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.