Glaucoma surgery and induced astigmatism: a systematic review
Chan and Kong Eye and Vision
Glaucoma surgery and induced astigmatism: a systematic review
Helen H. L. Chan 0
Yu Xiang G. Kong 0
0 Royal Victorian Eye and Ear Hospital , 32 Gisborne St, East Melbourne, VIC 3002 , Australia
Background: The refractive outcomes of glaucoma surgeries, particularly their effect on astigmatism, are incompletely understood. Main body: Trabeculectomy is associated with a considerable amount of with-the-rule astigmatic change in the immediate postoperative period. This is followed by a gradual against-the-rule shift. These changes are altered with the use of mitomycin C (MMC). Non-penetrating surgery such as deep sclerectomy is also associated with a similar or smaller degree of induced astigmatism. Minimally invasive glaucoma surgery appears to be astigmatically neutral. There is no clear evidence regarding refractive outcomes of glaucoma drainage device surgery. Conclusions: Induced astigmatism may account for a reduction in unaided visual acuity in the early postoperative period following a successful trabeculectomy. These changes appear to stabilise at 3 months, and it would be prudent to defer the prescription of new glasses until this time. If sequential cataract surgery is to be performed, toric intraocular lenses can be a useful option for astigmatic correction.
Filtration surgery; Trabeculectomy; Astigmatism; Refractive outcome
Glaucoma is the leading cause of irreversible blindness
]. Glaucoma surgery is necessary when
maximally tolerated medical therapy is not sufficient to
control disease progression [
]. Trabeculectomy is the
most commonly performed glaucoma operation [
Other surgical techniques include glaucoma drainage
devices, non-penetrating surgery such as deep
sclerectomy, and newer minimally invasive glaucoma surgery.
Despite the popularity of such procedures, the impact of
glaucoma surgical techniques on refraction, particularly
astigmatism, are incompletely understood. It is common
for patients undergoing glaucoma surgical procedures to
have a brief period of reduced visual acuity in the early
postoperative period, and for some patients, this can
persist long-term. While the change in vision can be due to
the reduction in intraocular pressure (IOP), several studies
have examined the direct effect of glaucoma surgery on
astigmatism, suggesting that changes to corneal
topography and refractive status can play a role. The purpose of
our review is to summarise the existing literature on the
effect of glaucoma surgery on astigmatism and its
significance for vision rehabilitation.
Type and amount of induced astigmatism
Hugkulstone was the first to examine corneal astigmatism
following trabeculectomy [
]. Keratometry was performed
pre-operatively and at 1, 3, 14 and 28 days
postoperatively in 10 patients who underwent trabeculectomy.
He reported a statistically significant reduction in vertical
corneal radius from a mean of 7.71 mm preoperatively to
7.36 mm at day 1. This steepening of the vertical meridian,
that is, with-the-rule astigmatism (WTR), was observed at
all post-operative time points up to the last follow-up at
28 days. A corresponding increase in horizontal corneal
radius was transiently observed but disappeared by day 14.
There was no significant alteration in the angle of the
steep meridian. Of note, Hugkulstone found no
correlation between the keratometric measurements and IOP.
He also did not observe any correlation between the
changes in corneal radii and final visual acuity and
concluded that the induced astigmatism did not appear
These findings were replicated by Cunliffe et al., who
measured refraction and keratometry in 16 patients
undergoing trabeculectomy [
]. Vertical corneal radius
was reduced from a preoperative mean of 7.69 mm to
7.56 mm at week 1. This WTR change in corneal
astigmatism persisted at week 3 and 8 of follow-up but had
returned to preoperative values at final follow-up at
10 months. Horizontal corneal radius was increased at
weeks 1 and 3 however, had normalised from week 8
onwards. They reported no difference in the axis of the
vertical corneal curvature. In addition, Cunliffe et al.
demonstrated a myopic shift related to altered anterior
chamber depth (ACD), with a 1 mm change in ACD
giving approximately 2 diopters (D) spherical change.
They proposed that the induced refractive error, i.e.,
myopia and astigmatism, accounted for the reduced
unaided visual acuity commonly observed in the immediate
Later, Rosen et al. extended these findings when
comparing corneal topography and keratometry in 8
eyes from 6 patients undergoing trabeculectomy [
They confirmed a WTR shift of 1.5 to 2.5 D of cylinder
up to 12 weeks post operatively and further reported
that the amount of induced steepening was
underestimated by keratometry compared to topography.
In a larger study measuring corneal topography in 29
patients undergoing trabeculectomy, Claridge and
colleagues observed more complex changes [
they confirmed an overall trend towards vertical
steepening and WTR astigmatism, they identified 3 subgroups
characterised by 1) superior corneal steepening, 2)
superior corneal flattening, and 3) irregular changes.
