Original Article
Change in Central
Corneal Thickness after Trabeculectomy
Mustafa
Kamal Junejo, PS Mahar
Pak J Ophthalmol 2017, Vol. 33, No. 4
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See
end of article for authors
affiliations …..……………………….. Correspondence
to: Dr. P.S Mahar FRCS, Professor & Dean Isra postgraduate institute of ophthalmology Karachi Email:
Salim.mahar@aku.edu |
Purpose: To
determine the change in central corneal thickness after trabeculectomy. Study Design: Prospective
interventional study. Place and Duration of Study: Isra
postgraduate institute of ophthalmology, Al-Ibrahim eye hospital, Karachi.
January 2014 to June 2016. Materials and Methods: One
hundred Thirty eyes of 113 glaucoma patients who underwent trabeculectomy
were assessed for the change in central corneal thickness from January 2014
to June 2016. The study approval was granted by the institutional Ethical
Review committee. Patients’ characteristics, including demographics, clinical
details and management were recorded in a detailed Proforma. Results: The mean age of our patients was 53.3
± 12.3 years (Range 13 to 84 years). The mean CCT before trabeculectomy was 524.25 ± 38.53 µm. After 12 months follow-up the mean CCT
was 521.95 ± 38.25 µm with correlation of 0.855 which was not statistically
significant. Conclusion: In our study there was no statistically significant change in
central corneal thickness after trabeculectomy at the mean follow-up of 12
months. Keywords: Central Corneal Thickness,
Trabeculectomy, Mitomycin C. |
Intraocular pressure (IOP) is an important
factor in the management of glaucoma. Therefore, it is necessary that it should
be measured using a technique with a high degree of accuracy. As Goldman
applanation tonometry (GAT) is most commonly used and currently being the “gold
standard”for IOP measurement1, the variations in readings of IOP
measured with GAT are proportional to the central corneal thickness (CCT). However,
multiple studies have proven that there is variation in the mean CCT among
individuals with healthy eyes2,3, in patients suffering from
different types of glaucoma and due to presence of pseudo-exfoliation4.
In the absence of corneal disease, Goldman and Schmidt very rarely observed significant
CCT changes, thus assumed a normal CCT of 520 μm for their instrument5.
The corneal thickness can help to identify
the glaucoma suspects among primary open angle glaucoma (POAG), ocular hypertension
(OHT) and normal tension glaucoma (NTG)6. As CCT is a predictive
factor for the conversion of OHT to POAG that is why its measurement has been
recommended by the ocular hypertension treatment study (OHTS)7. Central
Corneal Thickness can help to diagnose the likelihood of disease progression8.
There are multiple treatment options
available for the management of glaucoma. Although medical therapy in the form
of topical eye drops is considered to be the treatment of choice9,
surgical intervention remains popular in selective cases. In developing
countries, where patients’ follow-up is poor and standard preparations of
topical medications are not available or expensive, surgery remains the first
line in management strategy10.
The most common glaucoma surgery performed
is trabeculectomy, a type of drainage surgery. The use of anti-metabolites
adjuncts like Mitomycin – C (MMC) has led to its improved success rate, but it
is not without its risks and complications11. Various randomized
trials have demonstrated the loss of corneal endothelial cells after MMC –
augmented trabeculectomy. The combined effect of MMC and per-operative surgical
trauma may lead to endothelial cell compromise, resulting in subtle corneal
edema, and subsequent alteration in CCT12. If this is proven then the
need for adjustment in postoperative IOP values corresponding to CCT change, or
use of Dynamic Contour Tonometry (DCT) which is less affected by alteration in CCT,
can be recommended13.
The aim of our study was to determine the change in CCT after MMC- augmented trabeculectomy in Pakistani
population. To the best of our knowledge no study till date has depicted the effect
of trabeculectomy on CCT in our population and with this sample size.
