Epileptic patients often experience a wide range of shoulder injuries, with a notable occurrence of rare conditions such as bilateral lesions and posterior dislocations. Increased glenoid retroversion has been observed in posterior shoulder instability, but whether these bony alterations are predictive or adaptive remains unclear. During tonic-clonic seizures, the shoulder is typically positioned in adduction, internal rotation, and flexion. This displaces the humeral head cranial and posterior on the glenoid fossa, driven by powerful contractions of the infraspinatus, teres minor, deltoid, latissimus dorsi, and teres major. These forces may lead to either acute posterior dislocation or plastic deformations, particularly relevant during skeletal development. This study aimed to investigate whether the frequency of tonic-clonic seizures influences glenoid version angles.

Patients with various forms of epilepsy, including those experiencing tonic-clonic episodes (generalized or bilateral), were recruited from a tertiary epilepsy center. The duration and frequency of their seizures were recorded. Glenoid version, glenoid vault angle and humeral posterior subluxation were measured on axial MRI reconstructions by two independent observers. The cumulative tonic-clonic seizure burden (TCSB) was calculated by multiplying epilepsy duration by the yearly frequency of tonic-clonic seizures. Patients were categorized into three groups: no TCSB, low TCSB (<40), and high TCSB (>40).

Data were collected for 91 patients. Of these, 29 had no TCSB, 34 had low TCSB, and 28 had high TCSB (age: 35 ± 11.6; 33.5 ± 10.6; 36.1 ± 12.3, p: n.s.; TCSB: 0; 10.9 ± 10.0; 207.4 ± 213.3, p < 0.0001). Compared to no- and low-TCSB groups, high-TCSB patients exhibited significantly greater glenoid retroversion (1.93 ± 4.01°; 3.78 ± 4.33°; 6.67 ± 4.73°; p < 0.0001; p = 0.0148), glenoid vault angle (13.37 ± 4.17°; 16.60 ± 4.32°; 21.07 ± 5.69°; p < 0.0001; p = 0.0138), and posterior subluxation index (51 ± 4%; 55 ± 5%; 56 ± 4%; p < 0.0001; p: n.s.). Intra-rater agreement was nearly perfect (0.807-0.864) for angular measurements and substantial (0.610-0.719) for subluxation. Inter-rater agreement ranged from substantial to nearly perfect (0.642-0.829) for angular measurements and moderate to substantial (0.492-0.656) for subluxation.

Patients with frequent tonic-clonic seizures show measurable changes in glenoid anatomy compared to those with fewer episodes. This may result from plastic deformation of the glenoid during growth, induced by repeated posteriorly directed muscular contractions during seizures.