Commentary: Decoding the intraocular pressure conundrum surrounding the core of COR.

Abstract

The concept of intraocular pressure (IOP) gained popularity around the 19th century. It was during that time when Albrecht von Graefe developed the first IOP measuring instrument, the Maklakoff applanation tonometer, in 1865. Somewhere around the 10th century, it started off as an assessment of the firmness of the eyeball.[1] Currently, we possess cutting-edge technology like dynamic contour tonometry to provide in vivo IOP measurements. IOP control is an important component of glaucoma management, and therein lies the importance of accurate IOP measurement and documentation. Consensual Ophthalmotonic Reaction The concept of consensual ophthalmotonic reaction (COR) has been a point of debate for several decades now.[2] There have always been claims for and against this phenomenon. Though inconclusive, this theory needs to be taken into account in view of the sensitive nature of the disease. The accompanying article represents the largest evaluation to date of the untreated fellow eye response after unilateral trabeculectomy. The author’s suggestion raises the possibility of a real risk of IOP increase in the fellow eye following ipsilateral glaucoma surgery.[3] Nearly a third of patients needed an intervention to control IOP in the entire study population. COR – The Gray Area Surrounding the How and Why The authors of the accompanying article report that contralateral eye IOP was increased following ipsilateral glaucoma surgery. However, some works of literature have reported conflicting results and noted a drop in IOP, while some agreed.[4–7] The pathophysiology for this increase or decrease of IOP following ipsilateral glaucoma surgery is unclear and not well known. Possible Hypothesis and Postulations Surrounding COR Several hypotheses have been made to explain COR, but all are not entirely accepted. Some suggest that topical medication either works on the fellow eye via the bloodstream or acts centrally on IOP control mechanisms after systemic absorption. Smith et al. disproved it with the use of pindolol and stated that the IOP falls after topical and intravenous administration of pindolol.[8] Pindolol treatment is used to help decrease IOP in the contralateral untreated eye, which is unlikely to have resulted from systemic absorption of the drug. This theory can also not explain the COR seen in mechanical alteration of IOP, such as following laser trabeculoplasty and tonography. Leplat hypothesized the existence of a nervous mechanism that causes changes in IOP in one eye to provoke a reflex alteration in pressure in the contralateral eye.[9] Moreover, the Collaborative Initial Glaucoma Treatment Study (CIGTS) showed a moderate drop in IOP in the contralateral eye, but the findings were not statistically significant and were found to be transient. Hence, it is inferred that this COR does not warrant any clinical management.[10] It is postulated that post-trabeculectomy, the increased IOP in the fellow eye could be due to a reflex increase in aqueous humor production, an effort of the eyes to compensate for the low IOP of the operated eye. The ocular central nervous system (CNS) reflex mechanism would trigger an increase in aqueous production both in the ipsilateral and the contralateral eyes, resulting in a buildup of IOP in the contralateral eye. It is also possible that the IOP fluctuations in the nonsurgical eye are due to changes in the blood flow or circulation to the eye or due to the body’s overall response to the surgery.[4] COR and Minimally Invasive Glaucoma Surgery Though trabeculectomy has been the gold standard for the past few decades, minimally invasive glaucoma surgery (MIGS) is now taking up space in the world of glaucoma. Currently, there is a paucity of data regarding the efficacy of MIGS, but it is a sought-after mode of treatment. So, studies also need to focus on COR in MIGS patients to see if any similar responses are seen in that category. Conclusion Changes in IOP are possible after glaucoma surgery due to a multitude of factors coming into play, suggesting that the fellow eye needs to be carefully monitored after glaucoma surgery in one eye. In many instances, the fellow eye is often the one with useful vision. Failure to recognize and monitor IOP changes carefully may unnecessarily delay treatment and appropriate management of these eyes. In addition, fellow eyes already damaged by glaucomatous processes may be at significant risk of glaucoma progression due to higher IOP levels. Although the evidence for the existence of COR is strengthened, larger studies with more patients in each glaucoma category may shed light on its exact biological role.