34 Corneal Problems Associated with Phacoemulsification Successful cataract surgery depends on a good understanding of the cornea. Optical clarity, refractive predictability, and the long-term stability and health of the cornea are necessary components for optimizing outcomes. Surgical prowess is in part determined by a good foundation in anatomy. The cornea has unique structural characteristics, which are important in maintaining a clear and stable “window” for the eye. Our brief anatomy review is intended to help the surgeon achieve better outcomes and fewer corneal complications when performing cataract surgery. The adult cornea measures 11 to 12 mm horizontally and 9 to 11 mm vertically. The thickness varies from ∼ 0.5 mm in the center to 0.7 mm in the periphery.1 It is convex and aspheric, normally steeper in the center and flatter in the periphery.2 The anterior surface is covered by the tear film, which consists of a gradient of three layers: a superficial lipid layer, an aqueous layer, and a mucinous layer.3 The outermost lipid layer of the tear film is produced by the meibomian glands and accessory secretory glands of the eyelid. The lacrimal gland and accessory lacrimal glands produce the aqueous layer, and the mucinous layer is produced mostly by goblet cells in the conjunctival epithelium.2 The central cornea consists of several layers. The most anterior layer of the cornea is stratified, squamous, nonkeratinized epithelium that is five to seven cell layers and ∼ 50 µm in thickness. The deepest of these epithelial cells consist of a single layer of columnar basal cells adherent to a basement membrane via hemidesmosomes. Posterior to the epithelium is Bowman’s layer, an acellular matrix of collagen fibers and proteoglycans with a smooth anterior surface, ∼ 12 µm thick. The stroma, posterior to Bowman’s layer, constitutes 90% of the corneal thickness, and is composed of layers of highly organized collagen fibrils embedded in a ground substance with keratocytes dispersed among these lamellae. Descemet’s membrane is posterior to the stroma and is the basement membrane of the corneal endothelium. It gradually increases in thickness from ∼ 3 µm at birth to ∼ 10 µm in adulthood. Of note, a pre-Descemet’s layer, Dua’s layer, is recently purported to exist between the corneal stroma and Descemet’s membrane, and may have clinical significance in lamellar keratoplasty techniques.4 The endothelium on the posterior side of Descemet’s membrane is a single cell layer ∼ 5 µm thick. The endothelial cells are responsible for ion transport and maintain the cornea in a state of deturgescence. Endothelial cell density decreases with age, and it is important to protect the endothelium during surgery because human endothelial cells do not proliferate.5 The normal cornea is avascular, with the exception of the vascular arcade in the limbal region, which has contributions originating from both the internal carotid and external carotid arteries. The avascular cornea is supplied with glucose by diffusion from the aqueous humor and oxygen by diffusion from the tear–air interface.2 The limbus is often the site of incisions for cataract surgery. It is ∼ 1 mm wide and marks the transition between the cornea and sclera, extending from the end of Descemet’s and Bowman’s to the point where corneal stroma merges into sclera. Sensory nerve fibers from the ophthalmic division of the trigeminal nerve enter the limbus from the perichoroidal space and, after losing their myelin sheaths, branch into the corneal stroma, penetrating Bowman’s membrane and ending as naked fibrils between epithelial cells. Only the most basic concepts are discussed here. Cataract surgeons need to be concerned about two major areas when assessing the optics of the cornea. The first is opacification, which is the direct blockage of light to the more posterior structures in the eye. In addition, corneal opacities may hinder line-of-sight visualization for the surgeon, which could affect safety and efficiency during the procedure. The second type of optical problem is disruption of light rays not by direct blockage but rather by distortion of light rays due to irregularities of the optical surfaces (refraction, diffraction, scatter, decreased contrast, etc.). In many instances the disruption of light rays due to irregularity may be more clinically significant in terms of visual outcomes and certainly is a much more commonly encountered problem. In general, irregularities of the anterior refractive surface create more difficulties as compared with irregularities of the posterior corneal surface. Fig. 34.1 The anatomic