This small eye suffered from angle closure and subsequent development of a white cataract. The angle closure was successfully treated with laser peripheral iridotomy but now the patient needs cataract surgery and the anterior chamber depth is less than 1 mm. How do we proceed?
Small eyes have anatomic features which can complicate cataract surgery. These eyes have a shorter axial length, often associated with a shallower anterior chamber. In addition, the corneal diameter may be less and the effective lens position of an IOL more difficult to calculate. But there is one great upside: these are hyperopic eyes, often highly hyperopic, and are dependent on glasses for all activities. When successfully performed, cataract surgery can provide the patients with the best vision of their lives.
Small eyes have a shorter axial length, typically under 22 mm, and accompanying hyperopia of +3D or more. The anterior chamber can be shallow, with measured depths of 2mm or less and associated narrow angles. In cases where there are advanced cataract changes, the lens can swell and further narrow the angles.
The cornea should be carefully evaluated for endothelial weakness since there is a higher chance of phaco induced cell loss due to the closer proximity of the phaco probe. Small corneal size can also mean that the standard size phaco incision may induce more astigmatic change at the time of cataract surgery.
When measuring the axial length, small errors can result in more of a refractive shift than in normal or large eyes. While an axial length that is wrong by 1mm in a normal eye may induce 3 diopters of error, in a short eye this can be increased to 4 or even 5 diopters. Difficulty predicting the final effective lens position of the IOL also means that the lens calculations are more like estimations. Certain formulae have been shown to be more accurate in these short eyes. I recommend using www.IOLcalc.com since it will make special adjustments for these small eyes. Another good choice is the Holladay 2, which incorporates data such as white-to-white size, refraction, and anterior chamber depth, in order to produce more accurate results.
Single piece acrylic IOLs as well as three-piece acrylic and silicone IOLs are appropriate choices since they all have flexible haptics which allow placement within the smaller than usual capsular bag. In some situations, eyes with very short axial lengths may need an IOL power which is particularly high such as +30 diopters, +35 diopters, or even more. Since different IOL designs come in different ranges, every effort should be made to source the appropriate lens. It is better to have a single IOL with the correct power than to piggyback using two IOLs to achieve the same dioptric strength. In the US, there is at least one single-piece acrylic IOL which is available in special order powers up to +40 diopters (Alcon AcrySof single-piece).
In the rare eyes where a power of more than +40 diopters is indicated, it may be advisable to do the surgery in two stages: cataract surgery with implantation of the maximum power IOL (+40 diopters) in the capsular bag, followed by a second surgery with implantation of a piggyback lens in the ciliary sulcus if there is room. This provides more refractive accuracy since the power of the second IOL, which is placed in the sulcus, is determined based on the post-operative refraction after the initial cataract surgery. Even if we simply implant the +40 D IOL and then leave the patient with residual hyperopia, it will be far better than the pre-op refraction.
There are certain risks, such as choroidal hemorrhage, which are more common in these smaller eyes. In addition, the shallow anterior chamber can make it more difficult to complete the capsulorhexis and atraumatically remove the nucleus. After creation of an initial paracentesis the anterior chamber can be inflated with a cohesive viscoelastic in order to create space and deepen the anterior chamber.
In some small eyes, particularly those with coexisting pathology, the anterior chamber can be so shallow as to preclude capsulorhexis creation. A partial pars plana anterior vitrectomy can be done in order to help deepen the anterior chamber. This procedure, however, is not without risk since it involves creation of a sclerotomy to access the vitreous cavity in a crowded eye. Using a small gauge vitrectomy unit, a little vitreous can be removed at a time. It is important not to remove too much vitreous as this can result in too deep of an anterior chamber. Slowly injecting viscoelastic while performing the anterior vitrectomy will allow precise control.
The corneal endothelium should then be protected with a dispersive viscoelastic. Since the volume of the anterior chamber is less, there is a limited amount of working room during phacoemulsification. The closer the phaco probe is to the cornea, the greater the potential for endothelial damage. For this reason, phaco techniques which are performed within the capsular bag are preferred to extra-capsular methods. In this case, we used iris hooks for good exposure and then trypan blue dye to stain the anterior lens capsule.
Once the cataract is removed and the new IOL inserted in the capsular bag, care should be taken to ensure that the incisions are watertight. Since these small eyes often have smaller corneas, the standard sized phaco incisions may encompass a larger area and arc length and may not seal as well.
These patients tend to be particularly happy in the post-operative period since their high degree of hyperopia has been corrected. But there are additional benefits such as restoration of the angle anatomy and lowering of the intra-ocular pressure which occurs since the 4 mm thick cataract has been replaced by a 1 mm thin IOL. With careful pre-operative planning and appropriate intra-operative techniques, we can successfully perform cataract surgery on these small eyes.
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