The clinical presentation is consistent with lateral scapular winging due to iatragenic injury to the spinal accesory nerve.
Scapular winging is a rare debilitating condition that leads to limited functional activity of the upper extremity. Causes include traumatic, iatrogenic, and idiopathic processes that most often result in nerve injury and paralysis of either the serratus anterior, trapezius, or rhomboid muscles. Serratus anterior paralysis, such as from the long thoracic nerve, results in medial winging of the scapula. This is in contrast to the lateral winging generated by trapezius and rhomboid paralysis. Most cases of serratus anterior paralysis spontaneously resolve within 24 months, while conservative treatment of trapezius paralysis is less effective.
The review by Kuhn et al. classifies scapular winging as primary, secondary, or voluntary. Primary scapular winging may be due to neurologic injury, pathologic changes in the bone, or periscapular soft-tissue abnormalities. Secondary scapular winging occurs as a result of glenohumeral and subacromial conditions and resolves after the primary pathologic condition has been addressed.
Romero et al described the Eden-Lange procedure with lateral transfer of the levator scapulae and rhomboid muscles which can be helpful for lateral winging. They report satisfactory long-term results for the treatment of isolated paralysis of trapezius, but in the presence of an additional serratus anterior palsy or weak rhomboid muscles, the procedure is less successful in restoring shoulder function.
Levy et al describe a clinical forward elevation lag sign for trapezius palsy with resulting "Triangle sign" in the prone position which differentiates this from serratus winging.
Levy O, Relwani JG, Mullett H, Haddo O, Even T. The active elevation lag sign and the triangle sign: new clinical signs of trapezius palsy. J Shoulder Elbow Surg. 2009 Jul-Aug;18(4):573-6.
PMID:19423363 (Link to Abstract)
Romero J, Gerber C: Levator scapulae and rhomboid transfer for paralysis of trapezius: The Eden-Lange procedure. J Bone Joint Surg Br 2003;85:1141-1145.
PMID:14653596 (Link to Abstract)
Kuhn JE, Plancher KD, Hawkins RJ: Scapular winging. J Am Acad Orthop Surg 1995;3:319-325.
PMID:10790670 (Link to Abstract)