OCT TECHNOLOGY ALLOWS PRACTITIONERS TO IDENTIFY GLAUCOMA EVEN EARLIER

THE DEFINITION of a game changer is a person, idea, technology or procedure that transforms the accepted rules, processes, strategies and management of something, according to the dictionary.

In glaucoma diagnosis, OCT has been called a game changer in that it has facilitated earlier diagnosis and, therefore, earlier management of glaucoma, enabling us to help patients to stave off vision loss.

Here, I discuss the latest advancements in this important technology.

SD-OCT

SD-OCT captures optic nerve head parameters (vertical rim thickness, rim area, vertical cup to disc ratio, etc.) and circumpapillary retinal nerve fiber layer (RNFL) thickness.

In addition, SD-OCT can provide ganglion cell analysis (measurements of the ganglion cell inner plexiform layer and ganglion cell complex thickness). Ganglion cell analysis was deemed nearly equivalent to optic nerve head parameters and circumpapillary RNFL thickness in the diagnosis of glaucoma, reports a 2012 study in Ophthalmology.

Thus, the measurements of all three diagnostic markers give you, the practitioner, a powerful tool to aid in glaucoma diagnosis.

OCT-A

The OCT-A provides 3D, high-resolution images of segmented retinal tissue layers, based on motion contrast of red blood cell movement across time, sans injection or dye. The retinal capillary plexus is in different layers of the retina: the superficial plexus in the ganglion cell layer, the intermediate plexus near the inner edge of the inner nuclear layer and the deep plexus in the outer edge of the inner nuclear layer. OCT-A allows you to analyze how different retinal capillary plexus are affected by different retinal diseases. You are then able to make an accurate diagnosis and the proper referral for the disease process present.

Further, the technology measures optic disc, peripapillary and retinal perfusion. Perfusion measurements, or OCT-A flow index of the optic disc, provide metabolic information of retinal ganglion cells and nerve fibers. This is important because optic disc flow has been shown to decrease 25% in glaucoma patients compared with “normal” eyes, reveals a 2014 study in Ophthalmology. Further, the flow index was highly correlated with the VF pattern standard of deviation for glaucoma, the study also shows.

In addition to optic nerve head imagery of perfusion, OCT-A can also image large vessels and microvasculature of the peripapillary region. The percentage of area occupied by those vessels is known as the peripapillary vessel density. Glaucomatous eyes had a smaller percentage of peripapillary vessel density compared with “normal” eyes, reports a 2015 study in JAMA Ophthalmology.

SS-OCT

Swept-source optical coherence tomography (SS-OCT) uses a short-cavity swept laser, which has a fast scanning speed. This allows for high-resolution images and larger scan areas compared with SD-OCT. Another difference: The circumpapillary retina nerve fiber layer and the ganglion cell layer can be evaluated in a single wide-field scan, overcoming difficulties encountered with patient alignment when obtaining separate images of those structures.

An interesting application of SS-OCT in glaucoma is its ability to image the lamina cribrosa, which undergoes morphological changes when glaucoma is present. A 2013 Investigative Ophthalmology & Visual Science study shows that full-thickness focal lamina cribrosa defects were more prevalent in glaucomatous eyes than healthy eyes. The defects corresponded with neuroretinal rim thinning, current or previous disc hemorrhages, abnormal circumpapillary RNFL thickness and VF defects.

EARLY INTERVENTION

The earlier one can definitively detect glaucoma, the earlier one can prescribe the right treatment, monitor the condition and, hopefully, prevent loss of vision. The devices mentioned above facilitate the detection of glaucoma through new parameters, making early intervention and treatment even more feasible than before. OM

DR. SCHWEITZER specializes in advanced glaucoma, refractive surgical clinical care and anterior segment pathology at Vance Thompson Vision. He has had multiple articles published and has delivered lectures on glaucoma, anterior segment pathology, refractive surgical clinical care and surgical management. Email him at justin.schweitzer@vancethompsonvision.com.