Phacoemulsification in small eyes – Part One
Small eyes can be part of microphthalmos (short anterior chamber [AC] depth and short axial length), relative anterior microphthalmos (short AC depth and normal axial length) or axial hyperopia (normal AC depth with short axial length). Part 1 of this series deals with a description of the challenges in the microphthalmic eyes, and part 2 will deal with hyperopia and IOL-related issues.
Duke-Elder classified microphthalmos into nanophthalmos (short, otherwise normal eye); microphthalmos with coloboma and complex microphthalmos. Simple microphthalmos refers to a short eye. A nanophthalmic eye is less than two standard deviations below mean for age or <20.5mm in axial length. Nanophthalmos is bilateral and high hypermetropic errors (+8 to +20DS), an overall small eye with microcornea, small anterior segment, normal or increased lens thickness, marked iris convexity, crowding of the anterior chamber and shallow AC are seen. Blindness can occur if left untreated. Retinal problems such as macular hypoplasia can limit vision even after successful cataract surgery.
Problems encountered during cataract surgery are secondary to microcornea (<10mm), shallow central and peripheral AC, peripheral anterior synechiae, chronic angle closure glaucoma (CACG), poorly dilating pupil and thickened choroid and sclera. The thick and inelastic sclera decreases trans-scleral passage of proteins and compresses the vortex veins, leading to spontaneous as well as postoperative uveal effusions and exudative retinal detachments. Uveal effusions can cause anterior rotation of the ciliary body with shallow AC.
Microphthalmos with coloboma results from an incomplete closure of the embryonic fissure. This can range from small infero-nasal iris coloboma to orbital cyst. Retinal laser may be required preoperatively in eyes with choroidal colobomas. Iridoplasty may be required together with cataract surgery for coloboma repair.
Complex microphthalmos is often associated with congenital cataract, other anterior/posterior segment malformations and systemic abnormalities. Additional surgery for glaucoma or corneal opacity may be required. Postoperative vision may be limited by associated ocular comorbidity.
Relative anterior microphthalmos is more common. The normal-sized lens causes consecutive crowding of anterior segment, shallow AC and CACG. Since the eye is grossly normal, this may be missed on slit lamp. It is not associated with scleral abnormalities or uveal effusions.
Glaucoma if present may be managed by YAG peripheral iridectomy, laser gonioplasty, anti-glaucoma medications, sclerectomies, trabeculectomy or cyclodestructive procedures, depending on stage of the disease. Cataract surgery is challenging and carries risks, but has the benefit of decreasing anterior chamber crowding. Preoperative indentation gonioscopy or ultrasound biomicroscopy and complete glaucoma evaluation are important. Dilatation can precipitate angle closure and prophylactic YAG PI may be necessary before dilatation. Fundus evaluation to look for uveal effusions and B-scan to measure thickness of choroid and sclera are important.
Patients should be counselled regarding the increased risk of surgery and also about poor visual prognosis secondary to any associated retinal abnormalities or amblyopia. IOL implantation may be deferred in very severe microcornea where the IOL optic may cause crowding.
IOL power calculation is difficult in these eyes, with a higher chance of errors. Both immersion and optical biometry should be used. Hoffer Q or Haigis formulas are more reliable. Special high-powered IOLs may need to be customised.
An alternative option is to piggyback an IOL in the same sitting or at a second sitting after checking residual refractive error and available space in the sulcus. Preoperative acetazolamide, oral glycerol, IV mannitol and ocular pressure with Pinkie ball/Honan balloon should be used to decrease the IOP. General anaesthesia may be opted for to avoid an increase of the orbital volume from peribulbar anaesthesia. Blood pressure should be kept to the lower side during surgery.
Intraoperatively, prophylactic sclerotomies or sclerectomies are placed to decompress the vortex veins and allow fluid drainage without causing uveal effusions. Well-constructed tunnels are necessary to prevent iris prolapse. Sudden changes in intraocular pressure should be avoided. Pharmacological dilatation, viscomydriasis, synechiolysis, pupil stretch, mini-sphincterotomies or pupil expanders are used if indicated. The B-Hex ring is particularly suited in these small eyes as compared to other pupil dilators because of its low profile and thin nature.
Intraoperative challenges occur because of a lack of adequate manoeuvring space within the AC. Rhexis should be performed with microinstruments through a partially opened incision, using high molecular weight cohesive viscoelastics to maintain space and flatten the anterior lens capsule.
Bimanual phaco may be preferable in very small eyes. The soft-shell technique may be used using cohesive viscoelastic to maintain space and dispersive to protect endothelium. Soft nuclei may be partially hydroprolapsed and emulsified in parts. However, hard nuclei are often encountered because of propensity by both patient and surgeon to delay surgery. The bulk of the nucleus may be decreased by shaving away epinucleus within the bag and then using divide and conquer or crater and chop techniques. Nucleus is then broken into smaller fragments and emulsified.
Wound burns should be avoided, and chilled BSS may be used for irrigating the wound during phaco. Positive vitreous pressure and a shallow AC with inadequate manoeuvring space is tackled by increasing inflow by increasing bottle height or by using pressurised air infusion. Loose or defective zonules may cause vitreous hydration and further shallowing of AC.
If inadequate AC depth does not allow surgery, limited dry anterior vitrectomy with high-speed 25-gauge vitrector allows iris and lens to fall backwards and thus deepens AC. However, pars plana dimensions may be different from normal eyes and care should be taken while placing the sclerotomy. The eye should not be made excessively soft. Cortex aspiration and IOL implantation are performed as usual. Implanting very high-powered IOLs via injector is difficult and wound may need to be enlarged. Tight wound apposition at the end of surgery is important and wound sutures may be applied.
The risk of posterior capsular rent and endothelial damage is higher in these eyes because of positive vitreous pressure and a lack of surgical space. Uveal effusion, suprachoroidal haemorrhage, aqueous misdirection syndrome and prolonged uveitis are other complications. In case of a PCR, secondary IOL fixation may be done using a glued IOL (using small diameter optic and with trimmed haptics for tucking) or other preferred technique.
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 firstname.lastname@example.org.