MATERIALS AND METHODS
This prospective single group cohort study
involving patients who underwent trabeculectomy for the different types of glaucoma
was carried out from January 2014 to June 2016 at Isra Postgraduate Institute
of Ophthalmology/Al-Ibrahim Eye Hospital, Karachi. All phakic patients who
underwent trabeculectomy during study period were included in the study. Those
patients who had history of ocular trauma, any corneal disease, or those who
underwent any other ocular surgery were excluded from the study. Patients who
underwent combined phacoemulsification and trabeculectomy procedure and those
who had per-operative complications were also excluded from the study. Cairns-type
trabeculectomy was performed in all cases.
A detailed proforma was generated with
pre-operative details of patients’ demographics, best corrected visual acuity as
recorded by Snellen’s chart, IOP measurement using Goldman’s Applanation
Tonometer (GAT), anterior segment examination with gonioscopy and dilated
fundus examination (+ 90 D and + 78 D) with emphasis on optic disc evaluation.
Central corneal thickness was measured in
all patients by ultrasonic pachymetry (US 800 Nidek). Topical Proparacaine
(Alcain-Alcon) was used to anaesthetize the eye before the procedure. On an
average 10 readings of central cornea were taken as the reference CCT of
patients. All the measurements were taken between 9:00 am and 11:00 am by the
same technician. Similar details were recorded at 1, 6 and 12 months
postoperatively after trabeculectomy.
The statistical analysis
was done using SPSS program, version 19.0. Categorical variables like gender
were described in terms of frequencies and percentages. Continuous variables
like age and CCT were described in terms of mean ± standard deviation. Paired
t-test was used to analyze the change in central corneal thickness measured pre-operatively
and post-operatively. P value of < 0.05 was considered to be statistically
significant.
RESULTS
One hundred thirty (130)
eyes of 113 patients were included in the study. The mean age of patients was 53.3 ± 12.3 (range 13 – 84 years). Out of 113
patients 66.2% were male and 33.8% were female. The pre-operative
characteristics including age, refractive error and gender are summarized in
Table 1.
Table 1: Biographic data, frequency of eyes, gender predilection, refractive
error n = 130.
|
|
Frequency |
Percentage |
|
Eye |
|
|
|
Right |
64 |
49.2 |
|
Left |
66 |
50.8 |
|
Gender |
|
|
|
Male |
86 |
66.2 |
|
Female |
44 |
33.8 |
|
Refractive Error |
|
|
|
Emmetropia |
01 |
0.8 |
|
Myopia |
84 |
64.6 |
|
Hypermetropia |
45 |
34.6 |
The mean CCT of
our study population before trabeculectomy was 524.25 ± 38.53 µm. The mean CCT
postoperatively was 523.75 ± 38.12 µm, 521.53 ± 37.95 µm and 521.95 ± 38.25 µm
at 1, 6 and 12 months respectively. Using paired t-test the p value of 0.855 was obtained, suggesting that the
difference between CCT measurement before and after surgery at 12 months
follow-up was not statistically significant. The results are summarized in
Table 2.
The preoperative IOP in
our group of patients was 25.4 ± 8.0 with range of 16 – 39 mmHg. The mean IOP
at 12 months follow-up was 12.9 ± 4.0 with range of 8 – 19 mmHg. Using paired
sample t-test p value of 0.0003 was obtained which is statistically significant
(Table 3).
DISCUSSION
The corneal
thickness has been reported to change with the age and after intraocular
procedures such as cataract surgery. Multiple intraocular surgeries are also
shown to cause changes in CCT14. However, there are scanty reports
in the literature about changes in CCT after MMC augmented trabeculectomy.
Since 2002 when
OHTS was carried out, apart from the other factors, thin corneas were labeled
as poor prognostic factor for the progression of glaucoma15. In
2007, a report by American Academy of Ophthalmology concluded that CCT is a
risk factor for progression from ocular hypertension to POAG16. It
is now well recognized
Table 2: Pre-operative and post-operative central corneal thickness change.
|
Pre-operative |
Mean CCT |
524.25
± 38.53 µm |
|
Post-op Visit |
Mean CCT |
P value |
|
1
month |
523.75 ± 38.12 µm |
0.855 |
|
6
months |
521.53 ± 37.95 µm |
|
|
12 months |
521.95 ± 38.25 µm |
Table 3: Pre-operative and post-operative Intra ocular pressure change.
|
IOP (SD) mm Hg |
Mean |
Range |
p value |
|
Pre-operative (SD) |
25.4
± 8.0 |
16 –
39 |
0.0003 |
|
Post-operative (SD) |
12.9
± 4.0 |
8 – 19 |
that abnormally
thick corneas (> 530 µm) can overestimate the IOP and thin
corneas (< 520 µm) can erroneously register less IOP.
