Making the Diagnosis

To accurately assess and diagnose this patient’s condition, the clinician needs to eliminate other possible causes of shoulder pain from a nontraumatic injury such as rotator cuff tendonitis, impingement syndrome, shoulder dislocation, pathologic fracture, and acromioclavicular (AC) joint separation (also known as AC separation), among others. 

The patient’s radiography images are diagnostic. The first image shows a “light-bulb” sign indicating a posterior shoulder dislocation (Figure 1).  The light bulb appearance of the humeral head is due to internal rotation that is specific for a posterior shoulder dislocation. This is confirmed with a scapular Y view showing that the humeral head is posterior to the glenoid fossa, which lies at the confluence of the 3 arms of the scapula in a Y shape (Figure 2). Another option would be to take a radiograph of the axillary view; this typically provides a better view of the injury but can be more painful for the patient.  

Discussion

Posterior shoulder dislocations account for less than 5% of all shoulder dislocations. Like other dislocations, posterior shoulder dislocation present with pain and limited range of motion. The most common mechanisms of posterior shoulder dislocation involve a direct anterior blow, electrocution, or seizure. Some apparent dislocations are actually self-induced subluxations in patients with Munchausen syndrome. The physical examination tends to be remarkable for decreased range of motion and guarding in adduction, which is in contrast to anterior dislocation where guarding tends to occur in slight abduction.             


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Radiographic images for a posterior shoulder dislocation may have more subtle findings than an anterior dislocation, especially on anterior views in which all that may be seen is a light bulb sign. It is critical to order either an axillary or scapular Y view of the shoulder if posterior dislocation is suspected.

Reduction of the shoulder is by longitudinal traction aided by internal rotation in adduction. If the shoulder seems reduced then goes back out when the patient is awake, consider Munchausen syndrome and repeat radiography when the patient is asleep. If the shoulder injury goes in on its own without reduction during sedation then it is highly likely you are dealing with a case of Munchausen syndrome. Regardless, the patient should follow up with an orthopedist.

Brady Pregerson, MD, is an emergency physician at Tri-City Medical Center, Palomar Health System, and Scripps Coastal Urgent Care, all in San Diego, California.

References

Pregerson DB. Orthopedics chapter. Emergency Medicine 1-Minute Consult. 5th ed. 2017;5. http://www.erpocketbooks.com/emergency_medicine_reference_books/quick-essentials-emergency-medicine/

Murphy B, Carroll P, McColgan R, Molloy A, O’Shea K. Alleged recurrent traumatic shoulder dislocation in a young male patient – diagnostic and management considerations. Trauma Case Rep. 2021;37:100599. doi:10.1016/j.tcr.2021.100599