INTERNATIONAL GLAUCOMA REVIEW
Volume 13-4 2012
From The Royal Australian and New Zealand College of Ophthalmologists (RANZCO)
The paradox of early detection strategies is that we frequently detect artifact or slowly-progressive disease that will never affect our patients. Ophthalmology should draw lessons from other fields of medicine and recognize that technologies like OCT are poorly-suited for widespread glaucoma screening and are likely to over-diagnose patients with optic nerve or RNFL changes that will never lead to visual impairment.
M & T: As said before, assess ALL aspects of the glaucoma patient to maximize sound decision-making.
Visual Field Testing and Retinal Cell Ganglion
In a normal human retina, the visual field within 9° of fixation is served by one-third of the retinal ganglion cell population, but tested by less than 1/10 of the 24-2 test locations. The 10-2 tests 68 locations within 9° of fixation.
After the Ocular Hypertension Treatment Study (OHTS) demonstrated that central corneal thickness (CCT) is an important predictive factor for the development of glaucoma, ophthalmologists embraced pachymetry as an important diagnostic tool in the management of patients with glaucoma and ocular hypertension. However, how CCT should actually be used in day-to-day management of individual patients remains unclear. Some have chosen to use CCT as just one component of global risk assessment, whereas others have chosen to use one of the many published 'correction' nomograms to adjust IOP estimates acquired by Goldmann applanation tonometry (GAT).
Park, et al. found that adjusting GAT estimates using CCT-based formulae actually resulted in worse agreement between GAT and Pascal Dynamic Contour Tonometer (DCT). This finding was especially true in eyes with increased CCT. The authors' data strongly suggests that clinicians should not used so-called correction nomograms in individual patients. The fact that the agreement between GAT and DCT is worst among eyes with thick corneas should make us particularly careful about considering an ocular hypertensive with thick central corneal thicknesses to be low risk without fully assessing the optic nerve and visual field.
M & T: True, but "fully assessing the optic nerve" is the single most important aspect of the glaucoma evaluation.
24-Hour IOP Profiles in Primary Open-Angle Glaucoma (POAG)
A paper by Wang et al., contributes unique information of 24-hour intraocular pressure (IOP) profiles in subjects with primary open-angle glaucoma (POAG) identified from a population-based study in Handan, China. This is the first such evaluation of a population-based sample of newly-diagnosed and untreated POAG subjects. Notably, more than 80% of Chinese subjects with untreated POAG in this study had a peak IOP of <21mmHg on 24-hour phasing. In unilateral cases, the IOP in the glaucomatous eye was similar to the IOP in the contralateral non-glaucomatous eye. The finding of the majority of POAG patients having IOP within the statistically normal population range has also been reported in several population-based studies around the world, and it is noteworthy that within Asia, this has been found in Singapore, Japan, Korea, India and China (although none of these involved 24-hour IOP profiles). The study findings suggest the possible role of non-IOP related risk factors in the pathogenesis of glaucomatous optic neuropathy. Another implication is that there is no obvious cut-off of IOP where glaucoma occurs, and IOP cannot be used as a screening tool for POAG in the general population.
M & T: Again, no single parameter is sufficient; the evaluation must be comprehensive.
Retinal Nerve Fiber Layer (RNFL) Imaging
OCTs have become an important part of the glaucoma evaluation, and are commonly used to evaluate and manage glaucoma/glaucoma suspects. Health insurance data from the United States indicates that more OCTs are being reimbursed than retinal photographs, which may be an artifact due to reimbursement rates and how claims data are coded. Still as many clinicians use OCTs instead of retinal photographs to evaluate glaucoma suspects and glaucoma, it is encouraging to find evidence that they perform at least as well as the photographs in identifying and measuring RNFL loss.
M & T: If trying to decide which to purchase, go with the OCT.
Blood Flow and Glaucoma
Recent large population based studies have found ocular perfusion pressure (OPP) to be an independent risk factor for the development and progression of glaucoma.
M & T: Ocular perfusion pressure is essentially the diastolic blood pressure minus the IOP. If this number is less than 50mmHg, there is increased risk for progression. It now appears that assessing patients' blood pressures is yet another metric we need to insert into the comprehensive glaucoma workup. This may be especially true for patients who exhibit glaucoma when IOP is normotensive.
Alternate Daily Use of Travoprost in the Treatment of Ocular Hypertension
A randomized double-blind clinical trial comparing daily use to alternate daily use of travoprost 0.004% in the treatment of ocular hypertension concluded that alternate daily use was equal in safety and efficacy as daily use in lowering IOP.
M & T: A study in 2004 found that once-daily or once-weekly dosing of latanoprost provided equal IOP-lowering at 3-month follow-up, so this finding regarding travoprost is in no way surprising.