Safeguarding the Cornea

Expanding uses for AS-OCT

A diagnostic powerhouse for the front of the eye.

By James S. Lewis, MD

Posterior segment optical coherence tomography (OCT) dominates the mindshare of eyecare professionals. Macular disease diagnosis and management was the “killer-application” responsible for introducing OCT to the well-equipped office, and glaucoma specialists soon recognized that nerve fiber layer assessment was further justification to acquire an OCT system.

In contrast, anterior segment OCT (AS-OCT) technology has been all but neglected by the marketing divisions of most major manufacturers.

Here, I demonstrate some of the primary uses of this expanding technology.

ANALYZING SURGICAL RESULTS

Zeiss released the Visante in 2005, only a few years after its successful posterior segment unit. I owned one of these first machines, and a reluctant potential buyer asked me, “Does a corneal specialist really need this device? It shows some of my LASIK flaps measuring almost 200 microns — it can’t be accurate.” (Figure 1)

Figure 1: 200-micron flap (old-style mechanical microkeratome circa 2000) without visual or structural degradation.

Unfortunately, many of our LASIK flaps in that era were painfully thick. Our feeble efforts to use intra-operative ultrasonic pachymetry were flawed, and we generated unexpectedly thin residual stromal beds with too many associated cases of ectasia. AS-OCT demonstrated the consistent flaps from IntraLase and undoubtedly helped grow its popularity.

I used the Visante to measure central corneal flap thicknesses in 500 consecutive LASIK cases. These images showed how Moria’s One Use Plus (OUP) SBK, a next-generation mechanical microkeratome, created reliable, reproducible thin central corneal flaps (Figure 2).

Figure 2: 110-micron flap Moria One-Use-Plus (OUP) (Anton, France) with semi-planar edge profile and small break in epithelium. Imaged immediately after LASIK surgery.

Thick flaps and outdated microkeratomes would not survive AS-OCT’s arrival (Figure 3).

Figure 3: Perfect planar 105-micron flap. The perpendicular incision manifests a 25-micron gap (femtosecond furrow) immediately after surgery. Unlike blade-based microkeratomes, femtosecond lasers excise tissue.

Descemet’s membrane stripping endothelial keratoplasty (DSEK) procedures have thrived under the guidance of AS-OCT (Figure 4). The donor tissue’s meniscus shape helped explain the hyperopic adjustment needed for IOL power calculation. In fact, our obsession with thinner and thinner tissue can be traced back to precise measurement of central corneal thicknesses.

Figure 4: Meniscus shape of DSEK tissue resulting in hyperopic surprise.

Also, AS-OCT can help determine when a sluggish post-op DSEK might benefit from re-bubbling. In fact, chronic haze in seemingly well-positioned grafts is easily explained when revealed by AS-OCT.

Seeing CK post-ops with AS-OCT confirmed my intuition that this hyperopic surgical maneuver should never become part of my armamentarium (Figure 5).

Figure 5: One week after successful CK for +2.00, this patient was pleased with his visual results. Tissue changes taunt the endothelium.

Anwar’s big-bubble deep anterior lamellar keratoplasty (DALK) procedure was demonstrated as superior to full thickness penetrating keratoplasty thanks to this same technology (Figures 6, 7). Although intrigued by femtosecond-assisted keratoplasty, this author has not been enamored by the technology (Figure 8).

Figure 6: Late postoperative image of DALK with excellent overall corneal contour with only mild and regular topographic astigmatism.

Figure 7: Wound mismatch is seemingly perfect full thickness penetrating keratoplasty.

Figure 8: Wound dehiscence in femtosecond-assisted keratoplasty.

AIDING DIAGNOSES

AS-OCT is not limited to the role of faithfully critiquing surgical results. It has helped make some important diagnoses (Figure 9). For example, identifying a slow-growing, non-melanotic elevated lesion at the limbus helps solidify the value of this diagnostic modality (Figure 10). Also, diffuse lamellar keratitis (DLK) after LASIK is a common complication with a well-established but invasive management protocol. In one case, AS-OCT prevented the unwarranted lifting of a patient’s flap (Figure 11) and helped avoid excessive steroids use.

Figure 9: A young woman with a slowly progressive pupillary appearance and unilateral glaucoma was diagnosed with ICE-syndrome after this image was obtained.

Figure 10: AS-OCT allowed for an early diagnosis in this case of conjunctival intra-epithelial neoplasia.

Figure 11: A healthy college student with uneventful LASIK developed severe decreased central acuity. AS-OCT helped eliminate the DLK diagnosis and prevented the interventions planned. This case of central toxic keratopathy is being managed conservatively.

AS-OCT’S POTENTIAL

I have owned six OCT units capable of anterior segment assessment, and each iteration bests the previous unit in resolution, speed and usability. Like the phoropter, slit lamp and topographer, I turn to the AS-OCT in virtually all of my patient encounters thanks to these expanding uses. OM