VHF ULTRASOUND

Early keratoconus with epithelial compensation - Figure A: Eye with apparently normal front surface topography; Figure B: Epithelial thickness profile on Artemis scanning shows a localised region of epithelial thinning; Figure C: Epithelial changes are coincident with a posterior surface apex elevation. Courtesy of Dan Reinstein MD, MA (Cantab), FRCSC, DABO, FRCOphth, FEBO

Imaging and measurement of internal corneal and posterior chamber (PC) anatomy is the next frontier for better understanding of the cornea, both before and after refractive procedures as well as for optimising cataract and phakic intraocular lens (IOL) surgery. Very high-frequency (VHF) digital ultrasound can obtain unique biometry measurements of both the cornea and anterior segment to provide a wide range of applications.

VHF Digital Ultrasound

The Artemis (ArcScan Inc., Morrison, Colorado, US) uses a broadband 15-60MHz digital ultrasound scanning probe used with the patient sitting with the eye immersed in a soft rimmed eye-cup filled with warm sterile normal saline (33°C). The transducer sweeps in an arc approximately following the contour of anterior or posterior segment structures of interest. Three-dimensional scan sets are also possible. The tear-film is not incorporated into measurements, unlike current epithelium mapping optical coherence tomography (OCT) systems. Captured data is then digitally processed.

Digital signal processing localises interfaces such as Bowman's layer, LASIK flap etc to 0.87µm. Captured data is interpolated between meridional scans to produce pachymetry maps with high repeatability for epithelium (0.58µm), stroma (1.78µm), cornea (1.68µm), flap (1.68µm) and residual stromal bed (2.27µm). Posterior segment is measured using software for anterior segment scans.

Keratoconus Screening

Studies have shown that epithelial thickness profile changes in keratoconic eyes to a “doughnut pattern”, characterised by epithelial thinning over the cone surrounded by an annulus of epithelial thickening. Corneal epithelial thickness profile may be useful in early keratoconus screening owing to the ability to map epithelial thickness to the nearest micron.

Dr Dan Z Reinstein, who developed the original VHF digital ultrasound technology with co-workers at Cornell University in the early 1990s, and later the Artemis device for which he holds a proprietary interest and several related patents, says: “The epithelial doughnut pattern masks the extent of any stromal surface cone on topographic anterior elevation best-fit sphere (BFS) and curvature maps and can in some cases completely mask small degrees of stromal front surface ectasia.

“Anterior surface topography might thus miss the diagnosis and especially when posterior surface elevation mapping is also equivocal, early keratoconus may be missed. Not all posterior elevation BFS changes are due to keratoconus. Therefore, a diagnostic tool to confirm or exclude a diagnosis of keratoconus in eyes with eccentric posterior elevation BFS is needed. In the absence of epithelial doughnut pattern, underlying stromal surface cone can be excluded as cause of suspect inferior steepening or suspect eccentric posterior elevation. Epithelial thickness mapping thus allows patients to be deemed suitable for corneal refractive surgery who otherwise would have been denied treatment by topography or tomography alone.

“The converse may also occur. For example, the figure shows apparently normal front surface topography (Figure A), but epithelial thickness profile shows localised region of epithelial thinning (Figure B) coincident with posterior surface apex elevation (Figure C). This indicated presence of early keratoconus where epithelium was able to compensate fully for the stromal cone, resulting in an apparently normal front surface corneal topography.”

Epithelial Compensation

Compensatory epithelial thickness changes also have a significant impact in irregular astigmatism. The epithelium effectively acts as a low pass filter for both local and global changes in stromal surface curvature so that it becomes thinner over relative peaks and thicker over relative troughs in stroma.

Dr Reinstein explains: “This is summarised by my Law of Epithelial Compensation for irregular astigmatism: ‘irregular astigmatism results in
irregular epithelium’.

“Therefore, if a patient presents with stable irregular astigmatism, by definition epithelium has reached maximum compensatory function. Front corneal surface topography and aberrometry have been the mainstay of diagnostic testing in complicated LASIK. However, neither understanding of optical defect or front surface shape of cornea will necessarily provide a diagnosis for cause of underlying problem and this may lead to sub-optimal treatment planning.”

Improving Phakic IOL Sizing

A major advantage of ultrasound over optical (OCT, Scheimpflug) techniques is the ability to image the whole PC through iris. Optical instruments cannot image PC because optical path is blocked by iris pigment epithelium.

“As PC phakic IOLs gain in popularity, accurate PC biometry becomes increasingly important. Traditionally, IOL sizing is chosen using a formula based on horizontal white-to-white (W-to-W) diameter, which assumes a correlation exists between W-to-W and PC dimensions – sulcus-to-sulcus (S-to-S) diameter in particular.

“However, multiple studies using ultrasound biomicroscopy have demonstrated that nil to very weak correlation exists, and therefore lens sizing inaccuracies are not uncommon enough.

“Given that the vast majority of IOL complications are related to lens sizing error (oversized lens can cause angle closure or iris chafing and pigment dispersion; an undersized lens can cause cataract or damage to zonules with dislocation of IOL), it seems that using direct S-to-S measurement is the correct way forwards for improving PC phakic IOL safety. Indeed, a number of surgeons are now doing this with very impressive results,” says Dr Reinstein.

Future Potential

An outstanding source of error in calculating IOL power in cataract surgery, particularly after previous refractive surgery, is predicting the effective lens position (ELP). Because of inability to image the crystalline lens outside the pupillary boundary, it is almost impossible to predict the ELP accurately enough with current optical technology. Newer developments will also measure position and volume of crystalline lens, thus allowing better predictability of ELP and hence improved refractive predictability for cataract surgery.

Summary

In summary, a huge amount of important information can be obtained by the ability to visualise and measure internal corneal layers and anterior segment structures using VHF digital ultrasound, which cannot otherwise be appreciated by optical or external measurements alone. It is possibly time that this type of technology becomes more widespread among anterior segment and refractive surgeons.

* Dr Soosan Jacob is Director and Chief - Dr Agarwal’s Refractive and Cornea Foundation, at Dr Agarwal’s Eye Hospital, Chennai, India and can be reached at dr_soosanj@hotmail.com