They also demonstrated that in some patients, the
topographic changes induced by surgery are present for
up to 12 months.
The role of mitomycin C
More recent studies have examined corneal astigmatism
following augmented trabeculectomy. Use of mitomycin
C (MMC) appears to be associated with longer lasting
Hong et al. compared 18 eyes undergoing
trabeculectomy with MMC, to 14 eyes having trabeculectomy
without MMC at 1 week, 1, 3 and 12 months postoperatively
]. Use of MMC was associated with less induced WTR
astigmatism in the early postoperative period (1.01 D
with MMC compared to 2.63 D without MMC at
1 month). Both groups subsequently underwent a
further against-the-rule (ATR) change, however, the group
without MMC stabilised at 3 months, while the group
with MMC continued to experience ATR shift to time of
final follow-up at 12 months (0.22 D with MMC versus
1.24 D without MMC at 3 months; −0.34 D with MMC
versus 1.42 D without MMC at 12 months).
Further, Hong and colleagues compared 5 eyes having
the MMC-augmented triple procedure (extracapsular
cataract extraction and trabeculectomy) to 9 eyes having
the triple procedure without MMC. Use of MMC was
associated with a smaller amount of induced WTR
astigmatism at 1 week postoperatively (1.81 D versus
4.50 D) however, both groups were similar by 1 month
(1.41 D versus 1.13 D). The triple procedure/MMC
group continued to exhibit ATR shift up to time of last
follow-up at 12 months, while the amount of induced
astigmatism in the triple procedure/no MMC group
stabilised at 3 months (−0.49 D with MMC versus 0.05
D without MMC at 3 months; −1.73 D with MMC
versus 0.13 D without MMC at 12 months). Hong et al.
postulated that the induction of WTR astigmatism was
due to wound healing and that use of MMC may reduce
the amount of induced astigmatism by inhibition of
fibroblast proliferation and other effects on the wound
Use of MMC may also affect the duration of
observable astigmatic change following glaucoma surgery.
Kook et al. examined 18 eyes of 16 patients at 1, 3, 6,
and 12 months following trabeculectomy using MMC
]. A concentration of 0.4 mg/mL was applied for 2 to
5 min. Confirming previous studies, they found a mean
WTR change in corneal astigmatism of 1.23 D at
3 months postoperatively, followed by a period of ATR
change. Interestingly, changes in corneal astigmatism
were still present at 6 and 12 months (0.94 D and 0.65
D respectively), much longer than previously
demonstrated without MMC. All eyes were found to
conform to this pattern of early WTR followed by
persisting ATR change, regardless of whether they had
WTR or ATR astigmatism preoperatively.
In contrast, a larger study of 47 eyes demonstrated
earlier resolution of astigmatic change with MMC use.
Delbeke et al. reported only a small mean WTR change in
corneal astigmatism of 0.35 D at 1 month, which had
almost disappeared at 3 months (0.18 D) and remained
stable at 6 months postoperatively [
]. MMC was used at
a concentration of 0.2 mg/mL for 2 min, compared to
0.4 mg/mL for 2 to 5 min, and the authors suggested that
this lower dose and exposure time accounted for the
differences in residual MMC action on wound healing.
Ex-press miniature glaucoma implant
Ex-PRESS Miniature Glaucoma Implant surgery (Alcon
Inc.) is a commonly performed minimally invasive
glaucoma surgery. While its implantation also requires
formation of a scleral flap and conjunctival suturing, the
surgery appears to be astigmatically neutral in the long
term. Hammel and colleagues examined 19 eyes
undergoing Ex-PRESS implant surgery with Pentacam at
regular intervals up to 3 months postoperatively [
They reported a transient increase in both anterior and
posterior corneal astigmatism on day 1 (from 2.6 to 4.7
D anterior corneal astigmatism; 0.4 to 0.9 D posterior
corneal astigmatism) that resolved by the time of
followup at 3 months. Of note, they demonstrated that the
anterior and posterior corneal astigmatism were correlated
with IOP and ACD, suggesting that the short-lived
corneal changes were due to IOP fluctuations.
Glaucoma drainage device
To our knowledge, there are presently no studies
examining refractive outcomes following glaucoma drainage
device (GDD) surgery.
Francis et al. studied axial length following Baerveldt
tube implantation but did not specifically examine
refractive outcomes [
]. They found a variable reduction in
axial length dependent on the amount of IOP reduction
and final postoperative IOP. They emphasised that this
alteration in axial length must be taken into account when
considering intended refractive outcome, particularly in
conjunction with cataract surgery as changes in axial
length postoperatively can lead to refractive surprise.
More recently, Tzu et al. examined refractive outcomes
in combined phacoemulsification and glaucoma surgery
(22 eyes combined trabeculectomy, and 21 eyes
combined GDD) [
]. 77% of the combined trabeculectomy
eyes and 71% of the combined GDD eyes achieved their
defined target refractive outcome (between −1.00 D and
+0.50 D) at 6 months postoperatively, compared to 85%
of a control group who had phacoemulsification alone.
