Role of Ocular Response Analyzer in the Diagnosis of Normal Tension Glaucoma

Abstract

SUMMARY G laucoma is a disease presenting with progressive optic neuropathy, leading to progressive visual field loss. It is one of the leading causes of blindness. Normal tension glaucoma is an important subtype of glaucoma, constituting 10-30% of glaucoma cases. Several theories explain IOP-independent mechanisms for glaucomatous damage in NTG. Certain genetic mutations were found to be associated with NTG, and involved in apoptosis of the retinal ganglion cells and optic nerve damage. Decreased ocular blood flow is another risk factor for glaucomatous damage. Hence, systemic hypotension was found to be a risk factor for NTG, and NTG was also associated with vascular diseases like migraine and Alzheimer’s disease. Finally, autoimmune factors may be involved in glaucomatous damage. Although IOP is within normal range in NTG, IOP reduction, either by medical or surgical methods, remains the mainstay of therapy for NTG and the main way of preventing progressive damage. Therefore, accurate IOP measurement is of utmost importance in NTG diagnosis and management. GAT is the most common method and the gold standard for IOP measurement. IOP measurements are affected by corneal properties, including thickness and biomechanics. Algorithms are present that correct the GAT measurements for CCT. However, they remain insufficient to achieve an accurate IOP value. Newer methods of IOP measurement that take corneal biomechanical properties into account have been developed, in an attempt to increase accuracy. Ocular Response Analyzer is a new device for IOP measurement. It depends on a bidirectional applanation process. The device measures corneal hysteresis, which is a measure of the viscous damping of the cornea, or its ability to absorb and dissipate energy. This reflects corneal resistance to deformation during the applanation process. Corneal resistance factor is an index of corneal elasticity based on CH. The device also gives 2 IOP values; Goldmann-correlated IOP (IOPg), and corneal compensated IOP (IOPcc), which is an IOP value that is little affected by corneal biomechanical properties. It is intended to be a more accurate indicator of true IOP. Studies showed that CH and CRF are significantly lower in patients with NTG. IOPcc was also found to be significantly higher than GAT IOP in NTG patients. This may suggest that IOP values measured by GAT could be underestimated in NTG patients. Low CH could also be involved in the pathology of NTG, reflecting a weaker lamina cribrosa and thus an eye more prone for glaucomatous damage. It was also shown that eyes with lower CH show greater glaucoma progression. In patients with asymmetrical NTG, the eye with lower hysteresis usually showed more damage. Eyes with lower CH showed greater field changes, larger CD ratio and thinner RNFL. Patients with lower CH also showed greater IOP reduction after medical treatment. However, these patients require greater IOP reduction to avoid progression, since IOP is underestimated in patients with low CH. In conclusion, corneal biomechanics became an integral part of glaucoma assessment. It is essential in predicting the risk of NTG development and progression, prognosis and response to treatment.