Occipital and external acoustic meatus to axis angle: a useful predictor of oropharyngeal space in rheumatoid arthritis patients with atlantoaxial subluxation

Objective: Dyspnea and/or dysphagia is a life-threatening complication after occipitocervical fusion. The occiput-C2 angle (O-C2a) is useful for preventing dyspnea and/or dysphagia because O-C2a affects the oropharyngeal space. However, O-C2a is unreliable in atlantoaxial subluxation (AAS) because it does not reflect the translational motion of the cranium to C2, another factor affecting oropharyngeal area in patients with rheumatoid arthritis (RA) who have reducible AAS. The authors previously proposed the occipital and external acoustic meatus to axis angle (O-EAa; i.e., the angle made by McGregor's line and a line joining the external auditory canal and the middle point of the endplate of the axis [EA line]) as a novel, useful, and powerful predictor of the anterior-posterior narrowest oropharyngeal airway space (nPAS) distance in healthy subjects. The aim of the present study was to elucidate the validity of O-EAa as an indicator of oropharyngeal airway space in RA patients with AAS.

Methods: The authors investigated 64 patients with RA. The authors collected lateral cervical radiographs at neutral position, flexion, extension, protrusion, and retraction and measured the O-C2a, C2-C6, O-EAa, anterior atlantodental interval (AADI), and nPAS. Patients were classified into 2 groups according to the presence of AAS and its mobility: group N, patients without AAS; and group R, patients with reducible AAS during dynamic cervical movement.

Results: Group N had a significantly lower AADI and O-EAa than group R in all but the extension position. The O-EAa was a better predictor for nPAS than O-C2a according to the mixed-effects models in both groups (marginal R2: 0.510 and 0.575 for the O-C2a and O-EAa models in group N, and 0.250 and 0.390 for the same models, respectively, in group R).

Conclusions: O-EAa was superior to O-C2a in predicting nPAS, especially in the case of AAS, because it affects both O-C2a and cranial translational motion. O-EAa would be a useful parameter for surgeons performing occipitocervical fusion in patients with AAS.