Keratoconus suspects, form fruste and eye rubbing

Forme Fruste Keratoconus (FFKC)

All you need to know about forme fruste keratoconus.

MUCH ADO ABOUT KERATOCONUS TERMINOLOGY

The currently used phrases « forme fruste », « early subclinical » or « keratoconus suspect » in keratoconus terminology is fraught with confusion. The problem exists because keratoconus represents a spectrum of corneal deformation, and especially in its early stages, cannot be classified in binary terms into whether a person has or does not have the disease. What is certain, however, is that corneal deformation exists, and any attempt to draw a line to segregate healthy eyes from eyes with early disease is arbitrary.

The existence of these « forms fruste » stages of the disease is, in fact, another proof of the non-truly dystrophic and genetic nature of keratoconus, which is essentially caused by a repeated mechanical trauma. As we will detail below, the earliest forms of keratoconus simply correspond to moderate deformations caused by moderate frictions. Forme fruste keratoconus is the topographic consequence of « forme fruste rubbing ».

Several terms have been proposed to describe early forms of keratoconus which may be difficult to detect. The term « forme fruste keratoconus (FFKC) » was first proposed by Amsler in 1961, and then refined by Klyce et al. to define the apparently normal contralateral eye in unilateral KC, the forme fruste being « an incomplete, abortive, or unusual form of a syndrome or disease ».

From our point of view, there are strictly unilateral forms of keratoconus. They are rare but occur in patients who rub one eye, and never the other. In fact, considering that the un-rubbed eye (and whose cornea is not deformed) presents a forme fruste keratoconus is not accurate.

In the case of mild acquired permanent corneal deformation corresponding to early forms of keratoconus (which may be unfairly classified as a disease entity, as corneal deformation is mild and visually and topographically silent), it may not be justified to look for a line which would separate the affected from the unaffected eyes because such distinction has little relevance. It would be more practical, especially in the context of Refractive Surgery, to consider these cases in terms of the risk for further weakening or deformation after a refractive surgical procedure eg LASIK. (See SCORE Analyzer below, which is a software that scores each cornea with regards to the risk of post LASIK ectasia in myopic eyes).

What is of paramount importance, however, is to detect these early forms of corneal deformation, to arrest their progression via the search and deliverance of proper information about eye rubbing, and to prevent the acceleration of the corneal weakening by inadvertent LASIK surgery.

While LASIK is contraindicated in corneas which have already been weakened by years of eye rubbing and exhibit even minor deformations, photorefractive keratectomy (PRK) may be a viable option for the correction of low to mild myopic refractive errors in these cases.

In their earliest stages, subtle corneal deformations can be totally silent in terms of visual or local symptoms. The presence of minor topographic abnormalities without any significant clinical manifestation corresponds to what has been called « subclinical keratoconus ». This is an example of a « grey area » of medicine, and there is a persistent ambiguity regarding the exact definition of a « subclinical » keratoconus. Some automated detection software use the term « keratoconus suspect (KCS) » to characterize corneas in which some subtle deformation can be evidenced using corneal topography, but not pronounced enough to deserve with certainty the diagnosis « keratoconus ».

As stated above, forme fruste keratoconus (FFKC) has sometimes been defined as a cornea that has no clear abnormal findings by both slit-lamp examination and Placido-based corneal topography, while the patient’s fellow eye has an evident form of clinical keratoconus, because most physicians believe that keratoconus is always a bilateral disease. Following this theory, every fellow eye which appears normal while the other eye has frank keratoconus can be labeled a « FFKC » eye. In such eyes, the clinician should not find any clinical signs of clinically manifest keratoconus, like Vogt’s striae and Fleischer’s ring, nor should this eye have any significant topographic features like asymmetric bow-tie or skewed axis deviation, which would suggest an early form but yet clinically detectable keratoconus.

