Discussion

Posterior dislocation of the shoulder, whether unilateral or bilateral, is nearly always pathognomonic of a seizure in an atraumatic setting.4 During posterior shoulder dislocation, the humeral head impacts on the posterior aspect of the glenoid and due to rotational movements, grinding can occur resulting in cortication and a secondary cartilaginous injury. Therefore, timely diagnosis and definitive treatment are imperative to avoid resulting morbidity.

To the best of our knowledge, the exact sequence of events outlined in this case report of an indeterminate intracranial mass lesion causing a witnessed tonic–clonic seizure, leading to unilateral posterior dislocation of the shoulder, has not been described previously in the literature. In 1990, Elliott described a case of a patient who woke up with pain in the right shoulder and was subsequently found to have a two-part posterior fracture dislocation of the right shoulder.5 In recognition of the fact that in the absence of trauma, these injuries usually occur secondary to a seizure, the patient underwent a CT head scan which revealed a space occupying lesion in the left frontal lobe. Later excisional biopsy confirmed a diagnosis of benign macrocystic meningioma.

Kilicoglu et al reported a patient presenting with headache and bilateral shoulder pain, who was found to have bilateral posterior shoulder dislocation with impaction fractures of both humeral heads.6 Again, although this patient had no history of seizures, he underwent cranial MRI imaging which revealed a mass lesion in the right temporal lobe involving the uncus. Intralesional excision of the tumour and subsequent histological analysis confirmed Grade II atsrocytoma.

These cases highlight the need to consider a seizure-related aetiology for posterior shoulder dislocation in the absence of trauma. Diagnostic delay of posterior shoulder dislocation is common and can be due to late presentation, non-compliance with clinical examination and imaging, and inadequate or suboptimal radiological investigation.7

Acute posterior dislocation of the shoulder is characteristically painful. However, ability to illicit pain on palpation of the affected shoulder joint may be reduced in the postictal state or in the presence of acute intoxication. The affected shoulder is likely to be held in internal rotation.7 There will be a mechanical block to external rotation caused by engagement of the anterior humeral head defect on the posterior aspect of the glenoid. Rotator cuff function and neurovascular status should also be assessed to exclude any concurrent rotator cuff or neurovascular injury.

In the present case, despite anteroposterior (AP) and lateral radiographs of the shoulder, posterior dislocation of the left shoulder was only identified serendipitously from the staging CT scan. This illustrates the necessity for specific axillary views (apical oblique, Velpeau or modified axial) especially when the local protocol consists of only AP and lateral views. These radiographs are often suboptimal as the patient may be unable to abduct the arm. Obtaining an axillary view has the added practical benefit that it can be obtained with the arm in a sling, without the need for abduction. Additionally, classically described radiographic signs of posterior dislocation on AP radiographs are unreliable.7 Notably, a centre that adopted the protocol of obtaining an AP and axillary view at the initial assessment, reported a delayed diagnosis in only 9% of patients who presented acutely.2 Additional views, including the Bernageau (glenoid profile) view, are effective in demonstrating the degree of bone loss of the glenoid or humeral head.8 Indeed, obtaining these views bilaterally can be useful to highlight potential underlying abnormalities such as posterior dislocation. CT is also useful to confirm the diagnosis and is essential prior to any attempt at closed reduction.3 Non-contrast MRI is effective in diagnosing soft tissue pathologies including labral, rotator cuff and glenohumeral ligament injuries.9 MR arthrography is a reliable alternative but its use is limited by its invasive nature and relative lack of availability compared with CT.9 10 Historically, non-contrast CT has been used to assess bony Bankart and Hill-Sachs lesions, but more recently, MRI has been shown to be equivalent to CT to assess bone loss of the glenoid and humeral head.10 CT is unable to assess labroligamentous or rotator cuff injuries which has implications if MRI is not performed preoperatively. Understanding the severity and extent of soft-tissue injury may influence the surgeon’s decision-making process on optimal surgical management.

There is no evidence-based consensus on the optimal surgical management for such injuries.7 Despite this, disimpaction and bone grafting for small defects (<25% of thehumeral head surface) within 2 weeks of the initial injury is advised. Small defects presenting 2 weeks after initial injury can be treated with subscapularis transfer. Larger defects (40%–50% of the humeral head surface) should be filled with a structural allograft regardless of the time from injury to provide stability. Shoulder stability needs to be closely monitored postoperatively and residual instability may be due to capsulolabral disruption which may necessitate further repair.