Use of Ocular Response Analyzer Output Data in the Management of Low Intraocular Pressure after Glaucoma Filtration Surgery

CASE DESCRIPTION

A 78-year-old lady with normal tension glaucoma underwent right trabeculectomy with mitomycin C. The postoperative course over the 1st month was routine, with IOP measuring 12–24 mm Hg. The temporal releasable suture was removed at postoperative week 3, following which the IOP measured 18 mm Hg. One week later, the nasal releasable suture was also removed, reducing the IOP to 2 mm Hg, albeit with no anterior chamber (AC) shallowing or clinical signs of hypotony. A 0.2 mL bolus of Healon^® was injected intracamerally, increasing the IOP to 13 mm Hg. After 2 days, the IOP was measured as 1 mm Hg and a further 0.25 mL of intracameral Healon® was injected, increasing the IOP to 36 mm Hg. The IOP was then gradually reduced, measuring 5 mm Hg after 1 week. Best-corrected visual acuity (BCVA) remained 6/9 with no clinical signs of hypotony (shallow AC, macular folds or choroidal detachment). And, 7 weeks postoperatively, the IOP was also measured with the ORA. The IOPcc was found to be higher, 8.7 mm Hg, with a CH of 9.3 mm Hg. The IOPcc of the contralateral eye at this appointment was 23.9 mm Hg, and the CH was found to be lower than the operated eye at 7.1 mm Hg. There were no clinical features of hypotony and the patient was therefore managed conservatively. At 5 months, the GAT-measured IOP was 4 mm Hg, whilst ORA-measured IOPcc again was found to be higher (7.4 mm Hg) with a CH of 7.7 mm Hg. At 9 months, GAT-measured IOP was 3 mm Hg, whilst ORA-measured IOPcc was 6.8 mm Hg with a CH of 7.2 mm Hg. There were no signs of clinical hypotony observed at any time point and BCVA remained stable throughout. Figure 1 shows images taken at twelve months postoperatively, when GAT-measured IOP was 2 mm Hg, whilst ORA-measured IOPcc was 7.7 mm Hg.

Using the IOP measurement obtained with GAT, numerically, this patient fulfills the criteria for hypotony (the World Glaucoma Association defines hypotony as pressure ≤5 mm Hg).² Clinically, this patient had no AC shallowing, maculopathy or choroidal detachment, and vision remained stable throughout. In addition to measuring IOP, the ORA also calculates corneal biomechanics, such as the CH, which are used in the calculation of the IOPcc.¹ CH is an indicator of the viscous properties of the cornea and has a normal range of 9.6–12.2 mm Hg.³ Our patient had an average postoperative CH of 8.0 mm Hg, compared to 7.6 mm Hg in the unoperated contralateral eye. A recent study demonstrated that the IOP-lowering effects of GFS cause a postoperative increase in CH.⁴ The average IOPcc on three postoperative visits, in this case, was 4.3 mm Hg higher than the GAT-measured IOPs and more in keeping with the clinical picture. The IOPcc values, in this case, strengthened the clinician’s decision not to intervene surgically unnecessarily.