Refractive changes after posterior segment surgeries (Dr. Nesma, TA ophthalamology, Al Azhar university)
REFRACTIVE CHANGES AFTER POSTERIOR
Refractive changes caused by alterations in axial length
after retinal surgery.
Induced astigmatism after retinal surgery.
Refractive changes from alterations in corneal topography
after retinal surgery.
Refractive changes in children undergoing retinal surgery.
Refractive and biometric changes related to silicone oil use.
Refractive changes following retinal cryotherapy and
Refractive changes after retinal surgery may include a
hyperopic or myopic shift, as well as induced regular or
irregular astigmatism. The significance of these
changes has been debated and may be related in part
to surgical technique.
The eyes of infants and adolescents respond differently
to retinal surgery than do those of adults.
Retinal surgery in infants and children may have
profound effects on ocular development, producing an
extreme alterations in postoperative refraction.
Early reports focused on alterations in axial length and
the resultant spherical changes induced after retinal
detachment repair with scleral buckling techniques.
Scleral resection reduces axial length and produces a
hyperopic shift .
Theoretically, scleral buckling should increase the
axial length of the eye, producing a myopic shift;
however, reported postoperative outcomes have been
It was reported that a consistent axial elongation
of nearly 1 mm in all eyes examined, with a
resultant refractive change of 2.5 D.
Elongation occurred in the vitreous cavity, with
minimal changes in anterior chamber depth or
They found that the type of buckle element and
technique directly affected postoperative refraction.
Progressive horizontal shortening with a thin solid
silicone band (no. 41) increased buckle height and
Thicker elements (no. 287 tire, no.505 sponge, no. 507
sponge) increased axial length when horizontally
shortened but decreased axial length when applied by
It was found that a correlation between buckle type and
axial lengthening .
The encircling scleral bands increasing axial length by an
average of 0.99 mm and producing 2.75 D of refractive
Non encircling procedures increased axial length by 0.26
mm and induced 0.31 D of refractive change
Contrary to the belief of most surgeons, "high“
equatorial indentations produce a paradoxical
shortening of the axial length which changes the
induced refractive error toward hyperopia.
Highly unpredictable alterations in keratometric
power and corneal curvature can be produced by
scleral buckling procedures.
It is not clear from the literature whether these
astigmatic changes are transient or permanent.
Induction of severe persistent irregular
astigmatism after scleral buckling with episcleral
Radial buckles were significantly more likely to
produce astigmatic errors greater than 2 D.
It was reported that although small amounts of
astigmatism usually represented transient
changes, larger amounts of induced astigmatism
(>3 D) usually persisted.
Also it was reported occurance of astigmatism after pars
Most of these cases required lysis of the suture at the
vitrectomy site to relieve the astigmatism.
Risk factors include total air or gas tamponade and tight
wound closure .
Induced astigmatism is typically mild and transient,
Use of the 25-gauge sutureless vitrectomy system is
likely to reduce astigmatism induced by scleral
suturing. This system may result in more rapid visual
recovery in these patients.
Refractive changes from alterations in corneal
topography after retinal surgery
Although keratometry detects global changes in
corneal shape, topography can identify irregular
astigmatism–focal alterations in corneal curvature
that may not be translated to the entire corneal
meridian but nonetheless can diminish visual
These descriptors include the SRI, or surface
regularity index, and the SAI, or surface
asymmetry index .
The SRI is a measure that describes the regularity
and optical quality of the central cornea and is
correlated with best spectacle-corrected visual
The SRI and SAI values increase, indicating
increased surface irregularity and asymmetry .
It was found that circular buckles alone induced
only 0.4 D steepening over the entire cornea but
over 2 D in the central cornea by the first
These changes were transient; they became apparent by the
first postoperative week but normalized by week twelve in
all cases .
Topographic analysis has shown two patterns of
corneal steepening after the placement of scleral
Encircling buckles produce either uniform central
steepening or coupled steepening and flattening in the
Local or segmental buckles produce steepening in the
corresponding meridian .
Vitrectomy had a minimal effect on overall corneal
topography; however, it induced 1 to 1.5 D of central
corneal steepening, which correlated with the location of
the entry port.
Suture diameter, wound closure, and cautery were
related to corneal curvature changes after pars plana
Recommendations to to lessen these effects are , using
small diameter absorbable suture, avoiding excess
suture tightening during closure, and minimizing
Some loss of best spectacle-corrected visual acuity
after scleral buckling procedures or pars plana
vitrectomy, previously attributed to macular
dysfunction, may, in fact, be due to irregular corneal
This is to recommend a delay in prescribing
spectacles until 6 or more months postoperatively.
Refractive changes in children undergoing
Infants and children are most susceptible to large,
variable, and fluctuating postoperative refractive
results owing to effects on ocular development.
Scleral buckling has a profound effect on the
development and emmetropization of the eyes .
In human infants, it was found that an induced
myopia greater than 11 D with scleral buckling.
This induced myopia was reduced by one half after
division of the buckle .
They concluded that a scleral buckle prevented normal
ocular development, and that children under the age
of 10 years were particularly susceptible to this effect.
Refractive and biometric changes related to
silicone oil use
Silicone oil was chosen as a tamponade owing to
its high surface tension, transparency, stability,
and relatively low toxicity; however, it induces
refractive changes when it occupies the vitreous
cavity and it creates difficulties in obtaining
accurate measurements for intraocular lens
calculations after it is injected into the eye .
silicone oil has a variable effect on refraction
dependent on the phakic status of the eye .
