INTRODUCTION:
There
is a world-wide increase in the number of individuals opting for corneal
refractive surgery procedures, such as photorefractive keratectomy (PRK) and Laser-Assisted
In Situ Keratomileusis (LASIK). Since these patients are started on intense
steroid eye-drops in the immediate post-operative period, it is practical to
understand the implications of steroid-induced ocular hypertension in this
scenario.
RISK
OF GLAUCOMA IN MYOPIC PATIENTS :
Myopic
patients have a high risk of glaucoma. This is attributed to the thinned out
and stretched lamina cribrosa and scleral canal.
Intraocular
pressure (IOP) exerts a force on the lamina situated at the scleral canal. The
scleral canal, where the lamina is located, represents a relatively weak area
in the wall of the globe.
IOP,
as a force, can be considered to consist of two vector components. First, there
is a posterior force vector compressing the laminar plates or pushing it
outwards through the scleral canal. There is a second force vector contributed
by the stress in the eye wall which pulls radially on the scleral insertion of
the lamina. This latter component contributes, in large part, to the stress
within the scleral wall.
LaPlace's equation for a spherical shell relates the pressure and radius to the
wall stress:
s = (pi-pc)R/2h
In
the equation:
s = stress
(pi-pc) = transmural pressure (or difference between internal and external
pressure)
R = radius of sphere
h = thickness of sphere
It follows from LaPlace's equation that, for a given transmural pressure, the
larger the radius of the globe (i.e., the greater tile axial length), the greater
the wall stress and, hence, the greater the
potential distorting force on the optic nerve.
This may in part, explain why eyes with axial myopia may have increased risk
for developing glaucoma.
STEROID
INDUCED OCULAR HYPERTENSION FOLLOWING CORNEAL REFRACTIVE SURGERY:
Myopic
patients usually undergo corneal refractive surgeries, which involves
application of laser to the cornea in order to change the contour of the
cornea. In this procedure high power laser energy is delivered to the
cornea to ablate it. This causes an inflammatory reaction which needs to be
controlled by intensive topical steroids, such as every two-hourly per day.
Looking
at the risk of myopic patients to develop glaucoma, it is of practical importance
to keenly follow-up these patients for any steroid-induced ocular hypertension.
REFERENCES:
In
a study from Iran, myopic PRK was performed on 506 eyes of 269 patients. Preoperatively,
spherical equivalent refractive error ranged from −1.00 to −5.00 diopters (D)
and cylinder was less than 4 D.
Ocular
hypertension developed in 40 (7.9%) eyes overall, which occurred in 16 eyes
(40%) 2–3 weeks postoperatively (mean IOP=23.5±3.0mmHg), in 20 eyes (50%) after
4–6 weeks (mean IOP=25.1±4.2 mmHg) and in 4 eyes (10%) 8–12 weeks following PRK
(mean IOP=29.0±3.1 mmHg). There was no correlation between the level of IOP
rise and preoperative spherical equivalent refractive error. IOP recovered to
normal in all eyes after discontinuation of topical steroids and initiation of
anti-glaucoma medications. Mean duration of IOP normalization was 28.5±27.7
(range 7–108) days and no instance of steroid-induced glaucoma was observed in
any patient.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3589213/
In
another study from Hungary, 43 paients developed ocular hypertension following
PRK. They were divided into three groups. The first received only timolol, the
second timolol and dorzolamide and the third only dorzolamaide. The second
group treated with a combination of dorzolamide and timolol had the best ocular
hypotensive response.
https://pubmed.ncbi.nlm.nih.gov/11489570/
Steroid
induced IOP elevation has been noted by Seiler as well as Machat,
occurring in 8 to 32% of treated eyes. Shimizu et al reported the
incidence of post-PRK IOP rise (>21mmHg) to be 8.9%. Gartry has reported a
post-PRK steroid response of 12% in their series.
Seiler
T, Holschbach A, Derse M, Jean B, Genth U. Complications of myopic
photorefractive keratectomy with excimer laser. Ophthalmology. 1994;101:153–160.
Machat
JJ, Tayfour F. Photorefractive keratectomy for myopia: preliminary results in
147 eyes. Refract Corneal Surg. 1993;9(suppl):S16–S19.
Shimizu
K, Amano S, Tanaka S. Photorefractive keratectomy for myopia: One-year
follow-up in 97 eyes. Refract Corneal Surg. 1994;10(Suppl):S178–187.
Gartry
DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive
keratectomy.18-month follow-up. Ophthalmology. 1992;99:1209–1219.
CONCLUSION:
In
view of the significant number of patients developing elevated IOP in post-PRK
patients, these individuals should be monitored no later than 2 weeks after
initiation of corticosteroid treatment.
Javadi
recommends substituting potent steroids such as betamethasone with weaker
agents with lesser propensity for IOP elevation such as fluorometholone.
