Shoulder pain 2: remember the exceptions
What causes isolated teres minor atrophy?
In the last post we introduced a case of shoulder pain in a competitive swimmer. Here’s a recap of what we found when we scanned her shoulder:
The examination indicated: Subdeltoid/subacromial bursal fluid may indicate bursitis. Teres minor muscle atrophy. Supraspinatus and subscapularis tendons bilateral mild tendinopathy, likely within the normal range for a competitive swimmer.
We also posed ourselves two questions. This post will offer a brief discussion of the first of them, “what causes teres minor atrophy?”
Figure A: the left teres minor muscle, reduced size and increased moderate increased echogenicity.
Figure B: the right teres minor muscle comparison, no evidence of change.
We think you will probably agree that both the bursal thickening and tendon changes should not be excluded as pain drivers and must be considered in management planning. However, given the bilateral nature of these findings, the probability of their being the source of pain decreases.
What causes teres minor atrophy?
In answering this question, it is easier to exclude a possible cause before attempting to include them. Rotator cuff tears1 or ruptures are known to cause muscle atrophy, and a tear wouldn’t be unlikely given the competitive level of swimming the patient participated in. However, recall that the findings did not indicate a tear, let alone tendon rupture. Teres minor denervation, occurring in up to 5% of patients presenting for routine imaging of the shoulder2, is an attractive differential diagnosis.
Quadrilateral space syndrome, in which the humeral artery and axillary nerve are thought to be compressed within the quadrilateral space3, is thought capable of causing teres minor denervation2, appears a reasonable hypothesis. The quadrilateral space is bounded by the teres minor at the superior aspect, the teres major inferiorly, the humerus laterally and the long head of the triceps medially4. The contents are the posterior circumflex humeral artery and axillary nerve, which pass through the space, entering the posterior compartment of the upper arm4.
Quadrilateral space syndrome has a clinical presentation in keeping with that described in this case, and is typically described as being caused by fibrous bands within the space, or less commonly by a space occupying lesion3.
Closer examination of the literature suggests that in fact, fibrous bands are unlikely to be present, as their existence is questionable; they appear to have been discovered after poor dissection of the area, where improper technique cased the appearance of the bands5. Rather, the angulated path of the primary motor nerve to teres minor, along with a stout fascial sling, may cause compression5. Alternatively, traction of the nerve or poor humeral control appears more likely to cause nerve injury2, suggesting that quadrilateral space syndrome is not a good candidate to explain teres minor atrophy.
Is this an answer to the question of what causes isolated teres minor atrophy? Perhaps it is part of an answer, as we have a possible mechanism, and are getting closer to understanding what is less likely to be the cause.
Do we know what caused isolated teres minor atrophy for this particular patient? Again, at best we have part of an answer, but we’d be grateful for your opinion; let us know what you think.
References:
Chung SW, Kim SH, Tae S-K, Yoon JP, Choi J-A, Oh JH. Is the supraspinatus muscle atrophy truly irreversible after surgical repair ofrotator cuff tears? Clin Orthop Surg. 2013 Mar;5(1):55–65.
Wilson L, Sundaram M, Piraino DW, Ilaslan H, Recht MP. Isolated teres minor atrophy: manifestation of quadrilateral space syndrome ortraction injury to the axillary nerve? Orthopedics. 2006 May;29(5):447–50.
Flynn LS, Wright TW, King JJ. Quadrilateral space syndrome: a review. J shoulder Elb Surg. 2018 May;27(5):950–6.
Hong CC, Thambiah MD, Manohara R. Quadrilateral space syndrome: The forgotten differential. J Orthop Surg (Hong Kong). 2019;27(2):2309499019847145.
Chafik D, Galatz LM, Keener JD, Kim HM, Yamaguchi K. Teres minor muscle and related anatomy. J shoulder Elb Surg. 2013 Jan;22(1):108–14.