It has been
observed that CCT is greater in the early morning at the wakeup time due to
possible hydration of the cornea during the night but it goes back to its
normal thickness 2 hours after eye opening17. For this reason, in
our study CCT measurements were taken between 9 am to 11 am to avoid any
diurnal variation.
Multiple authors
have investigated the effect of IOP lowering drugs on the CCT. Sen et al18
in their study of group of patients using Latanoprost and Bimatoprost found
mean reduction of 2.7 ± 6.9
µm at 6 months, 6.6 ± 7.4 µm at 12 months and 10.7 ± 14.2 µm at the end of 24 months
follow-up in patients using Latanoprost. The amount of CCT reduction for Bimatoprost
group at same visits were 6.8 ± 5.7
µm, 11.3 ± 11.9 µm and 15.8 ± 10.2 µm. Lass and colleagues19
found the decrease of approximately 6 µm in CCT with the use of Latanoprost and
fixed combination of Timolol and Latanoprost at the end of 12 months follow-up.
Whether decrease in CCT by antiglaucoma medication is due to direct effect of
the compound or due to lowering of the IOP is not known. If the change in CCT
is due to lowering of the IOP then this should also occur after trabeculectomy.
In our cohort of 113 patients, however, there was no change in the CCT at the
end of 12 months follow-up irrespective of postoperative IOP variation.
Williams and
coworkers20 postoperatively evaluated patients undergoing primary or
repeat trabeculectomy with measurement of CCT and corneal endothelial cell
count. Their conclusion was that neither CCT nor endothelial cell count changed
to statistically significant degree as compared to their pre-operative value.
A study by
Wickham and colleagues21 has shown that the measurement of CCT requires
multiple readings to avoid any significant variability. For this reason, an
average of 10 readings was taken for our patients.
Ultrasonic
pachymetry has been shown to be accurate and reliable22. As it can
be subject to inter observer bias so all our CCT readings were carried out by
the same technician.
Soro-Martinez et
al23 assessed CCT changes in 80 eyes of 62 patients between 60 to 83
years of age. Mean (± SD) CCT was 542 ± 38.15 µm in the control group and 522 ± 34.70 µm, 540 ± 34.22 and 51.9 ± 32.91 in group I (Trabeculectomy), group II
(Combined Trabeculectomy with phacoemulsification) and group III (Trabeculectomy
followed by phacoemulsification). There was no significant difference between
all groups showing changes in CCT postoperatively. Storr-paulsen and coworkers24
looked at 14 eyes of 14 patients undergoing MMC augmented trabeculectomy in
regard to change in the corneal endothelial cell density and CCT. The
pre-operative value of CCT was 527 ± 32 µm and remains at 531 ± 30 µm at 12 months after surgery suggesting no significant
difference statistically.
Though there was significant decrease in IOP in our patients at 12 months
follow -up but this was not related to the change in CCT.
CONCLUSION
In our prospective study we found that at 12
months follow-up there was no statistically significant change in central corneal
thickness. As this is a single center study, we
think that similar studies should be conducted onlarge number of patients’population
and in coalition with other centers to confirm our findings.
Author’s Affiliation
Dr. Mustafa Kamal Junejo
MRCS, Senior instructor/ Assistant clinical
& Academic coordinator.
Dr. P.S Mahar
FRCS, Professor &
Dean
Role of Authors
Dr. Mustafa Kamal Junejo
Data collection &
writing part of manuscript.
Statistical analysis
Dr. P.S Mahar
All glaucoma surgery
& writing part of manuscript.
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