Optical coherence tomography has moved from being a tool primarily utilized by the retina subspecialist to one increasingly present in the comprehensive ophthalmologist’s office.
Time-domain OCT (TD-OCT) is a non-contact test that can be done by an ophthalmic technician to image the optic nerve, macula or retina. This is the initial instrument our clinic acquired, and it has become an indispensable part of our anterior segment practice. We recently acquired our first spectral-domain OCT (SD-OCT), which is a higher-speed, higher-definition instrument that gives even better images, allowing resolution that includes individual cell layers including the retinal pigment epithelium (RPE), photoreceptors, as well as a larger area of the retina or macula. The newest SD-OCT algorithms even allow 3-D reconstructions, and at one meeting I saw a presentation in which the ophthalmologist could step into a room and stand inside a large 3-D image of the entire retina and optic nerve head and look at highly magnified detail, including individual cells in every direction. To be direct, this demonstration was utterly amazing.
While interpretation of patterns, as we do when looking at an MRI or topography color map, are informative, as knowledge grows, reduction to mathematical equations and numbers that help delineate normal from abnormal usually emerge. This evolution is occurring in TD-OCT and SD-OCT as we learn to interpret retinal and macular thickness, RPE elevation and drusen size/volume in microns. In the future, it is anticipated that even cellular metabolic rates will be measurable. Of course, similar technology is being applied in the anterior segment as well, promising great advances in our diagnostic capability. For a consultative cornea and glaucoma practice such as ours, anterior segment OCT is also becoming a necessity.
A few thoughts on the posterior segment applications of OCT pertinent to the comprehensive ophthalmologist. The classical abnormalities in dry age–related macular degeneration include drusen formation, pigment dispersion/clumping and focal atrophy of the RPE. Each of these findings can be observed, documented and measured with OCT. In addition, with SD-OCT, drusen size and volume can be measured, and the actual RPE cells at the edge of an area of geographic atrophy can be seen, allowing one to measure the rate of progression. Photoreceptor atrophy and changes in the choriocapillaris often accompany these changes and can be visualized. In wet AMD, one can visualize the area where abnormal vessels penetrate Bruch’s membrane, causing fluid or blood to accumulate under the RPE and resulting in a pigment epithelial detachment (PED) or fluid accumulation in the subretinal space or within the retina. The size and volume of a PED can be measured, as can the overlying retinal thickness, potentially allowing earlier diagnosis and, when treated, monitoring of the response to therapy.
For the cataract surgeon, preoperative screening allows the detection of epiretinal membranes, vitreomacular traction syndrome, diabetic macular edema and rarer macular abnormalities such as macular schisis, resulting in proper preoperative, intraoperative and postoperative therapy and timely patient counseling. Postoperative macular thickness is a very useful tool in the diagnosis of postsurgical cystoid macular edema and is also helpful in monitoring the response to therapy. TD-OCT imaging and SD-OCT imaging of the optic nerve head are becoming a critical component in the early diagnosis of glaucoma and in monitoring the patient for early signs of progression.
Ophthalmology has always been heavily dependent on technology to enhance the diagnostic and treatment acumen of the clinician, and despite the cost of OCT acquisition in the face of increasing economic pressure, it is hard for me to imagine a busy ophthalmic practice without OCT or SD-OCT in 5 years.