Phacoemulsification in high myopic eyes
Everything you ever wanted to know about phacoemulsification in high myopic eyes
Lens-iris diaphragm retropulsion syndrome (LIDRS): A,B: Infusion pushes iris against anterior capsule causing reverse pupillary block; C,D: Lifting the iris with a rod allows fluid to equilibrate between anterior and posterior chambers.
Eyes with high myopia refer to those with an axial length greater than 26mm or -6D of myopia. These eyes often develop nuclear cataract earlier and surgery has added advantages of decreased dependence on high-powered glasses. However, surgery can be more difficult and should be carried out with care.
Associated co-morbidities such as open-angle glaucoma, chorioretinal atrophy and macular pathology may result in less than desirable results. Complete preoperative retinal examination including periphery is important and if required any peripheral retinal lesions should be lasered. For dense cataracts precluding retinal examination, B-scan is indicated followed by a dilated retinal examination as soon as possible after surgery. Bilateral myopes should be counselled regarding need for surgery in the second eye to prevent anisometropia.
Retrobulbar and to some extent peribulbar block poses the risk of inadvertent globe perforation in high myopes because of larger eye size and thinner sclera. Large eyes have lower scleral rigidity. Equatorial and zonular stretch on instilling viscoelastic or initiating irrigation causes lens-iris diaphragm retropulsion syndrome.
Increased anterior chamber (AC) volume causes a reverse pupillary block by causing 360-degree irido-capsular contact. Young age and previous vitrectomy can compound this effect. Sudden backward movement and deepening of AC can cause discomfort to the patient if surgery is under topical anaesthesia. An excessively deep AC necessitates holding instruments more vertically and increases range of focus required.
Reverse pupillary block can be neutralised by lifting the iris edge off the anterior capsule and allowing fluid to equilibrate between anterior and posterior chambers, thus bringing the iris, zonules and lens back to a more normal position. This manoeuvre generally brings the pupil back to original size, though miosis may sometimes occur because of prostaglandin release. The reverse pupillary block repeats on initiating infusion each time and should be tackled in a similar manner. It can be prevented by placing a rod between iris and anterior capsule each time before initiating infusion.
There are also other intraoperative problems specific to large eyes. Short corneal tunnels may not be self-sealing because of lower scleral rigidity, especially if combined with other risk factors such as young age and the need to work vertically thereby distorting the wound. Sutures should be used when required. Too long a tunnel also results in the need to place instruments more vertically leading to corneal distortion and decreased visibility.
The cystitome shaft needs to be more vertical to prevent depression of the posterior lip and escape of viscoelastic. Repeated refilling of viscoelastic may still be required. The rhexis size may appear deceptively small due to a larger corneal diameter. An unintentional large capsulorhexis can result in incomplete overlapping of the anterior capsule around the optic edge, thereby leading to anterior movement of IOL optic and a myopic shift as well as increased risk of posterior capsular opacification, the need for YAG capsulotomy and consequently increased risk of retinal detachment.
Repeated shallowing of the AC, especially in the presence of vitreous syneresis, also increases risk of retinal detachment. Gradual pressurisation and depressurisation should therefore be done by injecting viscoelastic with the left hand before withdrawing the phaco or I/A probe. New phaco machines have active fluidics and can decrease some of the problems faced.
Despite breaking the reverse pupillary block, instruments may still need to be placed more vertical than usual, leading to greater difficulty in nucleus removal and cortex aspiration techniques. For soft nuclei, prolapse and supra-capsular phacoemulsification may be easier than in-the-bag nuclear disassembly techniques.
These eyes have a higher risk for anterior and posterior capsular tears and zonular dehiscence. IOL implantation should be done after refilling the bag with viscoelastic in order to prevent haptic snag on a lax capsule. Laterally displaced IOLs and rotation of toric IOLs may occur because of a bag-IOL diameter mismatch. Complete removal of viscoelastic and implanting a prophylactic capsular tension ring may help. Avoiding anterior capsular polishing may promote earlier sandwiching of a toric IOL into place; however, this will not be helpful against the early rotation that can occur.
Very high myopes may be associated with progressive zonulopathy and sometimes late in-the-bag IOL subluxation. A subluxated three-piece IOL from initial surgery allows easy closed chamber translocation to secondary scleral fixation in the form of a glued IOL. An in-the-bag single-piece acrylic IOL may also be refixated using sutured segments or the sutureless glued capsular hook technique described by the author.
IOL power calculation is very important because of a higher chance of error in large eyes. Current optical biometers provide inaccurate axial length measurement in eyes longer than 26mm as it uses the same refractive index for all eyes. However, as posterior staphylomas can cause gross errors with ultrasound biometry, optical biometry is still preferable, even in eyes with axial length more than 30mm, despite lower accuracy. IOL power may be in low plus or rarely even in the negative range.
Hyperopic shifts may be seen with third-and sometimes even fourth-generation formulae. The Holladay 2, Olsen, Barrett Universal II and Haigis formulae are good. The SRK/T formula may be effective between 27-29mm where a negative IOL may be required; however, very steep or flat keratometric values can confound results. If low-range single-piece acrylic IOLs are not available, a three-piece IOL may be placed.
Femtosecond gives an accurately sized and centred rhexis. It also allows easier disassembly of the nucleus and helps overcome challenges of in-the-bag chopping and cracking manoeuvres in a deep AC.
Post-op complications that may occur include pseudophakic retinal detachment. Postoperative retinal examination is therefore a must. Steroid response may be seen, especially if also associated with young age. Weaker steroids may be used postoperatively.
Though high myopic patients can be brought close to a plano refraction, loss of the easy reading ability may create unhappiness. Micromonovision may be aimed for if bilateral surgery is being performed. IOL calculation errors may require a second surgery and patients should understand this. Macular pathology may limit postoperative visual acuity attained.
Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India and can be reached at email@example.com