Contact Lens Spectrum

June 2013 Online Photo Diagnosis

By Patrick J. Caroline, FAAO, & Mark P. André, FAAO

Tissue Changes in Keratoconus

In 1931, Professor Alfred Vogt at the University of Zurich described in detail the classic biomicroscopic findings in keratoconus. Von der Heydt and Appelbaum classified the corneal changes into seven distinct types of tissue alterations. These changes may appear at varying periods throughout the course of the condition and may not be present in all cases of keratoconus.

Vertical Striae

This month’s online photo shows vertical striae, a series of parallel, whitish lines in the deep stroma. They are most likely tension lines caused by apical stretching of the corneal lamellae and are most often orientated vertically; however, they can often be aligned in the meridian of the greatest curvature (Figure 1).

Figure 1. Vertical striae represent tension lines caused by apical stretching.

Most often the striae are seen in the region of the corneal apex before it becomes densely scarred. Crossing systems of striae may produce a lattice-like design (Figure 2).

Figure 2. The presence of vertical striae is often the first positive slit lamp finding noted in keratoconus.

As a rule, the lines do not cross at the same level. Vertical or Vogt's striae can often be the earliest slit lamp finding noted in keratoconus. Some clinicians believe that the diagnosis of keratoconus cannot be made without the presence of vertical striae.

Apical Thinning

In the early stages of keratoconus, apical thinning is often difficult to detect with the slit lamp. Other positive slit lamp findings often precede that of apical thinning, and therefore other findings may be more helpful in detecting early forms of the condition.

When an eye that has advanced keratoconus is viewed in optic section, the thickness of the corneal apex may be reduced to one-third that of the periphery (Figure 3).

Figure 3. Optic section of a keratoconus eye with significant apical thinning.

It may be difficult to keep the entire section in exact focus at one time because of the excessive corneal curvature. In the later stages of the condition, this can lead to Munson’s sign, in which an angular curve is present by the lower lid margin when the patient looks down (Figure 4).

Figure 4. Munson’s sign in advanced keratoconus.

Fleischer’s Ring

Fleischer's ring is a yellow-brown or olive green pigmented line that partially or completely encircles the base of the cone (Figure 5).

Figure 5. Yellow-brown or olive-green pigmented iron line (Fleischer’s ring) in moderate keratoconus.

It is the result of a deposition and collection of iron (haemosiderin) anterior to Bowman's layer in the adjacent epithelium. The broken or interrupted ring occurs in approximately 50 percent of keratoconus cases, and the ring is often best viewed under blue light illumination with the slit lamp (Figure 6).

Figure 6. A Fleischer’s ring is often best visualized under the blue light illumination of the slit lamp.

Ruptures in Bowman’s Layer

These opacities form at or near the apex of the cone and represent structural breaks in Bowman's layer resulting in irregular superficial opacities and scars (Figure 7).

Figure 7. In this case of unilateral keratoconus, the corneal opacities seen on the right eye (left images) represent structural breaks in Bowman’s layer. Note the normal thickness and optical clarity of the left eye (right images).

The opacities begin as grayish dots located at the level of Bowman's layer. Later, the spaces between the opacities become opaque, and an irregular superficial opacity forms (Figure 8).

Figure 8. Corneal histology shows the breaks in Bowman’s layer and the subsequent accumulation of fibrillar connective tissue within the spaces.

These changes result when fibrillar connective tissue fills in the spaces where there are ruptures in Bowman’s layer. In advanced cases, these may account for a considerable loss of visual acuity secondary to the induced, irregular astigmatism and loss of corneal clarity (Figure 9).

Increased Visibility of the Corneal Nerve Fibers

The corneal nerve fibers may become more visible in certain cases of keratoconus, seen as a network of grayish lines with corpuscle-like nodes at the point of branching (Figure 10).

Figure 10. The corneal nerve fibers in keratoconus may be more easily visualized.

It is likely not because the nerve fibers are more numerous, but only that they are more easily seen due to density changes of the corneal nerve fibers. Because a similar clinical picture is often seen in both normal corneas and in keratitis (Figure 11), an increased visibility of the corneal nerve fibers cannot be considered a singular distinction of keratoconus.

Figure 11. The increased visualization of the nerves is made possible by an increase in fibril density of the superficial nerve. In some cases the fibers can create localized elevations in the epithelium that will result in a negative staining pattern with fluorescein.

Ruptures in Descemet’s Membrane

Spontaneous ruptures in Descemet's membrane occur in approximately 5 percent of patients who have keratoconus. The ruptures are characterized by a crescent-shaped tear in Descemet's membrane and endothelium at the apex of the cone (Figure 12).

Figure 12. Ruptures or tears in Descemet’s membrane permit aqueous to pass into the stroma, resulting in significant corneal edema and opacification.

Aqueous from the anterior chamber passes through the tear, resulting in corneal edema and opacification (hydrops) (Figure 13).

Figure 13. Corneal histology of a rupture in Descemet’s membrane.

The extent of the opacification varies with the size and extent of the rupture. In most cases the endothelium recovers, and within days begins a slow but steady deturgescence of the corneal opacification, a process that can take weeks to months (Figure 14).

Figure 14. In most cases the endothelium recovers and the tear closes. The cornea then begins the slow weeks-to-months process of deturgescence.

Following resolution of the hydrops, the rolled edges of the tear in Descemet's are visible. Corneas that do not regain transparency may require corneal transplant surgery (Figure 15).

Figure 15. In some cases the cornea fails to clear and surgical intervention, in the form of a corneal transplant, is required.

Endothelial Cup Reflex

This brilliant reflex is seen at the apex of the cone and accounts for the characteristic "dewdrop" or crystalline appearance. The intensified reflective properties are related to the increased curvature of the posterior corneal surface that may appear as a convex mirror (Figure 16).

Every Keratoconus Case is Different

Throughout the years, the slit lamp has provided practitioners with tremendous insight into the multitude of corneal changes that occur in keratoconus. However, it is important to remember that the various slit lamp findings may appear at different stages throughout the course of the condition. In addition, not all of the classic slit lamp findings may appear in every case of keratoconus.

References:

Vogt, A. Textbook and Atlas of Slit Lamp Microscopy of the Living Eye, J. Springer, Berlin 1931.

Von der Heydt, R. Slit Lamp Observation in Keratoconus, Transactions of the American Ophthalmologic Society 28: 352-361 1930.

Appelbaum, A. Keratoconus, Achieves of Ophthalmology, 15 (5): 900-921, 1936