Paralabral Cysts: a common, yet often forgotten mechanism of scapular muscle wasting
Paralabral cysts are synovial-filled ganglion cysts that form near an antecedent labral tear (which may or may not be seen on imaging). The cysts form as the synovial fluid from the glenohumeral joint extravagates through a small hole created by the labral tear. If a ball-valve configuration is present at the site of the tear, fluid collects outside the normal anatomic boundary of the glenohumeral space.
A relatively common incidental finding on shoulder MR examinations, problems can arise from these benign cysts if they expand causing entrapment neuropathy of the suprascapular nerve.
The suprascapular nerve is a branch that comes off the superior trunk of the brachial plexus (C5/6). It then travels posteriorly through the suprascapular notch and descends to the suprascapular fossa sending branches to the supply the supraspinatus muscle. It then continues laterally where it rounds the spinoglenoid notch on route to the infraspinous fossa where it innervates the infraspinatus muscle.
When synovial-filled ganglion cysts involve the spinoglenoid notch they can produce atrophy of the infraspinatus and/or the supraspinatus muscle, secondary to suprascapular nerve entrapment. Isolated infraspinatus atrophy is associated with more posteriorly located ganglion cysts and dorsal suprascapular nerve entrapment. Supraspinatus and infraspinatus muscle atrophy are seen in association with anteriorly located masses and proximal nerve entrapment.
Clinical findings may or may not include typical ‘labral tear’ symptoms. Nerve entrapment signs can range from mild to severe wasting in the suprascapular and/or infrascapular fossa. As noted above, it is most common to have wasting in the infraspinatus only as the cyst expands into the spinoglenoid notch. It can however also include the supraspinatus only if the cyst expands anteriorly into the suprascapular notch (less common).