To diagnose keratoconus at its earliest stages, efforts have been made using available technologies such as corneal specular topography, tomography (corneal thickness maps), or instruments evaluating corneal biomechanics, in patients with vague visual symptoms, rapid evolution of myopic astigmatism or routinely during risk assessment of candidates for refractive surgery. Nowadays, combining parameters from corneal tomography, wavefront aberration, and biomechanics may be useful to improve the diagnostic ability of subclinical keratoconus. This is particularly important in the assessment of refractive surgery suitability because procedures such as LASIK or SMILE can further compromise corneal biomechanics and accentuate corneal deformation.

Schematic representation of groups of interest, the distribution of the values of the index, and the verdict based on the value of the index with respect to a cut-off in the context of automated keratoconus screening based on corneal topography indices. Forme fruste keratoconus is not detected, at a given limit of detection, by a validated test recognized by the ophthalmic community. Corneas with a value just beyond the cut-off may correspond to true keratoconus, normal corneas (false-positive) or true subclinical keratoconus (true positives).

The following table sumarizes the main features of this classification :

The term “forme fruste” often designates topographic forms that raise little or no suspicion, but which are known to constitute a minor form of the disease, either because of the minimal Placido topographic abnormalities, i.e. below the accepted limit of detection for keratoconus-suspect, but other suggestive tomographic, topographic or pachymetric abnormalities exist, or because of a suggestive clinical context. For example, it has been proposed that in a patient presenting with keratoconus in one eye, when the cornea of the fellow eye presents a negative test based on Placido topography data, this cornea can be considered to have forme fruste keratoconus, even if the phenotypic expression of keratoconus in this eye is below the limit of detection according to the same criteria. Similarly, progression of a cornea initially considered to be normal (Placido topography negative for keratoconus) towards a clinical form of keratoconus can lead to a retrospective interpretation of the initial examination as forme fruste keratoconus. Detailed topographic documentation of these clinical situations is essential, as it allows the definition of new cut-offs or new criteria (indices). It is also crucial to understand the reason of any subtle corneal deformation. The keratoconus deformation is initiated by vigorous and repeated eye rubbing. Before reaching an advanced and detectable change, the cornea undergoes progressive permanent shape alteration. The keratoconus « forme fruste » chase corresponds to the detection of the earliest shape changes.

These diagnostic subtleties are always, however, very dependent on arbitrary thresholds and definitions. It is quite difficult to draw a line between what is « normal » and what is « abnormal » within corneal topography maps. Some practitioners or automated diagnostic algorithms restrict their interpretation to the anterior surface of the cornea, while others consider the posterior surface and variations of the corneal thickness in their analysis. Algorithms and threshold may vary between topographic instruments of various manufacturers.  (more about keratoconus detection indices). Hence, there may be discrepancies in subjective and objective analysis of the same cornea when analyzed by different instruments and/or interpreted by automated diagnostic algorithms or physicians.

There is a continuous spectrum of topographic features from corneas judged « very normal » to corneas that are obviously showing topographic abnormalities. While this  variability in the amount of corneal deformation (between patients and eyes of the same patient) may be difficult to explain if keratoconus would be an acquired or genetically inherited disease, our experience reveals that the variable techniques of eye rubbing (in type, duration, intensity, frequency) may explain this broad topographic spectrum. Some representative examples are presented later on this page.  In the light of this, the following hypothesis on FFKC could be coined:

Forme fruste keratoconus is the consequence of « forme fruste rubbing »: a less pronounced form of keratoconus due to less frequent and less pronounced eye rubbing.

THE SCORE ANALYZER (Bausch & Lomb TechnoLas, USA)

Using the data provided by the Orbscan (Bausch & Lomb TechnoLas, USA), we have conceived a software (the SCORE Analyzer) which uses 12 variables to estimate the level of corneal deformation and thinning, and quantifies this into a single discrete number (the « SCORE » of the cornea). The SCORE objectively categorizes the corneas into « positive for topographic risk of ectasia » after LASIK or « negative for topographic risk ». The threshold has been set at zero where a zero or positive numeral would indicate « positive for topographic risk of ectasia ». In the generation of the algorithm for the SCORE Analyzer, the corneas analyzed  had SCORE numbers varying from about -4 (normal corneas, thick and regular) to +25 (very deformed and thinned corneas), illustrating the presence of a corneal deformation continuum. Again, the choice of a threshold set to zero is arbitrary and corresponds from our point of view to the best compromise between sensitivity and specificity in the considered task : to segregate refractive candidates.

