Recognising corneal abnormalities with PS-OCT
Assessing cornea status can aid early recognition of disease-related changes.
Reviewed by Ritika Mullick, MBBS, MS.
Polarization-sensitive optical coherence tomography (PS-OCT) is a functional extension of the conventional OCT that exploits the polarization properties of light.
The human eye is known to have polarization altering tissues that using an ultra-high-resolution PS-OCT, Ritika Mullick, MBBS, MS, explained, helps physicians and scientists study the form birefringence caused due to the underlying fibril structures in the corneal stroma. This imaging modality can detect subnormal distributions of the collagen in corneal disorders.
This ability to assess the corneal status can help in the early recognition of disease-related changes before they can be seen topographically, according to Mullick is a fellow in Cornea and Refractive Services, Narayana Nethralaya Hospital, Bangalore, India.
In her clinic setting, this technology can demonstrate normal corneal biomechanics and clear-cut cases of abnormalities, such as the development of post-SMILE ectasia.
Usefulness of PS-OCT
The technology is especially useful in what Mullick describes as “confusing” cases that comprise about 35% of the cases that pass through her clinic. For example, one patient upon examination had a thin cornea but no keratoconus, another had a suspicious presentation but the biomechanics were normal, and yet another had normal topography but suspicious biomechanics.
This raises the question: should physicians rely on the corneal topography or the biomechanics in such cases?
This question led to the development of PS-OCT, which can image the spatial distribution of fibril structures and their orientation.
“The arrangement of collagen fibres in normal corneas exhibit a checkered arrangement centrally,” Mullick said. “In the images, the presence of dark blue/purple indicates the least phase retardation, that is, the least alteration of polarized light due to the uniform arrangement of the collagen fibres. This indicates a very healthy cornea.”
When corneas are diseased, irregular organization of the collagen fibres can be seen with more alteration of the polarized light, which indicates higher phase retardation as seen by the warmer colours in the image.
Corneal study
Mullick and her colleagues studied 135 corneas prospectively using PS-OCT, i.e., 50 that were healthy, 50 with keratoconus, and 35 suspicious corneas. All patients underwent corneal tomography, corneal biomechanics, and epithelial mapping in addition to biomicroscopy, fundus evaluation, and refraction. The evaluations were followed by PS-OCT.
The results were categorized based on the topographic and biomechanical analyses followed by in vivo analysis of the phase retardation (PR) distribution. She provided some examples of representative cases.
Mullick reported that the in vivo imaging by PS-OCT showed normal topography in 2 cases, but another case had a suspicious BAD-D. The Oculus HR Pentacam software program is inbuilt with the Belin-Ambrosio Enhanced Ectasia Display (BAD), which helps in the early detection of subclinical keratoconus changes.
However, the checkered arrangement in the centre of the image indicated that the 2 corneas were healthy and therefore would be suitable candidates for refractive surgery.
A third case had normal BAD-D and suspicious biomechanics; this patient had a partial loss of the checkered arrangement. In this case, refractive surgery was deferred.
The fourth case had a thin cornea and no keratoconus, but there was partial loss of the checkered arrangement.
In the fifth and sixth cases with keratoconus and post-SMILE ectasia, respectively, there was a complete loss of the checkered arrangement, which indicated that these were weak ecstatic corneas with a weak collagenous arrangement
This approach has facilitated the ability to diagnose disease earlier before it becomes apparent on topography. Mullick noted that PS-OCT can clearly identify differences in collagen fibre distributions between healthy and keratoconic corneas.
“Suspicious corneas had a unique distribution of collagen indicated early changes before they were detected on topography,” she explained. “The imaging technology can help diagnose keratoconus even in cases that have normal or borderline suspicious topography findings, and therefore can be an excellent screening modality.”
The technology can also be used to screen candidates for refractive surgery, identify forme fruste keratoconus, monitor keratoconus progression or follow cross-linking outcomes, perform post-graft imaging, or diagnose retinal disorders.
