Corneal Topography in Keratoconus

BY PATRICK J. CAROLINE, C.O.T., F.A.A.O., MARK A. ANDRE, F.C.I.S.A.& CRAIG NORMAN, F.C.L.S.A.
JULY 1997

Corneal topography and photokeratoscopy can help us distinguish the various manifestations of keratoconus and aid in selecting contact lens parameters.

In clinical practice, the term keratoconus describes an entire spectrum of diversely shaped conditions in which the only common denominator is central or paracentral corneal steepening. Yet, the topography beyond the central and paracentral cornea may significantly influence the success or failure of a contact lens design. Therefore, to better understand modern keratoconus contact lens fitting techniques, it's imperative that we take into consideration the diverse central and midperipheral topographical manifestations associated with this condition.

PROGRESSION OF KERATOCONUS

In early keratoconus, keratoscopy reveals a characteristic pear-shaped elongation of the central mires, with initial steepening occurring midperipherally below the corneal midline. This is characterized by the close proximity of the keratoscopy rings to one another. Above the midline, the curvature of the superior cornea remains relatively normal with radii similar to, or flatter than, the non-keratoconic cornea (Figs. 1a, 1b & 1c).

FIG. 1a: PHOTOKERATOSCOPY OF EARLY KERATOCONUS (OS). NOTE THE PEAR-SHAPED ELONGATION OF THE CENTRAL KERATOSCOPY RINGS AND THE CLOSENESS OF THE RINGS (STEEPENING) IN THE INFEROTEMPORAL QUADRANT.

FIG. 1b: TOPOGRAPHIC MAP OF THE PATIENT IN FIG. 1a.

FIG. 1c: DIOPTRIC POWER VALUES OF FIG. 1b.

As the condition progresses, the steepening can spread more nasally to include the inferior 6 o'clock and inferonasal cornea. In advanced keratoconus, there may be rotational steepening at and above the midline along a path that includes the temporal, superior temporal and superior 12 o'clock cornea (Fig. 2). The superior nasal quadrant of the cornea is usually the last area to be affected. Therefore, an island of normal cornea often remains superonasally (Figs. 3a & 3b).

FIG. 4a: PHOTOKERATOSCOPY OF NIPPLE KERATOCONUS (OS). NOTE THE NEARLY 360 DEGREES OF NORMAL CORNEAL TOPOGRAPHY THAT SURROUNDS THE STEEP CENTRAL ECTASIA.

FIG. 3a: PHOTOKERATOSCOPY OF ADVANCED KERATOCONUS (OD). NOTE ISLAND OF NORMAL CORNEA IN THE SUPERIOR NASAL QUADRANT.

FIG. 3b: DIOPTRIC POWER VALUES OF FIG. 3a. KERATOMETRY: 65.50/64.75 CENTRAL, 41.50 SUPERONASALLY

There are exceptions to this pattern of progressive topographical changes, but serial keratoscopy evaluations indicate that most keratoconic eyes follow this spiral progression.

Many of the important changes in keratoconus occur well beyond the three-millimeter area measured by keratometry. Therefore, photokeratoscopy and videokeratography are essential in identifying where the midperipheral cornea is steepest (where the contact lens will be loosest) and flattest (where the lens will be midperipherally tightest).

We believe keratoconus encompasses topographical presentations that can be classified as nipple, oval, temporal and globus. These shapes are most likely the result of simple apex location as well as yet unidentified stromal factors that control midperipheral corneal shape.

NIPPLE KERATOCONUS

The nipple form of keratoconus characteristically consists of a small, near-central ectasia, less than five millimeters in diameter. The most striking features of the nipple topography are: 1) an often high degree of with-the-rule corneal toricity confined to the central five millimeters of the cornea; 2) nearly 360 degrees of normal midperipheral cornea that surrounds the base of the cone (Figs. 4a, 4b & 4c); and 3) the occasional presence of an elevated fibroplastic nodule at the apex of the cornea, hence the name nipple keratoconus (Fig. 5).

