Reviewed by Dr Ritika Mullick.
Polarisation-sensitive optical coherence tomography (PS-OCT) is a functional extension of the conventional form of OCT and exploits the polarisation properties of light. According to Dr Ritika Mullick, a fellow in Cornea and Refractive Services at Narayana Nethralaya Hospital, Bangalore, India, this imaging modality can aid the early recognition of disease-related changes before they can be seen topographically.
Dr Mullick explained that the human eye is known to have polarisation-altering tissues; using an ultra-high-resolution PS-OCT helps us to study the form birefringence caused by the underlying fibril structures in the corneal stroma. This, in turn, can enable users to detect subnormal distribution of collagen in corneal disorders.
In a clinic setting, this technology can demonstrate both normal corneal biomechanics and clear-cut cases of abnormalities, such as the development of post-SMILE ectasia.
Development of PS-OCT
The technology is especially useful in what Dr Mullick describes as “confusing” cases, which comprise about 35% of those that pass through her clinic. For example, upon examination, one patient 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 spurred the development of PS-OCT, which can image the spatial distribution of fibril structures and their orientation.
As Dr Mullick explained, “The arrangement of collagen fibres in normal corneas exhibits a checkered pattern centrally. In the images, the presence of dark blue/purple indicates the least phase retardation: that is, the least alteration of polarised light, due to the uniform arrangement of the collagen fibres. This indicates a very healthy cornea.”
When corneas are diseased, however, irregular organisation of the collagen fibres can be seen, with more alteration of the polarised light; this indicates higher phase retardation, as seen by the warmer colours in the image.
Dr Mullick and her colleagues studied 135 corneas prospectively: 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 categorised based on the topographic and biomechanical analyses followed by in vivo analysis of the phase retardation (PR) distribution. The Belin-Ambrosio enhanced ectasia display (BAD) software, which helps in the early detection of subclinical keratoconuschanges, was used with the inbuilt Oculus HR Pentacam camera system.
Dr Mullick provided some examples of representative cases. She reported that the in vivo imaging by PS-OCT showed normal topography in two cases, but another case yielded a suspicious BAD deviation value (BAD-D).However, the checkered arrangement in the centre of the image indicated that the two 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 ectatic corneas with weak collagenous arrangements.
This approach has facilitated the ability to diagnose disease earlier before it becomes apparent on topography. “PS-OCT can clearly identify differences in collagen fibre distributions between healthy and keratoconic corneas. Suspicious corneas had a unique distribution of collagen, which indicated early changes before they were detected on topography,” Dr Mullick commented.
She added: “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.
Ritika Mullick, MBBS, MS
This article was adapted from Dr Mullick’s Subspecialty Day presentation at the 2021 American Academy of Ophthalmology annual meeting, New Orleans, Louisiana, United States. Dr Mullick has no financial interest in this subject matter.