Optical coherence tomography shows links with thickness

Naples, FL-Results of a retrospective study analyzing images acquired by spectral-domain optical coherence tomography (SD-OCT) (Spectralis, Heidelberg Engineering) show that when compared with healthy control eyes, the subfoveal choroid is thinner in eyes with glaucoma and thicker in eyes with ocular hypertension, said Emily McCourt, MD, and Malik Y. Kahook, MD, at the 20th annual meeting of the American Glaucoma Society.

The results showed that compared with the controls, mean subfoveal choroidal thickness was 32.4 μm thinner in the glaucomatous eyes and 74.4 μm thicker in the ocular hypertensive eyes (p <0.05 for both comparisons). The data also were analyzed with the subfoveal choroidal thickness values grouped by patient instead of by individual eyes. The results showed the same patterns in terms of the relative thicknesses of the glaucomatous and ocular hypertensive eyes compared with the controls, although the differences were no longer statistically significant.

"However, these initial findings suggest subfoveal choroidal thickness holds promise as a clinically useful metric for determining glaucoma risk, diagnosis, and progression as well as for providing insight into disease pathophysiology," he continued.

Looking for new markers

The researchers at University of Colorado undertook the investigation of choroidal thickness in ocular hypertensive and glaucomatous eyes motivated by the need to identify new markers for glaucoma diagnosis, prognosis, and management, and considering mounting evidence that dysfunctional blood flow to the eye is important in the pathophysiology of glaucoma.

"Many patients with glaucoma have progressive disease despite having low IOP while many patients with ocular hypertension never develop glaucomatous optic nerve damage," Dr. Kahook said. "Therefore, factors other than IOP are clearly involved in the pathophysiology of glaucoma."

Dr. McCourt, a second-year ophthalmology resident at the University of Colorado Denver, noted that "ocular blood flow and ocular perfusion pressure have been identified as possible links to the development of glaucoma. Since the choroid supplies blood to the prelaminar optic nerve head, choroidal thickness may be a surrogate measure of ocular blood flow."

Investigation of the potential association between choroidal thickness and glaucoma was first undertaken several decades ago, but those studies were done using ultrasound, which lacks adequate resolution to provide reproducible measurements, Dr. McCourt noted.

"Measurement reproducibility with ultrasound is no better than 25 μm, whereas it is on the order of 5 to 7 μm using the [proprietary SD-OCT], and this higher resolution enables identification of tissue layers and the boundaries of the anterior and posterior aspect of the choroid," she said.

Commenting on the potential relevance of the findings, Dr. Kahook postulated that glaucoma may develop in eyes with a thinner choroid that have less blood flow, whereas the thicker choroid in the ocular hypertensive eyes may represent more blood flow that is conferring a protective effect in the presence of higher IOP.

"We are not sure which comes first, the glaucoma or the change in choroidal thickness," he added. "Prospective longitudinal studies are needed to help answer this question and to validate our current findings."