Unfortunately, keratometry data was limited to 22 eyes.
They found a mean induced astigmatism of 1.31 D in
the combined surgery group however, this did not
differentiate between the combined trabeculectomy and
combined GDD populations.
Egrilmez and colleagues examined 10 eyes undergoing
deep sclerectomy with a non-absorbable implant
]. Compared to a trabeculectomy control
group, the deep sclerectomy group experienced less
induced WTR astigmatism in the early postoperative
period and less ATR shift thereafter (0.62 D versus 1.06
D at 3 months; 0.62 D versus 1.24 D at 6 months). They
postulated that the thin layer of trabecular meshwork
tissue that is left intact in non-penetrating surgery may
help reduce the amount of induced corneal steepening.
However, a larger study suggested that both deep
sclerectomy and trabeculectomy induced considerable
postoperative astigmatism. El-Saied et al. compared 60
eyes undergoing deep sclerectomy with MMC to 60 eyes
undergoing trabeculectomy with MMC [
]. They reported
that both groups exhibited a statistically significant and
similar WTR change in astigmatism of 0.67 D in the deep
sclerectomy group and 0.82 D in the trabeculectomy group
at 6 months postoperatively.
In addition, Corcostegui et al. studied refractive
outcomes following combined cataract surgery and deep
sclerectomy with an absorbable implant (SKGel®) in 38
eyes and reported that the mean change in astigmatism
was less than 0.50 D and not statistically significant [
Minimally invasive glaucoma surgery
Currently, there is insufficient information on the
astigmatic effects of other minimally invasive glaucoma
surgeries such as iStent (Glaukos), Cypass stent (Alcon) and
Xen gel-stent (Allergan). Given iStent is trans-trabecular
and Cypass stent drains into the supraciliary space, both
with no requirements for conjunctival suturing and
antimetabolite application, it is expected that it will not lead
to astigmatic or refractive changes per se, apart from the
surgically induced corneal flattening from a temporally
placed keratome wound, which often serves as the main
wound for a combined phacoemulsification procedure.
On the other hand, Xen gel-stents are often inserted
with MMC application. While there is no requirement
for conjunctival suturing or scleral flap formation,
filtration bleb formation is an intrinsic part of the surgery
and as such the long-term effects of the operation on
corneal topographic changes are unclear.
Mechanisms of astigmatism induction
The reasons for the WTR astigmatism observed after
trabeculectomy are incompletely understood. In fact,
Hugkulstone had originally anticipated that
surgicallyinduced wound gape would lead to an increase in
vertical corneal radius and therefore ATR change. However,
to explain the observed WTR changes, he proposed that
the posterior placement of the incision restricted the
amount of gape and that the overlying scleral flap and
sutures provided support limiting any flattening of the
vertical meridian [
Cunliffe et al. postulated that the removal of tissue
(2 × 4 mm block) from under the scleral flap allowed
the corneal edge of the trabeculectomy opening to sink
slightly, causing a steepening in the vertical corneal
The role of sutures and suture lysis in astigmatism
induction has been discussed in the literature. Dietze
suggested that over-tight scleral flap sutures at 12 o’clock
could cause local tissue compression and therefore
steepening in the vertical meridian [
Delbeke et al. found no difference in astigmatism between
groups that had undergone laser suture lysis of scleral
flap sutures (19 eyes in the first week, 17 eyes in the
second week), and those that did not undergo suture lysis
(of a total of 47 eyes) [
]. Rosen et al. also demonstrated
that suture lysis of the scleral flap sutures had no effect on
corneal astigmatism [
]. Instead, they suggested that
excessive cautery may have caused scleral contraction and
therefore steepening in the meridian of surgery.
Claridge et al. agreed that this superior steepening
could be attributed to a combination of excessive
cautery or over-tight sutures, and suggested that this
could be further influenced by a large drainage bleb or
induced ptosis [
]. Indeed, the pressure of the eyelid and
bleb on the cornea may cause changes to the corneal
curvature that are dependent on IOP, especially in the
early postoperative period [
The influence of IOP and hypotony on induced
postoperative astigmatism is of particular interest. Kook et
al. found a statistically significant correlation between
lower postoperative IOP and reduced postoperative axial
length (R2 = 0.40, P = 0.000) in their study of 18 eyes
following trabeculectomy with MMC [
]. However, they
did not specifically report on the relationship between
postoperative IOP and astigmatism.
