Yet another new revolution in ocular surface imaging

During the past two decades, an incredible number of imaging technologies have been developed for eye diseases, both for diagnosis and follow-up. Most of these developments aim to improve the investigation of retinal diseases with fluorescein or indocyanine angiography, scanning laser ophthalmoscopy (SLO) and, more recently, the revolutionary optical coherence tomography (OCT).

There have been similar developments regarding the cornea and ocular surface, although use of these innovations remains more restricted. As the cornea is easily and directly accessible to biomicroscopy, several techniques were developed in the 1990s but with poor resolution at that time and limited uses in routine practice.

A breakthrough came in the early 2000s with the major improvement of the in vivo confocal microscopy technique provided by the Rostock Cornea Module, a customised objective added to the Heidelberg Retina Tomograph, a descendant of SLO and an outstanding tool for optic disk examination, its use more circumscribed than OCT but still usable in practice.

Cornea imaging

This curious marriage of an imaging technique dedicated to glaucoma diagnosis and a device developed by innovative academic researchers gave rise to a brilliant tool that made direct histological-like examination of the cornea possible, not only the central cornea as with previous devices, but the entire ocular surface.

Infectious diseases, corneal dystrophies, dry eye and many other disorders have now become accessible. Direct cell examination is possible at unbelievable resolutions, close to 1 µm, so that individual goblet cells, dendritic or polymorphonuclear cells can be counted and subcellular elements identified.

However, despite its major advantages, this technique has remained somewhat restricted because it is time-consuming and requires highly skilled operators.

At the same time, OCT began revolutionising the imaging domain based on research throughout the world that has concentrated on developing this tool to make retinal structures easily, rapidly and noninvasively accessible to clinicians and researchers for the first time. The technique improved step by step, and superb devices with impressive resolution and incredibly short examination times became available.

Almost every two years a new, even greater machine is introduced on the market, and the competition is fierce, pushing companies to improve their techniques and extend the device’s indications and applications. Imaging the cornea and anterior segment has clearly advanced in this race for the technological grail.

OCT-A

Retinal imaging has recently taken a further step forward with the incredible development of a technique capable of providing high-resolution images of the eye’s blood vessels, without any injection of potentially toxic or allergenic compounds: OCT angiography (OCT-A).

OCT-A can map the vascular plexi layer by layer, to precisely show even the smallest vessels and their interconnections and thus identify abnormal vessels or ischaemic areas where blood flow is compromised.

OCT-A has revolutionised care for retinal diseases and now shows similar potential for anterior segment vasculature. However, the quality of the anterior segment scans is still not as good as retinal images.

For example, anterior segment OCT-A does not tolerate the slightest eye movement on the part of the patient because even micro-movements create transverse artifacts on the final images. And so we must wait for technical improvements.

Meanwhile, a new unexpected application is appearing. OCT-A scans are based on moving elements rather than optical densities related to colour or staining with a dye.

The clear discrepancy observed between vascular density under slit-lamp examination and in OCT-A strongly suggests that this new tool not only targets blood vessels, but also a totally nonvisible parallel vascular network: the lymphatic vessels.

We are now viewing structures and tissues we have never seen before or could only access using invasive techniques. A new, truly innovative approach to the anterior segment is now in our hands and will certainly advance with the promising technical progress to come. We just have to wait for the next revolution!

Dr Christophe Baudouin, MD, PhD, FARVO

E-mail: cbaudouin@15-20.fr

Dr Baudouin works for Quinze-Vingts National Ophthalmology Hospital & Vision Institute in Paris, France.