Eyes at the extremes of axial length can be challenging to operate on. Very large and highly myopic eyes usually have deep anterior chambers and are subject to trampolining of the lens-iris diaphragm, high perceived intraocular pressure at relatively low infusion bottle height, and lens power calculation inaccuracy. Nanophthalmic and highly hyperopic eyes often sit in deep-set orbits and are subject to shallow anterior chambers, a propensity for iris prolapse and chafing, scleral thickening that can result in uveal effusion, higher than normal posterior pressure, and lens power calculation inaccuracy.
In this month's column, Hart Moss, M.D., and Douglas D. Koch, M.D., define what constitutes a nanophthalmic eye and describe some of the associated conditions that are seen in nanophthalmos. They review some of the important considerations when working in an eye with a shallow anterior chamber. They discuss management strategies for handling high posterior pressure depending on the specific anatomic situation. They also discuss the issues surrounding lens power calculation and selection in highly hyperopic eyes.
Patients with nanophthalmos do not come around very often. When they do, however, they can be a surgical challenge. This article may prove to be a useful reference for that occasional patient.
Kevin M. Miller, M.D., Complicated cataract cases editor
Injection of a viscoadaptive OVD in a nanophthalmic 15.5 mm eye with a slit anterior chamber. Following removal of the cataract and insertion of the 40 D IOL, there was no room for the 12 D
piggyback IOL that was planned. Because of the anterior position of the single IOL, the post-op refractive error was only +3.00, and the post-op course was complicated by formation of
posterior synechiae and secondary glaucoma
Scleral cutdown to drain choroidal
effusion that developed intraoperatively (performed by Peter T. Chang, M.D., assisting the author in this procedure)
Injection of the 14 D piggyback IOL into the ciliary sulcus in front of the 40 D IOL in the capsular bag
Source (all): Douglas D. Koch, M.D.
Nanophthalmos can be
defined in various ways, including an axial length of two standard deviations below average or as an extremely short eye with a shallow anterior chamber. It presents an array of challenges even to seasoned cataract surgeons. Pre-op, short eyes raise uncertainty with IOL power calculation. Intraoperatively, a shallow anterior chamber, small corneal diameter, and positive posterior pressure can conspire to make maneuvers within the anterior
segment difficult and hazardous to the endothelium, iris, and posterior capsule. Extreme axial hyperopia, with high dioptric IOL requirements, may complicate lens implantation. Associated ocular conditions, such as increased scleral thickness, pseudoexfoliation syndrome,
angle-closure glaucoma, and retinal pathology, can also affect management. Amblyopia is more common in nanophthalmic eyes, and reasonable patient expectations should be ensured prior to proceeding. However, with appropriate measures, one can achieve good visual results and even improve angle configuration.
Shallow anterior chamber
These eyes have very small and crowded anterior segments, often with anterior chamber depths of less than 2 mm. This may lead to difficulty with capsulorhexis, iris prolapse, endothelial cell loss, and posterior capsular rupture. Meticulous wound construction and small incision sizes with appropriate sleeves are standard means to provide a stable and formed anterior segment. A viscoadaptive agent (Healon 5, Abbott Medical Optics, AMO, Santa Ana, Calif.) can be
instrumental in sufficiently deepening the anterior chamber for capsulorhexis and phacoemulsification, while minimizing progressive loss of viscoelastic during the surgery. Because the capsulorhexis is prone to tear toward the periphery, extra attention must be taken and the anterior chamber should be refilled with viscoelastic as necessary. Iris hooks or other expansion devices should be kept handy given the propensity for poor dilation and iris prolapse. During phacoemulsification, raising the bottle height can help to maintain an adequately formed anterior chamber. Of course, care must be taken to direct phaco energy and lens fragments away from the corneal endothelium, while phaco settings should be optimized to minimize energy output. As with capsulorhexis, periodically refilling the eye with a dispersive or viscoadaptive viscoelastic may be helpful. This can be done through the paracentesis while the phaco handpiece is still in the eye, to avoid repeated anterior chamber shallowing and transient hypotony.
