PATHOLOGY

Histologically, keratoconus shows the following:

• iron deposition in the basal epithelium at the base of the cone

• fragmentation or breaks of the Bowman layer

• thinning of the corneal stroma and overlying epithelium

• folds or breaks in the Descemet membrane

• variable amounts of apical scarring

CLINICAL PRESENTATION

Nearly all cases are bilateral, but asymmetry is common. Early biomicroscopic and histologic findings include breaks in the Bowman layer followed by fibrous growth through the break, leading to reticular scarring (Fig 7-25). As progression occurs, the apical thinning of the central cornea worsens, as does the degree of irregular astigmatism. There is generally no associated keratitis or corneal neovascularization.

Figure 7-25 Broad, oblique slit-lamp image (high magnification). Breaks at the level of the Bowman layer (white arrow) are apparent. The black arrow indicates where collagen has presumably filled in a previous break. Vogt striae are also seen. This is the mechanism of the superficial reticular scarring seen in keratoconus.

(Reprinted from Shapiro MB, Rodrigues MM, Mandel MR, Krachmer JH. Anterior clear spaces in keratoconus. Ophthalmology. 1986;93(10):1316–1319.)

Scissoring of the red reflex on retinoscopy is commonly associated with irregular astigmatism and is an early sign of keratoconus. Rizzutti sign, a focusing of the light within the nasal limbus when a penlight is shone from the temporal side, is another early but nonspecific finding. Munson sign, an inferior deviation of the lower eyelid contour on downgaze (Fig 7-26), is also nonspecific and is a late sign. Iron deposition within the basal epithelium at the base of the cone forms a Fleischer ring (Fig 7-27), best seen with the slit lamp using a broad, oblique beam and the cobalt-blue filter. The line becomes narrower and increasingly well defined as the disease progresses. Fine, parallel stress lines, Vogt striae, can be observed in the posterior stroma at the apex of the cone and may disappear with application of external pressure (Fig 7-28).

Spontaneous perforation in keratoconus is extremely rare. However, a tear can occur in the Descemet membrane, usually late in the disease course, resulting in the sudden development of corneal edema, or acute hydrops (Fig 7-29). Allergy and eye rubbing are risk factors for the development of hydrops, which is more common in patients with Down syndrome. The break in the posterior cornea usually heals spontaneously within 3 months; the corneal edema then disappears, but posterior stromal scarring occurs. Some patients experience improved vision following the resolution of hydrops because of apical flattening. The improved vision will depend on the extent and location of the scar. Occasionally, stromal clefts can be seen in association with hydrops. Even large clefts usually close, but corneal neovascularization can develop.

Figure 7-26 Munson sign. Note the angulation of the lower eyelid with the eye in downgaze.

(Courtesy of Woodford S. Van Meter, MD.)

Figure 7-27 Fleischer ring (arrow), demonstrated using diffuse illumination with a cobalt-blue filter.

(Courtesy of James J. Reidy, MD.)

Figure 7-28 Vogt striae are fine folds in the posterior stroma at the cone apex. Striae become less apparent when external pressure is applied.

(Courtesy of Robert S. Feder, MD.)

Figure 7-29 Corneal edema in keratoconus due to a sudden rupture of the Descemet membrane (acute hydrops).

(Courtesy of Robert S. Feder, MD.)

EVALUATION

Keratometry can be used to detect keratoconus even at an early stage. Irregular astigmatism, indicated by the inability to superimpose the circular mires, is commonly seen. Inferior steepening, an early finding in keratoconus, can be detected by comparing measurements obtained with the patient in primary gaze to those in upgaze. Corneal topography and corneal tomography (Fig 7-30) have become indispensable in the management of keratoconus and are used by clinicians in several ways, such as detecting early keratoconus, following its progression, fitting contact lenses, and managing the postoperative patient. Important power map findings include superior flattening, inferior steepening, increase in I–S ratio (inferior to superior power), and significant skewing of the radial axes of the bow tie, in contrast to the normal symmetric bow-tie pattern of regular astigmatism. Findings obtained via slit-scanning devices, Scheimpflug imaging, or anterior segment OCT include decentered islands of elevation anteriorly and/or posteriorly. The elevation map is developed in reference to a computer-generated best-fit sphere.

These devices can also identify abnormal corneal thinning and/or decentration of the thinnest cornea, which is particularly significant when it is coincident with an island of elevation. Scheimpflug imaging and OCT have largely supplanted ultrasonic pachymetry. Accurate measurement of corneal thickness and elevation has been the cornerstone of recent advances in keratoconus detection programs. Examples of other advances are the various indices and graphic representations of both the change in thickness from thinnest cornea to limbus (more dramatic in keratoconus) and the impact of calculations of the best-fit sphere when the thinnest cornea is included or eliminated from the calculation. Serial evaluations over time are necessary to determine whether the changes noted are unlikely to progress (forme fruste keratoconus) or whether subclinical or suspected keratoconus is progressing and likely to become symptomatic. Corneal ectasia can also occur in association with ablative keratorefractive surgery (eg, LASIK or PRK). Risk factors for ectasia after LASIK or PRK are young age, high myopia, thin residual stromal bed, thin preoperative cornea, and abnormal preoperative contour. A young patient with a suspicious corneal contour who is considering refractive surgery should be observed over time to determine whether keratoconus will develop.

