The role of low muscle mass in acute neck infections is underexplored. This study aimed to validate the reliability of MRI-based masseter muscle area (MMA) measurements, derive normative age-related data in a diverse patient cohort, and evaluate the association between MMA measurements and infection severity and clinical outcomes in patients with acute neck infections.

In this retrospective single-center study, 526 patients with clinically confirmed acute neck infections who underwent emergency neck MRI had bilateral MMA measured on T2-weighted Dixon images at the mandibular foramen level. MMA was normalized to height squared (MMA/h²). Interobserver reliability of MMA was assessed between two readers using intraclass correlation coefficients (ICC). Among adults, associations with infection severity markers and clinical outcomes were examined using univariable and multivariable tests.

The interobserver agreement for MMA was excellent (ICC = 0.991). MMA/h² declined significantly with age (r = −0.21, p < 0.01). In adults, MMA/h² correlated negatively with age, maximal abscess diameter, and length of hospital stay (LOS; p < 0.01 for all); patients with abscesses had lower MMA/h² than those without (p = 0.002). Multivariable analysis confirmed that MMA/h² independently predicted abscess diameter after adjustment for age and C-reactive protein (CRP) (p = 0.001), but not LOS (p = 0.136).

Opportunistic MRI-based MMA measurements are highly reliable and associated with age, LOS, and abscess presence and size in acute neck infections. Our findings suggest a link between low masseter muscle mass and disease severity in patients with acute neck infections.