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Editors Selection IGR 16-4

Anatomical Structures: Choroidal changes after surgery

Comment by Gustavo de Moraes on:

53878 Changes in choroidal thickness, axial length, and ocular perfusion pressure accompanying successful glaucoma filtration surgery, Kara N; Baz O; Altan C et al., Eye, 2013; 27: 940-945

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Kara et al. investigated changes in choroidal thickness estimated with frequency-domain optical coherence tomography (fdOCT) measurements following significant intraocular pressure (IOP) reduction in eyes undergoing successful trabeculectomy. In addition to a significant increase in subfoveal, nasal, and temporal choroidal thickness, the authors found a significant decrease in axial length and increase of ocular perfusion pressure (estimated using brachial blood pressure). Moreover, changes in choroidal thickness were linearly correlated with these parameters, although IOP reduction was the only variable associated with choroidal thickness increase in the multivariable analysis.

The prospective, interventional design of the study added to an adequate sample size heightens the validity of the results.

As the authors presented in the introduction, one of the complications ‐ which some may prefer to call 'adverse events' ‐ during the early postoperative phase of trabeculectomy is serous choroidal detachment. This complication can occur within a wide clinical range, from small, peripheral areas of detachment to massive detachment leading to 'kissing choroidals' seen on fundoscopy. The authors' findings reveal a sub-clinical range now possible to be detected using fdOCT. The potential mechanisms underlying the increase in choroidal thickness after acute IOP reduction are thoroughly discussed in the paper and, in brief, may be the result of changes in the osmotic gradient between the choroidal vasculature and the extracellular space. Another hypothesis not addressed in the paper, and which is unrelated to the mechanisms described above, is simply that the outflow of aqueous humor from the anterior chamber to the sub-Tenon space leads to negative pressure in the anterior chamber, which in turn makes the irido-ciliary diaphragm shift anteriorly to equalize the pressure with the posterior segment. The resulting negative pressure in the vitreous cavity may exert a mechanical force on the retinal surface leading to the expansion of its most plastic structure: the choroid. Nevertheless, the study has some limitations to be considered. Firstly, all the variables tested are inherently correlated based on how they are measured and hence it is obvious that significant changes in one variable are also expected to be seen in correlated ones. This creates a statistical issue named 'colinearity', which should be avoided when running multivariable linear models. This is probably why only IOP remained significant in their model. For example, the axial length measured with ultrasound is correlated with the subfoveal choroidal thickness, therefore, if one changes the other has to change in the opposite direction; ocular perfusion pressure is calculated using a linear equation dependent on IOP, therefore IOP reduction obviously increases ocular perfusion pressure since blood pressure is very unlikely to change significantly before and after surgery (as there data show). Secondly, the authors stated that their findings support that "choroidal thickness is influenced by ocular perfusion pressure." The authors cannot make any conclusion regarding causality based on the current findings. As discussed above, it is actually more likely that increased ocular perfusion pressure may be a consequence of IOP reduction, which then leads to increased choroidal thickness. Finally, the clinical relevance of their findings is unclear. These microscopic changes in choroidal thickness should not be searched in every patient after filtering surgery as they have no clinical implications and often require no treatment. From a research perspective, however, Kara et al. helped clarify the relationship between IOP, choroidal thickness, and pressure changes in the ocular compartments after filtering surgery. The authors are encouraged to test whether their findings can be replicated following IOP reduction using topical anti-glaucoma medications.

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