Figure 7-30 Corneal contour map. Upper left: Scheimpflug-based imaging illustrates a power map with inferior steepening and superior flattening. Upper right: An isolated island of anterior elevation above a best-fit sphere coincident with the maximum steepness. Lower right: An isolated island of posterior elevation. Lower left: Marked thinning in an area coincident with steepening and elevation.

(Courtesy of Robert S. Feder, MD.)

Feder RS. Corneal topography. In: Feder RS, ed. The LASIK Handbook: A Case-based Approach. 2nd ed. Philadelphia: Wolters Kluwer; 2013:32–39.

Gomes JA, Tan D, Rapuano CJ, et al. Global consensus on keratoconus and ectatic diseases. Cornea. 2015;34(4):359–369.

Rao SN, Raviv T, Majmudar PA, Epstein RJ. Role of Orbscan II in screening keratoconus suspects before refractive corneal surgery. Ophthalmology. 2002;109(9): 1642–1646.

Weisenthal R. Optical coherence tomography. In: Feder RS, ed. The LASIK Handbook: A Case-based Approach. 2nd ed. Philadelphia: Wolters Kluwer; 2013:40–46.

Yeu E, Belin MW, Khachikian SS. Topographic analysis in keratorefractive surgery. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017: 1728–1735.

MANAGEMENT

Some cases of mild keratoconus can be successfully managed with glasses. Patients should be counseled about the risk of progression with continued eye rubbing. Contact lenses can mask the associated irregular corneal astigmatism (Fig 7-31), and their use results in a dramatic improvement in vision in most cases. Although most patients will require hard or rigid gas-permeable lenses, some patients, particularly those with mild disease, may achieve improved vision and comfortable wear with soft lenses. Hybrid (rigid gas-permeable center with a soft lens “skirt”) or scleral lenses (standard or custom-made) may be helpful in more advanced disease. A central subepithelial scar can, on occasion, be removed (superficial keratectomy), allowing continued comfortable wear of contact lenses. Intrastromal corneal ring segments can be implanted to center the cone and facilitate successful contact lens wear. The procedure does not prevent progression, however, and is not intended to reduce dependence on glasses or contact lenses. Corneal crosslinking is used in patients with progressive disease. The risk of progression is greater in adolescents, who may benefit the most from this treatment. The greatest efficacy is achieved in mild to moderate cases. Corneal crosslinking may not work as well in cases of post-LASIK ectasia or in patients with more severe disease. Crosslinking may be performed in concert with intrastromal ring insertion. See also BCSC Section 13, Refractive Surgery.

Figure 7-31 Hard contact lens effect on the keratoconus power map; with lens (right side) and without lens (left side). Upper left: Apical scarring has resulted in flattening, as noted in the power map. Lower left: The corresponding videokeratoscopic image is irregular, and the rings are widely spaced. Upper right: The power map appears much more normal when acquired with the patient wearing a contact lens. Lower right: The videokeratoscopic image is also much more regular, with narrower spacing.

(Courtesy of Robert S. Feder, MD.)

Keratoplasty becomes an important option under the following circumstances:

• poor vision even with a comfortable contact lens fit (usually due to scarring)

• contact lens intolerance even with good vision

• unstable contact lens fit (even with good vision and tolerance)

• progressive thinning to the corneal periphery approaching the limbus, requiring a very large graft (with increased risk)

• corneal hydrops that fails to clear after several months

PK is still the most widely performed surgical procedure for the treatment of keratoconus, and the prognosis is excellent. Some surgeons prefer DALK for keratoconus, especially when tissue suitable for PK is in short supply. Endothelial cell counts are significantly higher at 6 months following DALK compared with PK. Endothelial rejection does not occur after DALK, but stromal rejection is still possible. Postostoperative wound integrity in the case of postoperative trauma is better with DALK than with PK. (See Chapter 15 for further discussion of PK and DALK.)

Hydrops is treated conservatively with topical hypertonic agents and/or a soft contact lens for several months. A cycloplegic agent may help with pain relief. Aqueous suppressants may decrease the flow of fluid into the cornea. Intracameral injection of air or gas (SF6 or C3F8) may help in speeding the resolution of hydrops. Pupil dilation and/or inferior peripheral iridectomy may reduce the risk of pupillary block. Hydrops is not an indication for emergency keratoplasty.

Feder RS, Neems LC. Noninflammatory ectatic disorders. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:820–843.

Kiliç A, Colin J. Advances in the surgical treatment of keratoconus. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2012, module 2.

Panda A, Aggarwal A, Madhavi P, et al. Management of acute corneal hydrops secondary to keratoconus with intracameral injection of sulfur hexafluoride (SF6). Cornea. 2007;26(9): 1067–1069.

Raiskup-Wolf F, Hoyer A, Spoerl E, Pillunat L. Collagen crosslinking with riboflavin and ultraviolet-A light in keratoconus: long-term results. J Cataract Refract Surg. 2008;34: 796–801.

Terry M, Ousley P. Deep lamellar endothelial keratoplasty visual acuity, astigmatism, and endothelial survival in a large prospective series. Ophthalmology. 2005;112(9):1541–1548.

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.