Aphakic eyes become less hyperopic by an average
of 6 to 7 D .
whereas phakic eyes become more hyperopic by an
average of 5.5 to 7.6 D .
Silicone oil has an index of refraction of 1.405
compared with that of vitreous (1.336).
In the aphakic state, silicone oil forms a convex anterior
surface relative to the corneal endothelium. This surface
decreases the overall hyperopia by acting as a plus lens.
In the phakic eye, the silicone oil forms a concave surface
behind the natural lens, which acts as a minus lens and
increases the overall level of hyperopia.
In pseudophakic eyes, silicone oil negates the effect of
the intraocular lens (if the refractive index is similar to that
of silicone), causing a myopic shift like that of an aphakic
The effect silicone oil has on refraction can also be affected
by head position . This effect is more pronounced in
When patients’ refractions were measured in the supine
position and then remeasured in the head-down position,
the spherical equivalent increased by approximately 6 D in
This difference was most likely caused by a shift in the
position of the oil bubble. In the head-down position, the
anterior surface is less convex than in the supine position,
resulting in more hyperopia in the headdown position.
In the same study, the cylinder axis was observed
to shift 11.5 degrees on average in aphakic eyes and
10.1 degrees on average in phakic eyes with
changing head position, but the cylindrical power
did not change. These changes can be noted by the
patient, who may complain of fluctuations in
vision with various activities.
Silicone oil has been noted to decrease the
accommodation of the lens in phakic patients, and,
typically, a +2.00 to 2.50 D bifocal is required in phakic
patients when the fellow eye requires none .
Biometry of the silicone oil–filled eye
To determine the axial length and intraocular lens calculation in a silicone oil–
filled eye, A-scan ultrasonic biometry requires an adjustment for the slower
speed of sound in oil. Sound waves travel at 987 m/s in silicone oil of viscosity of
1000 centistokes versus 1532 m/s in vitreous .
Silicone oil also causes poor penetration of the sound wave, complicating
measurements further .Because the A-scan uses time in microseconds from the
cornea to the retina to determine the length of the eye, if no adjustment is
made for the oil, an artificially high axial length is obtained, resulting in the
placement of a lower power intraocular lens than necessary and a hyperopic
To adjust for this effect, the axial length obtained
with silicone oil in place can be multiplied by a
correction factor of 0.71 .
When the A-scan measures the vitreous cavity
depth separately, that figure can be multiplied by a
correction factor of 0.64 and added to the anterior
chamber depth and lens thickness to obtain a true
axial length .
Another method based on a velocity conversion
equation, which provides for the correction of an
erroneous measurement that results from the use
of an incorrect sound velocity setting.
To determine the true axial length (TAL) using the
equation, the correct sound velocity (Vc) should be divided
by the incorrect sound velocity setting used for the
measurement (Vm) and then multiplied by the incorrect
(apparent) axial length reading (AAL) .
TAL ¼ Vc=Vm AAL
Owing to shifting of the oil bubble, the patient
should be measured upright so the oil fills the eye
from lens to macula.
The transducer should be oriented with the beam
perpendicular to the globe to reduce refraction .If
the axial length is out of the range of the A-scan
used, the B-scan may be used instead, although
this is known to underestimate the true length and
may induce more error in long eyes .
Posterior staphylomas were found in the eyes with the
greatest deviation from predicted values, and it has
been suggested that leaving these eyes aphakic, at least
initially, may be the best strategy.
Because the silicone oil, with its increased
refractive index compared with that of vitreous,
will remain in direct contact with the posterior
capsule, it becomes difficult to predict the
Owing to the difficulties in obtaining accurate
axial length measurements, many authorities have
suggested that A-scan ultrasonic biometry be
performed before silicone oil injection .
In a study by Grinbaum and colleagues  in
which the A-scan had been completed before oil
injection and after scleral buckling, extracapsular
cataract extraction and biconvex intraocular lens
implantation were successfully completed in eight
Refractive changes following retinal
cryotherapy and panretinal
Loss of accommodation, transient myopia, or both
have been reported following retinal cryotherapy and
Loss of accommodation can be secondary to the
retrobulbar block owing to mechanical injury to the
ciliary ganglion, its roots, or the short ciliary nerves.
Accommodative paresis and myopia have also been
reported without retrobulbar block following
cryotherapy, laser retinopexy of retinal tears, and
These symptoms are usually transient and tend to
resolve within 5 weeks without treatment.
A possible explanation could be damage to the short
ciliary nerves during treatment .
Retinal surgery can induce significant refractive errors.
These errors include spherical changes caused by
alterations in axial length after scleral buckle
placement and astigmatic changes induced by scleral
buckling or pars plana vitrectomy.
Focal alterations in corneal curvature can significantly
limit postoperative visual acuity when axial length and
keratometry values seem relatively normal.
Surgical technique may also influence the induction of
corneal surface irregularity, especially in highly
These refractive errors are usually transient, but suture
lysis and buckle transection may occasionally be
In very young patients, retinal surgery not only affects
refractive outcomes but also alters the course of
normal ocular development.
The adjunctive use of silicone oil can impose
alterations directly, by the oil’s interaction with the
other refractive elements of the eye, and
indirectly,through its effects on intraocular lens power
calculations for subsequent cataract surgery
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