Phacoemulsification in small eyes

[caption id="attachment_11316" align="alignleft" width="1024"] Relative dimensions of A: Normal eye; B: Microphthalmic eye with small anterior segment and short axial length[/caption] In the last issue, small eyes secondary to microphthalmos and relative anterior microphthalmos (RAM) were discussed, as well as surgical techniques for small eyes. In this issue we deal with hyperopic eyes, as well as IOLs for small eyes. The axial length is small and the anterior chamber depth is normal in axial hyperopia. It differs from RAM, where the axial length is normal and the anterior chamber depth is small. Phacoemulsification can generally be carried out using normal techniques but with extra care. Surgery can be done for cataractous hyperopic eyes or alternatively as clear lens extraction for correction of refractive error. Hyperopic patients have always had to use spectacles to see clearly for both distance and near. Therefore, they are generally very happy with the results. CLEAR LENS EXTRACTION This is a good choice in older hyperopic patients who have already started to lose accommodation. In this age group, it has numerous advantages over keratorefractive procedures – it is not associated with problems of regression and higher powers can be corrected with good predictability and fewer aberrations. Surgery is generally easy as the nucleus is soft and can easily be hydroprolapsed out of the capsular bag and aspirated. The risks of an intraocular procedure should be kept in mind, however, and surgery should proceed with all due precautions taken. Loss of accommodation is a major drawback, and one of the reasons why it is less popular in younger age groups. If opted for, the patient should be counselled appropriately preoperatively. In case of high hyperopia, customised IOLs may need to be ordered or piggybacking of IOLs may be used. IOL POWER CALCULATION Most eyes have an axial length between 22 and 24.5mm, and third-generation formulas such as the SRK-T, Holladay 1 and Hoffer Q estimate IOL power satisfactorily in these eyes. This is because the effective lens position (ELP) is predictable in eyes that fall within this range. However, this is not the case in shorter and longer eyes, which require special formulas. IOL power calculation in these eyes can be associated with a greater degree of error. Small errors in axial length measurements can get magnified while calculating IOL power in small eyes. Accurate measurements may be difficult with ultrasound biometric machines as these are calibrated for normal eyes with fixed anatomical proportions. In microphthalmic eyes, the normal-sized lens takes up a larger volume in the small eye. Theoretical formulas may be more accurate; however, most still err towards some residual hyperopia. [caption id="attachment_11317" align="alignleft" width="1024"] C: Relative Anterior Microphthalmos (RAM) with small anterior segment and normal axial length; D: Axial Hyperopia: normal anterior segment and short axial length[/caption] Measurement of axial length and prediction of ELP is very important and repeated measurements should be taken carefully. Immersion and optical biometry can be used. A relatively anteriorly placed effective lens position can lead to errors in calculation and the Holladay 2, Haigis and Hoffer Q formulas are better relied upon. It is advisable to calculate using different formulas and lean towards the Hoffer Q in deciding final IOL power. Intraoperative aberrometry (ORA Inc, Wavetec Vision) can also be useful. CHOICE OF IOL For lower degrees of hyperopia, standard choices may be made, but higher degrees require special IOLs. These can be either in the form of customised IOLs of high power or piggybacking of two IOLs. Piggyback lenses with the higher-powered one in the bag and the lower-powered one in the sulcus are reported to give lesser spherical aberration than a single IOL with very high power. Placing two IOLs in the bag should be avoided, as this can lead to interlenticular fibrosis, decrease in vision and late hyperopic shift. It should be remembered, however, that many of these small eyes may not have enough space or may not tolerate two IOLs with resultant crowding, uveitis-glaucoma-hyphaema syndrome etc. Also, interlenticular membranes may still develop even with one IOL in the bag and another in the sulcus, and even if made of different materials. If piggybacking is opted for, many surgeons prefer implanting the maximum-powered IOL that is available in the bag. The piggyback IOL can be done in a second stage after assessing postoperative refractive result. The power may be calculated by using the Gills nomogram: {(1.5 x Spherical equivalent) +1}. Acrylic IOLs have higher a refractive index and are thinner than both PMMA and silicone, and are therefore preferred to be placed in the bag. The piggyback IOL that is placed in the sulcus should be thin and non-reactive. Silicone IOL is a good option for this. Hyperopic eyes may have a large-angle kappa, and multifocal IOLs should be avoided in such eyes. SHALLOW AC A shallow anterior chamber can occur secondary to hypermetropia, primary angle-closure glaucoma, anterior rotation of the ciliary body, lax zonules allowing anterior movement of the lens such as pseudoexfoliation etc. It can also occur as part of microphthalmos and relative anterior microphthalmos, which were discussed in the last issue. In cases of narrow-angle glaucoma, the large lens, small pupil and shallow AC all contribute to difficult phacoemulsification. However, cataract extraction with IOL implantation generally treats the condition by decreasing overcrowding of the anterior segment and letting the iris fall backwards. This opens the angle and treats the glaucoma as well. Instilling viscoelastic will instantaneously deepen the AC in diffuse zonulopathy, leading to a shallow AC, whereas it may result in increased IOP and possibly iris prolapse in other cases. Positive pressure leading to a shallow AC can be tackled preoperatively by vitreous dehydration, and intraoperatively by doing a limited dry vitrectomy. Loosening the lid speculum also helps decrease positive pressure. FEMTOSECOND LASER-ASSISTED CATARACT SURGERY (FLACS) FLACS can be very useful in shallow anterior chambers where there is less space to manoeuvre. Longer tunnels should be programmed and positioned carefully to prevent iris prolapse. A femtosecond-created rhexis decreases the chances of a runaway and is a major advantage. Pre-treatment of a dense nucleus with femtosecond can break the nucleus into smaller fragments, thus making removal easier and thereby decreasing possible endothelial damage. 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 dr_soosanj@hotmail.com. Click here to see the live surgery.

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