OCT, HRT FOR CORNEA

Routine use of optical coherence tomography (OCT) and in vivo confocal microscopy could improve corneal disease screening, diagnosis, monitoring and research much as they have retinal disease, Laurent Laroche MD told the 4th EuCornea Congress in Amsterdam. “I still use the slit lamp as a first step for corneal examination. But higher resolution allows better visualisation of structural features of the cornea,” said Dr Laroche, of Quinze-Vingts National Ophthalmology Hospital, Paris, France.

Dr Laroche currently uses Fourierdomain OCT on all corneal patients, and confocal microscopy on a few. He uses OCT-RTVue (Optovue) with a corneal adaptor, which provides micron-scale crosssectional imaging and automatic corneal epithelium thickness mapping. For confocal microscopy, Dr Laroche uses HRT 3-RCM (Heidelberg Engineering), which provides high-quality images for assessing microstructural morphologic changes occurring in pathology – in vivo cytology, if you will. Both can be highly effective in refining diagnoses for infectious and non-infectious disease, and assessing treatment follow-up.

OCT can be helpful in confirming diagnoses of diseases that can be hard to visualise, such as Cogan’s dystrophy, Dr Laroche said. “Cogan’s disease sometimes is obvious on slit lamp examination, but sometimes you don’t really see the cysts very well. OCT cross-sections clearly reveal cysts and separations between the epithelium and Bowman’s membrane, while HRT can show detailed images of the cysts,” he noted. High-resolution imaging also can be useful for diagnosing and determining treatment for infectious keratitis, Dr Laroche said. For acanthamoeba keratitis, the slit lamp shows a characteristic immunological ring in the cornea, but OCT better shows infiltration sites while HRT can image cysts as well as trophozoites.

In fungal keratitis, OCT shows the depth of ulcerations and inflammation, Dr Laroche noted. “You can clearly see when Descemet’s membrane is involved, which is of utmost importance for treatment strategy.” OCT can be used to track inflammation and ulceration to determine the effectiveness of treatment. Similarly, OCT can show the extent of infection in bacterial keratitis. The depth of the scar will be important for deciding the right treatment, Dr Laroche said.

OCT also clearly shows features of Reis-Bucklers dystrophy, including involvement of the anterior stroma, and granular corneal dystrophy, involving the full stroma. Differentiating the disease is important for treatment. Perhaps the most significant recent application of OCT for corneal disease is in staging keratoconus, said Dr Laroche. He was one of a team of researchers who developed the scale now used (Ophthalmology Dec 2013; 120 (12):2403-2012).

OCT imaging

Blinded evaluations of 218 patients with keratoconus established five stages of keratoconus clearly identifiable based on OCT imaging. The criteria were found to be highly reproducible and specific and correlated well with clinical characteristics of keratoconus, including visual acuity, corneal epithelium and stromal thickness changes, as well as corneal topography, biomechanical corneal characteristics and microstructural changes observed on confocal microscopy, Dr Laroche noted.

Stage 1 demonstrates thinning of apparently normal epithelial and stromal layers at the conus. Stage 2 demonstrates hyper-reflective anomalies occurring at the Bowman's layer level with epithelial thickening at the conus. Stage 3 demonstrates posterior displacement of the hyper-reflective structures occurring at the Bowman's layer level with increased epithelial thickening and stromal thinning. Stage 4 demonstrates pan-stromal scar with thinning of residual stroma. Stage 5 demonstrates hydrops, with 5a detailing acute onset with Descemet’s membrane rupture and cyst formation, and 5b healing with pan-stromal scarring.

OCT images and the staging classifications can be very helpful in clinical practice, Dr Laroche said. “When you set the indication for corneal cross-linking in thin corneas, it is not the same problem if your cornea is 400 microns thick and epithelium represents 25 per cent of thickness compared with [a 400 micron cornea with] over 300 microns epithelium. The effect will be very different.” OCT is also helpful in cross-liking followup. Variations in patient response can be seen and tracked, infiltrates observed and monitored, and the effectiveness of crosslinking assessed.

“Sometimes you can see a demarcation line, and sometimes you can see increased response in some patients.” OCT is also useful to monitor results of intracorneal ring segments, including any response to the implants. In lamellar keratoplasty, OCT helps evaluate remaining bed in anterior grafts, and attachment of posterior grafts. Grafthost junction can be monitored after penetrating procedures. “When you have a giant astigmatism, it may give you some idea of how to treat it.”

OCT provides accurate pachymetry readings and epithelial thickness maps, which may provide early warning of forme fruste keratoconus. Follow-up after PRK helps track sub-epithelial healing, while post-LASIK imaging can determine the extent of epithelial ingrowth. High-resolution imaging routinely used is as informative for the cornea compared with slit lamp examination as OCT is for the retina compared with fundus examination, Dr Laroche concluded. He believes it will revolutionise corneal treatment and research.