JOJ Ophthalmology - Juniper Publishers
Opinion
A high degree of attention is commonly paid to the
accuracy in determining the amount of corneal astigmatism for toric IOL
implants, however when it comes to refractive laser surgery corneal
astigmatism does not receive such a high consideration in either the
treatment or the postoperative result. Most refractive laser treatments
are based on the refraction exclusively – manifest or wavefront,
regardless of pre-existing corneal astigmatism. Consideration of the
magnitude and the orientation of the astigmatism on the cornea
preoperatively should be done routinely so that the patient can be
advised if all the astigmatism can be corrected and the likely
astigmatic target outcome.
The difference between the refractive cylinder at the
corneal plane and the corneal astigmatism is termed the Ocular Residual
Astigmatism (ORA) [1,2] and is expressed in diopters together with an
axis. An ORA greater than 0.75D is considered significant [1,2]. If the
refractive laser treatment is based on refractive parameters alone then
all the ORA will be left on the cornea postoperatively to neutralise
this internal aberration. The effect of leaving excess astigmatism on
the cornea, and in 7% of cases this can be greater than the corneal
astigmatism measured preoperatively [1], is that the quality of vision
can be impaired due to glare, ghosting, starbursts and halos (known as
GASH). So, the patient may end up having 20/20 unaided vision in
daylight, however, is unhappy as a result of the quality of vision,
particularly under low light where these symptoms become more
problematic with a dilated pupil. The method of Vector Planning
systematically combines the corneal astigmatism and the refractive
cylinder in the treatment plan by first calculating the ORA and then
apportioning this to the cornea and the refraction to minimize the
amount of corneal astigmatism postoperatively without compromising the
refractive cylinder [3,4].
The combination of a high ORA preoperatively
(>1.00D), high corneal astigmatism (>1.00D) postoperatively and
any one
or more of these symptoms of GASH comes under the condition known as
PALS: Predictable - astigmatic outcomes if the ORA is calculated
preoperatively, Avoidable – as incorporating corneal astigmatism into
the refractive treatment, such as in Vector Planning, which will reduce
the corneal astigmatism remaining, LASIK - can apply to PRK and SMILE
procedures as well as LASIK, Surprise - unexpected GASH symptoms. There
have been a significant number of unhappy patients with symptom(s) of
GASH writing to the FDA and the New York Times expressing their
dissatisfaction after LASIK [5].
Dr Alpins presented a case study at the recent AECOS
meeting in Aspen to demonstrate this condition. A 34-year-old male
attended complaining of symptoms of glare, ghosting, starbursts and
halos as well as a reduction in contrast sensitivity after bilateral
LASIK. To reduce his symptoms, he shone his cell phone torch light
directly at his eyes to constrict his pupils and be able to drive at
night. His unaided vision was 20/20 in each eye however the amount of
corneal astigmatism postoperatively was >1.00D in each eye (Figure
1).
To reduce the incidence of PALS post excimer laser surgery it
is imperative that the ORA is calculated routinely preoperatively
and addressed using Vector Planning. Currently none of the
excimer laser systems consider how much astigmatism will
remain on the cornea after LASIK and don’t place any limit
on corneal astigmatism post LASIK. This can be achieved by
employing Vector Planning. The laser manufacturers need
to make this paradigm available to the surgeon so that the
maximum amount of astigmatism can be treated, reducing
the amount of corneal astigmatism remaining postoperatively.
Only then will the satisfaction rate of laser surgery consistently
improve closer to 100%. Until this facility is provided by lasers
currently in use, refractive surgeons can use a free application
at www.assort.com to calculate the ORA and the appropriate
Vector Planning treatment.
Prof. Noel Alpins and Mr George Stamatelatos report a
financial interest in ASSORT Surgical Management Systems
which holds trademarks in Vector PlanningTM.
To Know more about JOJ Ophthalmology
No comments:
Post a Comment