This algorithm has been validated in both Caucasian (read paper) and Asian eyes (read paper)

You can find out more about the SCORE Analyzer here

Example of the SCORE Analyzer display. The score is slightly positive, due to the combination of central thinning, and inferior steepening.

Here are some examples of minor corneal deformations which can all be explained by particular rubbing habits.

This 55 yo patient had been referred for refractive surgery evaluation (correction of hyperopia). Minor topographic abnormalities are seen in the right eye: a slight inferotemporal steepening (A) and a paracentral inferior thinning (B). The posterior surface prolateness is also increased (C).  The left eye topography is unremarkable. The right eye topographic features are commonly found in forme fruste keratoconus.

When asked about possible chronic rubbing habits, the patient admitted that he often wiped his right eye during the day because this eye felt irritated and watery. When questioned about his sleeping habits, the patient declared that he constantly slept on his stomach with the head rotated to the left, causing the right orbit to be compressed onto his right forearm.

In our experience, there is a striking correlation between the side on which patients sleep and the side where the most pronounced topographic abnormalities are evident.

Compression, heating and local contamination all may contribute to ocular irritation at night, leading to itching sensations during the day. The itch in turn triggers repeated eye rubbing leading to various levels of deformation depending on the native corneal properties and the duration, frequency, type, and intensity of eye rubbing.

In this case, the repeated rubbing has even caused the arms of the spectacle frame to be twisted!

Routine pre-operative corneal topographical evaluation led to the discovery of an aspect of minor corneal deformation characterized by a SRAX (skewed radial axes) pattern (deviation of the hemi-meridians of maximum curvature).

The 35 yo female patient was questioned about the possibility of ocular rubbing. She acknowledged that she was often vigorously rubbing her eyes in the evening and at night, after removal of make up and contact lenses.
The pulp of the index fingers was used to massage the eyelids and the adjacent globes quite vigorously.

The repeated movements are likely to explain the mild corneal deformation. As these movements were not exerted during the day, the deformation of the corneal dome remained moderate.

In women, the nails are sometimes long and varnished. As can be seen, this requires a special ocular rubbing technique which is performed with the pulp of the fingers (usually the index fingers).

In the context of discovering an aspect of very minor corneal deformation affecting the left eye; this 37 yo woman was questioned about her sleeping position and possible ocular rubbing. The patient declared that she used to sleep on the left side, placing her hands below her left cheek and orbit. She admitted that she often rubbed the left eyelid during the day, again using the pulp of her fingers (middle finger mostly) with lateral and circular movements.

topography of the cornea patient rubbing his eyes

The presence of corneal topography abnormalities in the left eye lead to the choice of laser surface ablation (photorefractive keratectomy – PRK) for the correction of mild myopia and astigmatism in this 45 yo patient. During the first pre-operative evaluation visit, the patient declared that he did not rub his eyes, nor was he aware of such a habit. We invited him to pay attention to possible unconscious rubbing episodes. The patient was seen on the day of surgery and was questioned again. He acknowledged that he was often sleeping with the left arm resting on the head, or the left side of the head in the pillow. He could distinctively feel the pressure exerted on the left eye at night. He realized that he was rubbing his left eye many times a day, as he felt a « heavy » sensation and itch in the left orbit.

In such cases of FFKC, cessation of eye rubbing prevents evolution to frank keratoconus.

Forme fruste keratoconus (FFKC) is often neglected and may go unnoticed. This may represent a real risk for eventual refractive surgery, in particular the risk of post-LASIK ectasia, The definition of forme fruste keratoconus will undoubtedly evolve in the future, as new tests become accepted as new standards for the detection of early subclinical forms of keratoconus.

Our real forme fruste keratoconus cases

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