Interestingly, Delbeke et al. demonstrated a significant
negative correlation between postoperative IOP and
astigmatism in their study of 47 eyes undergoing
trabeculectomy with MMC [
]. They reported a correlation
between lower IOP and greater amounts of induced WTR
astigmatism at 1 month postoperatively (R2 = −0.49,
P = 0.001). An IOP of 3 was associated with a WTR
change of approximately 1 D, whereas an IOP of 11 was
astigmatically neutral (Delbeke et al., Fig. 2). This
correlation was not present at 6 months postoperatively
(R2 = −0.07, P = 0.656). Delbeke and colleagues
hypothesised that the eye is likely more susceptible to deformation
and therefore greater astigmatic changes during times
when the IOP is lowest.
Intraoperative steps to avoid astigmatism induction
Several modifications to intraoperative technique have
been proposed to reduce the amount of astigmatism
induced during trabeculectomy. These steps aim to
mitigate the causes of astigmatism postulated above.
A smaller sclerostomy has been advocated to reduce
wound gape and “sinking” due to tissue removal [
This can be achieved by use of a standardised punch
such as a Kelly punch, using a “one punch only”
technique, or even implantation of an Ex-PRESS shunt. Care
may be taken to ensure a narrow, short scleral flap and
to keep the limbal ring intact when making the radial
Scleral flap sutures that are over-tight or of unequal
tension should be avoided [
17, 18, 20
Excessive cautery may be limited by reducing the
energy, or use of a point-tip or wet field cautery,
particularly at the limbus [
6, 10, 18, 19
As with all trabeculectomy procedures, overhanging
and intracorneal dissection of the bleb should be
avoided as this can cause significant astigmatism and
reduced visual acuity long-term. This may be helped
by the use of a fornix-based rather than limbus-based
conjunctival flap, and broader, more posterior
application of MMC [
7, 10, 21
The creation of corneal grooves in which to embed
conjunctival closure sutures did not appear to influence
postoperative astigmatism [
An aberrometer may even be used to assess any
induced corneal changes intraoperatively [
Postoperative management of induced astigmatism
Refraction may explain some of the reduction in unaided
visual acuity observed following trabeculectomy. It is
useful to know that the induced astigmatism tends to
stabilise, if not completely resolve, by 3 months
postoperatively. It is therefore prudent to advise patients of an
anticipated waiting period at least 3 months prior to a
stable refraction [
]. Additionally, after this waiting
period, the amount of residual astigmatism may not be
significant enough to affect visual acuity.
If astigmatic correction is required, the prescription of
new glasses is a useful option. Contact lens wear is best
avoided following trabeculectomy due to the possible
increased risk of infection [
Sequential cataract surgery, following a suitable
interval after trabeculectomy and careful assessment of bleb
function, allows the option of astigmatic correction with
the implantation of a toric intraocular lens (IOL) [
However, toric IOLs may be less suitable for patients
with pseudoexfoliative glaucoma due to the risk of
capsular bag decentration [
Incisional corneal surgery, such as limbal relaxing
incisions delivered by femtolaser laser, is another option.
However, this is associated with a transient elevation in
IOP that is higher in glaucomatous compared to
nonglaucomatous eyes [
]. It may be prudent to avoid
femtosecond laser treatment in glaucomatous eyes, particularly
if a contact applanation patient interface is used [
Similarly, if laser corneal astigmatism management is
chosen, PRK (photorefractive keratectomy) may be
preferred over LASIK (laser-assisted in-situ keratomileusis)
to avoid the IOP spike during LASIK flap creation [
Following the procedure, one must also consider the
impact of reduced corneal thickness on subsequent IOP
These strategies may be more difficult to employ
during combined surgery. IOL power calculations must take
into account the variable reduction in axial length
observed following combined cataract extraction and
trabeculectomy or GDD (dependent on postoperative
]. It may be helpful to set a slightly myopic
target refraction . Some authors suggest avoiding the
use of toric IOLs during combined surgery altogether
due to the difficulty in predicting the variable amount of
subsequent astigmatism [
]. Doctors need to be
diligent in preoperative counselling regarding refractive
outcomes after combined surgery, particularly in the
initial postoperative period. Complications, such as
hypotony, may cause even greater alterations in axial length
and induced astigmatism .
Trabeculectomy can cause a significant amount of
induced astigmatism. This may partially account for the
reduction in unaided visual acuity commonly observed.
WTR change occurs in the early postoperative period,
followed by gradual ATR shift. These effects can be
modulated with the use of MMC and its action on the
wound healing process and may depend on the
concentration and duration of application.
These changes appear to stabilise at 3 months, and it
would be prudent defer permanent astigmatic correction
until that time.
Deep sclerectomy can be associated with a similar or
smaller degree of induced astigmatism. MIGS appears to
be astigmatically neutral. There is no clear evidence
regarding the astigmatic outcomes following glaucoma
drainage device surgery.
Availability of data and materials
YXK conceived of the project. HC performed the review. Both authors read
and approved the final manuscript.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
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