Mouhcine El Bakkali
The capability of modern optical coherence tomography instruments to accurately distinguish the corneal epithelial layers from the underlying Bowman’s layer and stroma makes them an essential tool in the early diagnosis of keratoconus and also the treatment of the condition, said Mouchine Al Bakkali MD, Clinique de La vision de Rabat, Rabat, Morocco.
“In the past we were told that keratoconus starts in the posterior
surface because we didn’t have OCT, but now we know we can
detect it earlier on the anterior surface,” Dr Bakkali told the 24th ESCRS Winter Meeting in Marrakech, Morocco.
He noted that before the development of OCT, assessments of the cornea were less reliable, because the tomography technologies relied on the image extrapolation, using mathematical reconstruction that can miss some abnormalities. In addition, the older devices do not allow segmental assessment of the corneal layers and therefore cannot disassociate the epithelium from the stroma. Furthermore, tear film stability and resolution issues can undermine the reliability of the data the instruments provide.
EPITHELIAL MAPPING
OCT has greater sensitivity and specificity because it overcomes the deficiency in segmental analysis with spectral OCT providing a high resolution from 4-to-5 microns for spectral domain and 8-to-9 microns swept source, he said. It therefore can be a reliable tool for the diagnosis and followup of all corneal abnormalities. It also has a range of potential applications in the treatment of corneal disease. For example, surgeons can use OCT intraoperatively for placement of intracorneal rings segments (ICRS), before trans-epi PRK and also before and after corneal graft surgery.
In healthy corneas the corneal layers are aligned in a parallel fashion, but in eyes with keratoconus that alignment is disturbed. The corneal epithelium has a characteristic pattern of growth, wherein the epithelium grows more thickly at the base of the cone and more thinly at its apex in a classic donut image. In this way, the epithelium masks the cone and makes the change in the anterior surface less abrupt.
He added that since adopting epithelial mapping when assessing corneal refractive surgery candidates, he will always rule out LASIK in eyes with epithelial abnormalities despite normal topology and will instead opt for a surface ablation procedure.“Epithelial mapping in keratoconus could be an accurate device for early diagnosis, and more attention should be paid to the three types of data, namely a thinning variation with a standard deviation greater than six microns, a thinning point localisation outside the 2.0mm optical zone, and the index superior more than 5.0mm,” Dr Bakkali said
PREOPERATIVE AND INTRAOPERATIVE OCT
Dr Bakkali noted that epithelial mapping is mandatory before epi-off CXL, as it can determine whether the stromal thickness beneath the epithelium is superior to 500 microns and can therefore safely undergo the procedure. Corneal OCT also has a role in the follow up of CXL by providing accurate pachymetry showing the demarcation line as an index of efficient cross-linking.“Therefore, we can conclude that corneal OCT is essential before any classification we might make for keratoconus, and we should take into account the corneal OCT data of the epithelium, the Bowman’s membrane and the stroma. The Belin ABCD classification of keratoconus should be the ABCD E classification, taking into account the epithelium behaviour,” Dr Bakkali said.
OCT IN THE OPERATING ROOM
OCT can also be very useful in the operating room. It can be used for diagnostic purposes in patients who are not able to be examined in consultation, for example paediatric patients. Intraoperatively it makes transparent structures visible increasing surgical precision and allowing the surgeon to adapt their surgery in real time as unexpected situations arise. In addition, OCT may enable surgeons to perform new types of surgery, including new minimally invasive techniques that were not possible without OCT.
OCT can also play a useful role in the implantation of ICRS by insuring the three principle criteria for their safety and efficacy, namely correct pupil centration with regard to angle kappa, insertion at 80% of stromal depth and a healthy endothelium covering the ring to prevent extrusion.
Similarly, in eyes undergoing transepithelial PRK, OCT allows accurate measurement of epithelial thickness. Even in healthy eyes the epithelium thickness can range 48-to-57 microns. Relying on the supposed epithelial thickness provided by the literature increases the risk of over- or under-correction.
He added that in corneal graft surgery, swept-source OCT devices combined with a surgical microscope enhances the corneal surgeon’s ability to perform some of the more technically difficult but less invasive keratoplasty techniques such as deep anterior lamellar keratoplasty (DALK) l and Descemet’s membrane endothelial keratoplasty (DMEK).
“My take-home message is that we should keep in mind the cornea’s structure and function and by combining corneal tomography, topography, OCT and biomechanics, the question is are we able to identify all the risky corneas or not quite yet,” Dr Bakkali concluded.
