Purpose: To investigate the prevalence of fatty infiltration in normal orbits using magnetic resonance imaging (MRI) and its impact on muscle diameters.
Methods: A retrospective analysis was conducted on normal orbits in patients who underwent coronal T1-weighted MRI. Fatty infiltration was defined by a hyperintense signal within the extraocular muscle. Fatty infiltration area was calculated by dividing the cross-sectional fat area by the sum of the muscle and fat area in the coronal plane. Muscle diameters were measured perpendicular to the muscle belly at their maximum and the mean diameters of the infiltrated muscles were compared with the non-infiltrated muscles.
Results: Seventy-six orbits from 76 participants with a mean age of 55.7 ± 18 years were included. Fatty infiltration in at least one muscle was seen in 57 (75%) of participants. When infiltration was present, inferior rectus was most commonly involved (98%). The area of fatty infiltration ranged from 6% to 42%, with 5 participants in the <10% group, 36 in the 10-20% group, 20 in the 20-30% group, 9 in the 30-40% group and 2 in the 40-50% group. Participants with fatty infiltration were significantly older (mean 61 ± 15 years vs 40 ± 16 years, p < .01) and had a higher proportion of males (56% vs 16%, p < .01). No significant difference in muscle diameters were found between infiltrated and healthy muscles.
Conclusion: Fatty infiltration of the extraocular muscles is a reasonably common finding in normal orbits, most commonly involves the inferior rectus muscle and is associated with increasing age.
Keywords: Orbit; extraocular muscle; fat.