Clinical History: 35 year-old male weight-lifter with shoulder pain. T1-weighted coronal (1a) and T2-weighted fat-suppressed coronal (1b) and sagittal (1c) images are provided. What is your diagnosis?
Entrapment of the suprascapular nerve caused by a ganglion cyst in the supraspinatus fossa and spinoglenoid notch, resulting in denervation of the infraspinatus muscle.
MRI is an indispensable tool for the evaluation of rotator cuff and labral pathology at the shoulder. It is equally effective in diagnosing other surgically correctable causes of shoulder pain, which may clinically mimic a rotator cuff tear. Peripheral nerve injuries at the shoulder result in characteristic muscle denervation changes with a predictable distribution of muscle involvement. Familiarity with the neurovascular anatomy is essential in localizing the site of nerve entrapment.
Denervation changes of muscle can have several MRI appearances. In the subacute phase the affected muscle demonstrates increased T2-signal resulting from muscle edema. In the chronic stages, fatty infiltration usually accompanied by muscle atrophy represents irreversible muscle injury.
Entrapment of the suprascapular nerve may present with supraspinatus and infraspinatus weakness and pain which may be difficult to clinically differentiate from a rotator cuff tear. Suprascapular nerve entrapment most commonly occurs in the confined space of the suprascapular notch or spinoglenoid notch. At the level of the suprascapular notch, the suprascapular nerve contains motor and sensory branches to both the supraspinatus and infraspinatus muscles. Entrapment at this level may result in denervation changes of both muscles. If entrapment occurs more distally along the course of the suprascapular nerve, innervation of the infraspinatus muscle only is affected. The most commonly encountered cause of this condition is a ganglion, usually arising from a posterior labral tear. Other causes include trauma to the scapula, varicosities and tumors.
Quadrilateral space syndrome results from axillary nerve compression, most commonly caused by fibrous bands within the quadrilateral space (Figure 3). These fibrous bands are thought to be the result of scarring from previous trauma and are usually undetected by MR. However, muscle changes of denervation limited to the teres minor muscle point to the diagnosis of quadrilateral space syndrome (Figures 4). Ganglia or other space occupying lesions can also result in compression of the axillary nerve within the quadrilateral space (Figure 5).
Recognizing the MRI appearance of muscle denervation and the patterns of muscle involvement is important in correctly diagnosing nerve entrapment syndromes at the shoulder. Surgically correctable lesions which may result in nerve entrapment at the shoulder are readily diagnosed by MRI.
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