KEY CONCEPTS
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Keratoconus (KC) is described as a condition in which the cornea assumes a conical shape with coinciding corneal stromal thinning inducing irregular astigmatism, myopia, and protrusion.
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Forme fruste is an atypical or attenuated manifestation of a disease or syndrome, with the implications of incompleteness, partial presence, or aborted state.
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Suspicious topography is defined as a corneal topography that presents signs of abnormality that are not enough to claim keratoconus.
Keratoconus: Definitions
Keratoconus (KC) is a condition in which the cornea assumes a conical shape with coinciding corneal stromal thinning. This process induces irregular astigmatism, myopia, and protrusion, leading to mild to markedly impaired visual quality. , KC is a progressive disorder that typically affects both eyes, although disease presentation can be highly asymmetric, and only one eye may be affected clinically. Symptoms are highly variable and, in part, depend on the stage of the disease progression. Early in the disease there may be no specific symptoms, and KC may be noted by the ophthalmologist simply because the patient cannot be refracted to a clear 20/20 corrected vision. In advanced disease there is significant distortion of vision accompanied by profound visual loss.
The diagnosis of KC, in its earliest stages, remains challenging and controversial. In 2015 a consensus of ophthalmologists from around the world used a Delphi panel approach to put forward opinions regarding KC and ectatic diseases, focusing on definition, clinical management, and surgical treatments. The experts determined that KC is an ectatic disorder in which abnormal posterior ectasia, abnormal corneal thickness distribution, and clinical noninflammatory corneal thinning are essential findings for diagnosis.
Other authors, however, have questioned these diagnostic criteria. Neither . nor . found that posterior corneal or regional thickness metrics successfully identified the clinically unaffected eye in patients with highly asymmetric KC, whereas . and . both found combined metric strategies to distinguish between the clinically unaffected eye and a normal cohort. However, neither analysis found posterior corneal metrics or regional thickness metrics to be effective in their modeling.
Although the classical KC definition refers to it as a non inflammatory disorder, enough information exists for inflammation to be considered as a significant component of the pathophysiology of KC. The inflammation observed in KC resembles the chronic inflammation associated with systemic diseases rather than acute inflammation. This is evidenced by the fact that cytokines that make up the acute phase reaction, such as tumor necrosis factor alpha (TNF-α), are not overly expressed in KC, but chronic inflammation-associated target genes such as matrix metalloproteinase-9 (MMP-9) are significantly high. Also, KC corneas have a reduction in collagen fibril-maturating enzyme lysyl oxidase (LOX), a cross-linking enzyme involved in the formation of a fibrillar extracellular matrix through the oxidative linkage of collagen. Specifically, a significant reduction of transcript levels in KC corneal epithelia and LOX activity in KC tears correlated with disease severity have been reported. ,
Initially, the diagnosis of KC was based on slit-lamp examination and clinical signs, and then topographic signs were included as diagnostic criteria. The diagnosis of clinical KC is based on classic corneal biomicroscopic and topographic characteristics including the presence of one or more clinical signs (corneal stromal thinning, Vogt striae, Fleischer ring, scissoring of the red reflex, or oil droplet sign identified by retinoscopy) and the presence of topographic characteristics (an increased area of corneal power surrounded by concentric areas of decreasing power, inferior-superior [I-S] power asymmetry, and skewing of the steepest radial axes above and below the horizontal meridian). Fig. 2.1 shows an advanced case of KC, in which all these topographic characteristics are present.
More recently, with the advent of Scheimpflug imaging analysis and anterior segment optical coherence tomography (OCT), early diagnosis of the disease was achieved with information such as pachymetry, epithelial, elevation, and aberrometry data ( Figs. 2.1 and 2.2 ). As a result of the early diagnosis provided by these new devices, many terms have come to life such as subclinical keratoconus (SCK), and forme fruste keratoconus (FFKC). A variety of indicators have been suggested to diagnose these entities; however, there is a lack of unified criteria to define them.
SUBCLINICAL KERATOCONUS
The term “subclinical” by dictionary definition is “not detectable or producing effects that are not detectable by the usual clinical tests.” Currently, the usual tests for KC diagnosis include corneal topography and tomography. Therefore although many papers today may use the term “subclinical” to mean disease evident on imaging but not on “clinical” examination, this definition appears to be incorrect and outdated.
Henriquez et al, in a review concerning the definitions used in published articles for the terms SCK and FFKC, noted that the most frequently cited criteria combinations used to define SCK are normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy plus I-S asymmetry and/or bow-tie pattern with skewed radial axis plus diagnosis of KC in the fellow eye. Table 2.1 shows the frequency of parameters used in the definition of SCK in published studies. Table 2.2 shows the most commonly used combinations to define SCK in the studies.
| Parameter | Frequency | ||
|---|---|---|---|
| Diagnosis of KC in the fellow eye | 72.72% (24/33) | ||
| Normal-appearing cornea on slit-lamp biomicroscopy, keratometry, retinoscopy, and ophthalmoscopy | 45.45% (15/33) | ||
| Inferior-superior (I-S) asymmetry and/or bow-tie pattern with skewed radial axes | 36.36% (12/33) | ||
| Lack of any KC-related findings/signs in the slit-lamp biomicroscopy | 33.33% (11/33) | ||
| No history of contact lens use, ocular surgery, or trauma | 21.21% (7/33) | ||
| Corneal topography showing an abnormal localized steepening or central/inferior steepening or asymmetric bow-tie pattern or claw-shaped pattern on topography | 21.71 (7/33) | ||
| One of the following signs: steep keratometric curvature greater than 47 D, oblique cylinder greater than 1.5 D, central corneal thickness less than 500 μm | 12.12% (4/33) | ||
| I-S lower than 1.4 D and/or maximum keratometry lower than or equal to 47 D | 12.12% (4/33) | ||
| Keratoconus percentage index (KISA) between 60% and 100% in the SCK eye | 9.09% (3/33) | ||
| Keratoconus percentage index (KISA) lower than 60% | 9.09% (3/33) | ||
| Normal topography (with no asymmetric bow tie and no focal or inferior steepening pattern) | 9.09% (3/33) | ||
| No topography finding significant enough to be diagnosed as clinical | 6.06% (2/33) | ||
| Keratoconus severity score (no specified number was used for SCK definition) | 6.06% (2/33) | ||
| Abnormal biomicroscopic findings including Vogt’s striae and Fleischer ring >2 mm or skewed radial axis (SRAX) >21 degrees or >20 degrees, or keratoconus predicting index (KPI) >30% or >0.3 or keratoconus severity index (KSI) >30%, and abnormal keratoconus index (KCI) | 6.06% (2/33) | ||
| Corrected distance visual acuity (CDVA) of 20/20 or better (Snellen) | 6.06% (2/33) | ||
| Keratoconus severity score (KSS) 0, 1, or 2 | 6.06% (2/33) | ||
| Paracentral I-S dioptric asymmetry difference in 1.4 to 1.9 D gradient | 6.06% (2/33) | ||
| Belin/Ambrósio Enhanced Ectasia total deviation index (BAD-D) from the Pentacam <1.60 standard deviations | 3.03% (1/33) | ||
| Corvis biomechanical index (CBI) score of greater than 0.5 in both eyes | 3.03% (1/33) | ||
| Simulated central corneal power greater than 47.2 D but less than 48.7 D | 3.03% (1/33) | ||
| Maximum keratometry ≥47 D/47.2 D | 3.03% (1/33) | ||
| Elevation of the posterior corneal surface (unspecified quantitative value) | 3.03% (1/33) |
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