Inflammation of the temporalis muscle and adjacent nerve tissue in giant cell arteritis: expanding the spectrum of inflammatory lesions.

Objectives: To investigate the histopathological features of the temporalis muscle (TM) and adjacent nerve tissue in active cranial giant cell arteritis (C-GCA).

Methods: Temporal artery biopsy (TAB) specimens containing fragments of the TM from patients with active C-GCA fulfilling the 2022 ACR/EULAR classification criteria (n = 11) were assessed by conventional histology and immunohistochemistry in comparison with non-GCA controls (n = 3). Clinical, laboratory and imaging features based on patient charts at time of biopsy were retrospectively recorded.

Results: The majority of the studied TAB specimens showed inflammation of the TM (10/11) and adjacent nerve fascicles (7/11) that was characterized by prominent endomysial lymphomonocytic infiltrates, whereas controls showed no inflammatory lesions and no disruption of the local architecture. Association of active C-GCA with sarcolemmal MHC class I (8/8) and MHC class II (6/11) upregulation suggests primary inflammation of the TM in a subset of patients. αB-Crystallin positivity (10/11) highlights areas of pre-necrotic myofibres within the TM. The presence of endomysial fibrosis, signs of atrophy and variations of muscle fibre size suggest a rather longstanding and potentially subclinical process of myoinflammation.

Conclusion: Our results expand the spectrum of inflammatory lesions known to be associated with active C-GCA. Specifically, inflammatory infiltration of the TM and adjacent nerve structures could contribute to localized symptoms of the temporomandibular region and may be included in future concepts of pathophysiology.