FIG. 4a: PHOTOKERATOSCOPY OF NIPPLE KERATOCONUS (O3). NOTE ISLAND THE NEARLY 360 DEGREES OF NORMAL CORNEAL TOPOGRAPHY THAT SURROUNDS THE STEEP CENTRAL ECTASIA	FIG.4b: TOPOGRAPHIC MAP OF FIG.4a DEMOSTRATING THE STEEP, CENTRALIZED, WITH THE RUEL ASTIGMATISM SURROUNDED BY NEARLY 360 DEGREES OF NORMAL MIDPERIPHERAL CORNEA
FIG. 4c: DIOPOTRIC POWER VALUES OF FIG.4B	FIG.5: ELEVATED FIBROPLASTIC NODULE NOTED IN THE NIPPLE FORM OF THE KERATOCONUS

The rapid change in curvature from the steeper center to the more normal midperipheral cornea makes the nipple form of keratoconus the most difficult to manage with rigid contact lenses. A rigid lens will often erode the superficial fibroplastic nodules, so these patients often require RGP/soft piggyback contact lens designs, custom keratoconus soft lenses or surgical removal of the nodule by manual superficial keratectomy or phototherapeutic keratectomy.

OVAL KERATOCONUS

The most common shape in advanced keratoconus is oval, whereby the corneal apex is displaced well below the midline, resulting in varying degrees of inferior midperipheral steepening. This 'pushing out' of the cornea inferiorly creates an island of normal or flatter-than-normal superior cornea exactly 180 degrees away (Figs. 6a, 6b & 6c).

FIG. 6a: PHOTOKERATOSCOPY OF OVAL KERATOCONUS (OS). NOTE THE ISLAND OF NORMAL CORNEA ABOVE THE MIDLINE FROM 9 O'CLOCK TO 2 O'CLOCK.

6b: TOPOGRAPHIC MAPPING OF FIG. 6a.

FIG. 6c: DIOPTRIC POWER VALUES OF FIG. 6B.

TEMPORAL KERATOCONUS

In 1990, Rodger Kame, O.D., described a variation of the oval topographic presentation of keratoconus in which the apex is significantly displaced temporally. This form, which he termed temporal keratoconus, is hallmarked by significant temporal steepening and subsequent nasal flattening (Figs. 7a, 7b & 7c).

FIG. 7a: PHOTOKERATOSCOPY OF TEMPORAL KERATOCONUS (OD).

FIG. 7b: TOPOGRAPHIC MAP OF FIG. 7a.

FIG. 7c: DIOPTRIC POWER VALUES OF FIG. 7b.

GLOBUS KERATOCONUS

The globus form of keratoconus is the largest, encompassing nearly three-quarters of the corneal surface. Due to its size, nearly all of the keratoscopy rings will be encompassed within the area of the ectasia. Unlike the advanced forms of nipple or oval keratoconus, the globus cone has no island of normal midperipheral cornea above or below the midline (Figs. 8a, 8b & 8c).

FIG. 8a: PHOTOKERATOSCOPY OF GLOBUS KERATOCONUS (OD). NOTE THAT ALL NINE RINGS OF KERATOSCOPY IMAGES ARE ENCOMPASSED WITHIN THE AREA OF THE ECTASIA.

FIG. 8b: TOPOGRAPHIC MAP OF FIG. 8a.

FIG. 8c: DIOPTRIC POWER VALUES OF FIG. 8b.

SUMMARY

There's little doubt that the various central and midperipheral topographies will influence the final contact lens parameters that we choose for our keratoconus patients. Therefore, it's imperative that we appreciate the diverse symmetrical and asymmetrical presentations of this condition.

Quantitative corneal surface analyses using photokeratoscopy and videokeratography will enable us to identify the steepest and flattest portions of the cornea. This information will help us clarify many of the peculiar fluorescein patterns we see during our diagnostic contact lens evaluations. CLS

Patrick Caroline is assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University, Portland.

Mark Andre is the director of contact lens services at the Casey Eye Institute and a research associate at the Oregon Health Sciences University, Portland.

Craig Norman, an advisor to the RGP Lens Institute, is director of the contact lens section at South Bend Clinic, South Bend, Ind.