotherapist, or consultant may help to diagnose QSS. However, if QSS is suspected then further investigations may be required to confirm this diagnosis.
MRI scans and MR arthrograms can be useful in visualising the shoulder better and to identify any labral tears, quantify muscular atrophy of the teres minor and deltoid, and identify any cysts or masses that may compress the axillary nerve or PHCA.
Angiograms and Doppler ultrasounds can be used to investigate potential vascular causes of QSS as they assess for blockage or narrowing of the PHCA.
Additionally, electromyography (EMG) or nerve conduction studies (NCS) can be performed to assess the function of the axillary nerve. These tests are designed to test the response of the teres minor and deltoid to the electrical stimulation of the axillary nerve.
Quadrilateral space syndrome can be seen to be “self-limiting” and a majority of individuals suffering with QSS will improve over a period of time through activity modification and a reduction in overhead activities, which allows for desensitisation of the axillary nerve.
In athletes, there may need to be complete rest from overhead activities for up to eight weeks in conjunction with a progressive rehabilitation program that is focused on throwing mechanics, shoulder strength and shoulder co-ordination. Analgesics or anti-inflammatories may be prescribed during this time to facilitate the rehabilitation process. In more severe presentations a consultant may even prescribe a nerve block injection. This can also be useful in confirming a diagnosis of QSS.
If symptoms of QSS persist for more than six months despite conservative management, or in cases with significant loss of function and weakness, surgical decompression may need to be considered. If the compression is caused by small cysts then keyhole surgery (arthroscopy) can be performed. However, if there are large masses or fibrous bands causing the compression then an open procedure would be recommended. Vascular QSS may require onward referral to a Vascular Surgeon for further management, but this is less common and not often indicated.