Therapeutic Corneal Laser Concepts
There are essentially 3 types of irregular corneas:
Localized
Global
Localized on Global
Localized Irregularity
Here we have small defects with micro-irregularities of the stromal anterior surface. This causes multiple small light deflections, resulting in poor vision. This supports the statement: irregularity is worse than opacity for vision.
The following is a case with a rare corneal dystrophy bilateral, Thiel-Behnke Corneal Dystrophy. This is a genetic condition and unavoidable. There is a deposition of a hyaline-like material within the basement membrane of both corneas. This lady has had bad vision for 15 years. BCVA (with best spectacles) tested at 0.4 (6/15). We only removed 8µm of tissue together with a smoothing laser. Day 1 she could already see 6/3 uncorrected (without spectacles). Note, the aim was not to remove the spectacles, but this proves that there was no global irregularity causing refractive error as the laser profile did not include a refractive component.
DIFFUSE LIGHT PHOTO BEFORE SURGERY. NOTE THE "CLEAR" NATURE OF THE CORNEA.
ZOOMED IN CORNEAL PHOTO OF "BUBBLES" CAUSING THE LOCALIZED IRREGULARITIES.
PRE-OP CORNEAL OCT. NOTE THE IRREGULARITY OF THE ANTERIOR STROMA BELOW THE IRREGULAR EPITHELIUM.
CORNEAL OCT IMMEDIATELY POST-OP. SMOOTH ANTERIOR STROMAL SURFACE. NOTE THE ABSENCE OF EPITHELIUM BUT WITH A CONTACT LENS IN SITU.
Global Irregularity
With this we have irregular astigmatism. The surface is mostly smooth, but the shape of the surface induces refractive errors that cannot be corrected with regular corrective systems like spectacles or contact lenses.
The following is such a case. This lady had RKs (Radial Keratotomies) in the 90's. The corneal shape is incompatible with normal means of correction. Pre-op she had poor quality BCVA 0.7 (6/9) with a refraction of +4.75/-2.00. Post-op BCVA improved to great quality 1.5 (6/3) with a refraction of -2.50/-1.50. Now we have plenty of options to correct the refractive error including spectacles, contact lenses, implantable contact lenses and lens surgery. We can also target specific focus points as needed with these surgeries with great accuracy. We do not target plano refraction with these corrections, because we aim to save as much corneal tissue as possible.
BEFORE (TOP) AND AFTER (BOTTOM) SURGERY CORNEAL TOPOGRAPHY OF THE SAME EYE. NOTE THE TRANSITION FROM 21D TO 105D BEFORE SURGERY. NORMAL CORNEAS HAVE <1.5D DIFFERENCES.
Localized on Global Irregularity
A combination of the two problems above.
The last case is of a 30year old male dentist who had LASIK 10 years ago. He then developed LASIK ectasia as a result of older flap technology. The old Hansatome flap created was simply just too deep for his cornea. He underwent crosslinking which stabilized the cornea beautifully (not much shape change with this process expected), but unluckily developed a corneal ulcer after the procedure. With scleral contact lense he was still able to get to 1.5 (6/3) vision. But became intolerant to them and seeked further help. BCVA with specs before surgery was 0.3 (6/18). Post-surgery he could see 1.5 (6/3) with spectacles or soft contact lenses.
The corneal ulcer left a "valley" in the cornea adjacent to the "peak" of the ectasia. Together this formed the global irregularity. The crosslinking (which needed to be done) or previous poor flap positioning caused a "flap crunch syndrome". Essentially the flap shrinks with microfolds forming at the level of Bowman's layer, just below the epithelium. A laser profile to take care of the global irregularity was done first, then a smoothing type of laser. All within 5 minutes.
We can consider a second laser to remove even more of the global irregularity. Attempting this at the first setting is possible, but can be tissue heavy. Safer to then split the two procedures.
Notice the difference with the second case. In the second case the global irregularity is much worse (21D to 105D). Here it is only 34D to 51D, but the vision in case 3 was worse before surgery. This is as a result of the microfolds causing local irregularities, as in case 1. This again proves: irregularity is worse than opacity for vision.
OCT OF A SINGLE MICROFOLD.
NOTE THE STEP AND IRREGULARITY IN BOWMAN BEFORE (TOP) AND AFTER (BOTTOM) CORNEAL TOPOGRAPHY. NOTE THE TRANSITION FROM 34D (ULCER AREA) TO 51D (ECTASIA AREA).
We have many options at our disposal. But, we are limited with tissue volume. Each individual case is different and needs an individual treatment plan.