Experienced cataract surgeons know that the most challenging cataract cases are often the nanophthalmic eyes. These tiny eyes have a very short axial length of less than 20 mm along with a shallow anterior chamber, less pupil dilation and a smaller anterior segment. Cataract surgery in these eyes can produce some of the most amazing outcomes because there is no better way to surgically address a high degree of hyperopia.
Cataract patients with nanophthalmos will come in wearing contact lenses or high-powered spectacles, often of +10 D or more. The contact lens patients should discontinue wearing them until the surgeon is satisfied with the ocular surface and corneal biometry. Spectacle-wearing patients may be used to the high degree of magnification that comes from wearing such a strongly hyperopic correction. When cataract surgery is performed and the focusing power is on the IOL, they will lose that additional magnification.
IOL power estimation in these eyes is less accurate when compared with an average eye. As the IOL power increases, a slight shift in the effective lens position makes a larger change to the refractive outcome. In addition, it is often hard to predict the effective lens position, particularly when the anterior chamber is not as shallow. This is because the IOL optic is now closer to the retina and the power requirement increases dramatically. Not all IOL designs are available in these higher powers, including toric, extended depth of focus and trifocal IOLs. Monofocal IOLs are available in the U.S. up to +40 D in power. If the IOL calculations indicate that a power above that is needed, often the best move is to just implant the +40 D IOL and have the patient wear spectacles for the remaining postoperative hyperopia. In some cases, a second procedure can be performed to implant a piggyback IOL with the remaining power.
When creating the capsulorrhexis, keep in mind that a 5 mm diameter is desired for efficient and safe nucleus removal. With the small anterior segment, shorter white-to-white diameter and lesser pupil dilation, it can be hard to accurately judge the capsulorrhexis size. Using forceps that are marked (Figure 1) can help gauge size during capsulorrhexis creation.
The lens nucleus is large compared with the smaller anterior segment and shallow anterior chamber (Figure 2). Using a stop-and-chop technique with a wide central groove helps to debulk the central nucleus in the capsular bag, thereby creating smaller hemi-nuclear halves. Using a chop technique can also work well; however, the two nuclear halves will be larger and more challenging to bring out of the capsular bag.
Maintaining stable fluidics can help to prevent choroidal effusion or hemorrhage, which is more common in nanophthalmic eyes. The high-power IOL optic will be thicker than the typical +20 D IOL and therefore may require enlargement of the incision for insertion (Figure 3). When the IOL is implanted in the capsular bag, it will look comparatively large because of the small anterior segment size. The incisions are also relatively larger compared with the smaller corneal diameter, and they may not seal as well.
There may be more inflammation and even corneal edema because the small anterior segment and shallow anterior chamber mean that the corneal endothelial cells are closer to the ultrasonic energy from the phaco probe. As the capsular bag contracts in the postoperative period, the IOL optic may shift slightly, which can change the refractive outcome. The patient may require a longer period of time to achieve the final visual outcome, but they are very happy to finally have a permanent cure for their lifetime of high hyperopia.