relationships of the cornea and limbus. As cataract surgery has continued to improve over the past three decades, our threshold for surgical intervention has gradually been lowered and we are more willing to operate at earlier stages of disease. The vast majority of patients seem to present in the 20/30 to 20/50 range, with significant decline in their ability to perform activities of daily living. Sometimes patients with corneal disease present at earlier stages of cataract formation, and it is imperative for the surgeon to determine the relative negative effect of the corneal pathology before performing surgery. The following subsections provide some general guidelines for managing commonly encountered conditions in cataract surgery patients. If treatment is required to improve or optimize the ocular surface before cataract surgery, allow a minimum of 4 to 6 weeks for stabilization. Serial topography should be reproducible on two separate evaluations before the final biometry is performed. Dry eye and blepharitis are the most common “corneal” comorbidities and must be identified, especially if “premium” (multifocal or toric intraocular lens [IOL]) cataract surgery is performed. In general, even 1–2+ punctate staining in the visual axis or tear film instability may be enough to adversely affect the biometry or the visual outcomes. Fig. 34.2 Anterior/epithelial basement membrane dystrophy. (Courtesy of James Gilman.) Anterior/epithelial basement membrane dystrophy (ABMD) is common and may be subtle but needs to be identified and discussed with the patient prior to surgery (Fig. 34.2). Severe cases will affect the biometry and the visual outcome and may require treatment (superficial keratectomy, diamond bur, photo-therapeutic keratectomy) before cataract surgery. Pterygia may induce irregular astigmatism or in severe cases obstruct part of the visual axis. Depending on the extent, combined or separate surgery may be appropriate. In many older patients, it is very appropriate to not intervene surgically for long-standing, inactive pterygia before cataract surgery. However, if it is anticipated that pterygium surgery will be required in the future, it is advisable to perform the pterygium surgery first and allow the cornea to heal before obtaining biometry for cataract surgery. Salzmann nodular degeneration is generally easy to recognize and more likely to be found superiorly or obliquely in a peripheral or paracentral location. Depending on proximity to the visual axis and induced astigmatism, intervention may or may not be needed prior to cataract surgery. Similar to pterygia, however, if it is clear that surgical removal will be needed in the future, this should be performed well before biometry and cataract surgery to enable the most accurate IOL calculations (Figs. 34.3 and 34.4). Fig. 34.3 Irregular topography due to Salzmann nodular degeneration. Note pattern of irregular flattening. Keratoconus is characterized by a spectrum of severity and may occasionally be diagnosed for the first time in older patients being screened for cataract surgery. This entity is typically detected using topographic and tomographic corneal mapping. The differential diagnosis includes contact lens “warpage,” which may cause mild irregular astigmatism usually noted in a pattern of inferior steepening. Certainly rigid contact lenses may temporarily or permanently “mold” the cornea, and this is a point of consideration when performing biometry for IOL calculations. Toric IOLs may be used successfully in some stable keratoconus patients after careful evaluation. The cylinder must be regular or near regular for toric IOL implantation. But toric IOLs should not be used for irregular astigmatism or in patients expecting or needing to wear rigid contact lenses after cataract surgery. Great care should be taken in performing astigmatic keratotomy or limbal relaxing incisions in patients with thin or structurally unstable corneas. Clear corneal incisions (CCIs) may be used successfully in most patients, but some surgeons prefer scleral tunnel incisions in keratoconus patients. Very careful wound design is required in the case of CCI, and suturing may be advisable for optimum safety. Patients with severe keratoconus and cataracts may need combined or sequential keratoplasty to achieve acceptable visual outcomes. This is addressed in more detail later in the chapter.
Anatomy of the Cornea (Fig. 34.1)
Optics of the Cornea
Helpful Hints
The Cornea and Indications for Cataract Surgery
Surface Disorders
Helpful Hints
Stromal Disorders
Ento Key
Fastest Otolaryngology & Ophthalmology Insight Engine
