Orbital Fat Volume After Treatment with Topical Prostaglandin Agonists

Abstract

Purpose: Topical prostaglandin analogs (PGAs) are common treatment for primary open-angle glaucoma (POAG) but reportedly may cause adnexal fat atrophy. We asked if patients with POAG treated with PGAs have abnormalities in orbital fat volume (OFV).

Methods: We studied 23 subjects with POAG who had never experienced intraocular pressure (IOP) exceeding 21 mm Hg and were treated long term with PGAs, in comparison with 21 age-matched controls. Orbital volume, non-fat orbital tissue volume, and OFV were measured using high-resolution magnetic resonance imaging.

Results: Subjects with POAG had been treated with PGAs for 39 ± 19 months (SD) and were all treated within the 4 months preceding study. In the region from trochlea to orbital apex, OFV in POAG was significantly less at 9.8 ± 1.9 mL than in the control subjects at 11.1 ± 1.3 mL (P = 0.019). However, between the globe-optic nerve junction (GONJ) and trochlea, OFV was similar in both groups. Width and cross sectional area of the bony orbit were significantly smaller in POAG than in controls (P < 0.0001). Posterior to the GONJ, the average orbital cross-sectional area was 68.2 mm2 smaller, and the orbital width averaged 1.5 mm smaller throughout the orbit, in patients with POAG than in controls.

Conclusions: Patients with POAG who have been treated with PGAs have lower overall OFV than controls, but OFV in the anterior orbit is similar in both groups. Lower overall OFV in POAG may be a primary association of this disorder with a horizontally narrower bony orbit, which may be a risk factor for POAG at nonelevated IOPs.

Figure 1.

Quasicoronal MRI in the mid-portion of a representative right orbit, whose bony walls are outlined in green. Intraorbital structures including extraocular muscles, ON, and blood vessels, are outlined in white and shaded blue. Orbital fat represents the remaining orbital area excluding structures shaded in blue.

Figure 2.

Axial MRI of representative orbit in pseudophakic subject, with contiguous 2 mm thick image planes arranged from inferior at mid-globe at left, to superior at right with the plane including the trochlea. GONJ, globe-optic nerve junction; IOL, intraocular lens.

Figure 3.

Horizontal width of each quasicoronal image was sampled 15 times (red lines) at uniformly-spaced vertical intervals spanning the middle 60% of the vertical extend of the orbit (white arrow). These measurements were averaged to compute orbital width in each image plane.

Figure 4.

Bland-Altman plot of agreement between two independent observers measuring OV in all subjects. Abscissa plots the averages of each pair of measurements, and ordinate the differences within these pairs.

Figure 5.

Orbital fat volume (OFV) (A) OFV in controls and patients with POAG without abnormally elevated IOP who were treated with topical PGAs. While total OFV from apex to trochlea in POAG was significantly less in patients than controls (left paired columns), the difference was driven by the posterior orbit from apex to the GONJ (middle paired columns). In the anterior orbit from the GONJ to the trochlea (right paired columns), OFV was similar in the two groups. (B) Total orbital volume (OV) of patients with POAG was significantly less than in controls. SD, standard deviation; N.S., not significant.

Figure 6.

Anteroposterior dimensions of the orbit and position of the GONJ, did not differ between patients with POAG and the controls. N.S., not significant.

Figure 7.

Quasicoronal MRI just posterior to the GONJ illustrating that the orbit of a typical patient with POAG is horizontally narrower that than of a representative control.

Figure 8.

Mean cross-sectional area of the orbit along its anteroposterior extent in patients with POAG and controls. Except at the extreme ends of the orbit, cross-sectional area was significantly less in POAG than in controls. Differences between groups were significant at P < 0.0001 by ANOVA. SD, standard deviation.

Figure 9.

Mean orbital dimensions along the anteroposterior extent of the orbit in patients with POAG and controls. (A) Width. (B) Height. Differences between groups were significant at P < 0.0001 by ANOVA. SD, standard deviation.