The increased scleral thickness seen in nanophthalmic eyes may lead to uveal effusions and marked intraoperative posterior pressure, which can compound an already shallow anterior chamber. Mechanical compression with one's palm or a Honan balloon may help decompress the vitreous body; intravenous mannitol (20%, 1-2 ml/kg) can also be used to dehydrate the vitreous and should be given 15-30 minutes prior to the incision. If surgery is delayed, make sure to give the patient access to a bathroom. To minimize positive pressure, a peribulbar block or topical anesthesia may be preferable to a retrobulbar block. There are two types of situations in which surgeons encounter problems with chamber depth, and they require completely different approaches for management: 1. If the chamber is excessively
shallow at the beginning of the surgery, this suggests that the problem is the thickened sclera and inherent posterior pressure. If the above-noted measures are not adequate and if one is comfortable with the procedure, a limited pars plana vitrectomy is a definitive means to reduce posterior pressure and deepen the anterior chamber when otherwise impossible. The incision should be placed around 3.25-3.5 mm posterior to the
limbus (less if an extremely short eye). Because the lens volume is still normal in these otherwise very small eyes, care must be taken to always direct instruments toward the optic nerve and away from the lens. Vitrectomy should be done with the highest cut rate available to limit vitreous traction; newer 25-gauge systems can now be used transconjunctivally as well. 2. If posterior pressure develops
during the procedure after the chamber was initially of sufficient depth, this indicates a choroidal effusion until proven otherwise. Unfortunately, this can be hard to visualize if it is shallow, so a scleral cutdown may be required. This can be done with a 54 Beaver blade, placing a 1 mm incision that begins 1 mm behind the
limbus and extending posteriorly. The Kelly punch can be used to create a window after the suprachoroidal space has been accessed. More than one sclerotomy may be required in some cases. To prevent a choroidal effusion, it is critical to maintain a stable and deep chamber throughout the surgery. Each time that the phaco or I/A tips leave the eye, the surgeon should inject balanced salt solution or OVD to maintain chamber depth.
Hyperopia and lens selection
Accurate biometry is especially important in short eyes, as small errors in axial length have a disproportionately large effect on the refractive outcome. IOL power formulas are also traditionally less accurate at the extremes of axial hyperopia. Newer generation formulas, such as the Holladay IOL Consultant or Hoffer Q, may provide improved accuracy. Indicated IOL powers can range from 40-60 diopters and are often higher than available in a single lens. Our preference is to implant the highest power foldable IOL in the bag (currently 40 D in the U.S.), placing a second IOL in the sulcus. Formulas for piggyback lens calculations are available. IOLs of two
different materials, or two silicone IOLs, may be preferable in order to prevent interlenticular opacification.
One exception is eyes with extremely shallow anterior chambers pre-op and intraoperatively. In these situations, the first IOL in the bag may sit so anteriorly that a) its effective power is much higher, and b) there is no space in the sulcus for the second IOL. If there is any doubt about the adequacy of the space for the second IOL, we recommend postponing its insertion. This allows one to assess both the dioptric
need and anatomic feasibility of a piggyback lens.
In some eyes with an extremely small anterior segment, it can be difficult to perform all aspects of the surgery, including IOL insertion. These eyes may have corneal diameters of 8 mm or less (our lowest is 6 mm), and complication rates of zonular dehiscence and capsular rupture are high. Extensive pre-op counseling is important.
Finally, patients with high
hyperopia and unilateral cataracts should be counseled that post-op anisometropia may require a contact lens or surgery in the other eye.
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Editors' note: Drs. Koch and Moss are affiliated with the Cullen Eye Institute, Baylor College of Medicine, Houston. Dr. Koch has financial interests with AMO and Alcon (Fort Worth, Texas). Dr. Moss has no financial interests
related to this article.