Use these clues and evidence that are associated with early KCN

Because keratoconus (KCN) is a progressive thinning and steepening of the paracentral cornea that can cause high myopia, astigmatism, and lead to permanent vision loss, optometrists need to keep an eye out — no pun intended — for it. (See “KCN: An Overview,” below.) That said, early stage KCN can be clinically elusive, due to its absence of inflammation and its subtle reduction in best-corrected vision.

Here, I discuss the corroborating clues and specific evidence associated with early KCN and the next steps after you’ve made a definitive diagnosis. This way, these patients can maintain satisfactory vision.

Corroborating clues

While none of the following symptoms are specific to KCN, our suspicion must be heightened when reported by patients in their early to mid-twenties (see sidebar), in whom other clinical explanations, such as diabetes, are much less unlikely.

• Complaints of reduced vision in one eye more than the other. While KCN is a bilateral condition, it typically progresses asymmetrically.
• Unsatisfactory prescription updates. KCN patients will present with a history of having their prescription updated frequently within the past year but with frustration that the changes do not quite result in improved vision.
• Glare
• Photophobia
• Distorted night vision

Specific evidence

The following confirms the KCN diagnosis:

• Scissoring of the red reflex via retinoscopy.
• Asymmetrically increased astigmatism, but visual acuity is not improved upon manifest refraction, as indicated by autorefraction.
• Irregular astigmatism with steepening, high-central corneal power. Central K larger than 47.8 D and an inferior to superior differential greater than 1.9 D indicate a clinical manifest KCN. Corneal power readings that follow these values are helpful guidelines: Normal <47.2 D/forme fruste KCN 47.2 D to 48.7 D/KCN >48.7 D.¹
• A posterior corneal surface elevation greater than 12 mm, via tomographical 3D image. A normal cornea will have a maximum elevation of 10 mm.
• Lower corneal hysteresis values, as per a dynamic bi-directional applanation tonometer.² There is an approximately 2.5 mm HG difference between normal values and KCN.
• Lower minimum epithelial thickness on OCT. One study shows that a thin epithelial thickness 52u less than the thickest point differentiated very early KCN from normal eyes.³
• The appearance of Vogt’s striae in the deep stroma, via slit lamp findings. These are bright, parallel stress lines caused by the tension of corneal stretching.
• Fleisher’s ring, via slit lamp findings. This is a pigmented ring resulting from iron deposition in basal epithelial cells. It may be detected in the peripheral cornea.
• Keratometry readings greater than 47 D.

Next steps

Timely referral for corneal collagen cross-linking is a main benefit of the diagnosis of early KCN. As a reminder, this procedure strengthens the cornea and stabilizes disease progression, improving best-corrected spectacle vision, as well as potentially reducing the need for corneal grafting.⁴

My contact lens management of KCN is guided by the patient’s correctable vision and severity of corneal irregularity. I begin with soft lenses when possible (either standard or KCN design) and move to hybrid and rigid lens designs when acceptable vision is no longer achievable. (I elect to start with soft lenses, as many patients prefer them for their convenience and reduced initial cost. Additionally, soft lenses give newly diagnosed patients an approach with less anxiety and a feeling of being more “mainstream.”)

Fortunately, scleral lenses have allowed most of my patients who have moderate-to-advanced disease to postpone or avoid corneal transplants.

Battling the bulge

As primary eye care providers, ODs are on the “front lines” of detecting and managing this visually debilitating condition. Early detection and timely diagnosis are critical to optimizing clinical outcomes and preserving patients’ quality of life.⁵ Pro tip: Consider complimentary screening of the adolescent children of your KCN patients to proactively diagnose and manage this life-altering condition. OM

References

1. Rabinowitz YS. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. 1995;;11(5):371-9. doi: 10.3928/1081-597X-19950901-14.
2. Cavas-Martínez F, De la Cruz Sánchez E, Nieto Martínez J, Fernández Cañavate FJ, Fernández-Pacheco DG. Corneal topography in keratoconus: state of the art. Eye Vis (Lond). 2016;3:5. doi: 10.1186/s40662-016-0036-8.
3. Zhang, X., Munir, SZ, Sami Karim, SAS, Munir WN. A review of imaging modalities for detecting early keratoconus. Eye (Lond). 2021;35(1):173-187. doi: 10.1038/s41433-020-1039-1.
4. Cifariello F, Minicucci M, Renzo FD, et al. Epi-Off versus Epi-On Corneal Collagen Cross-Linking in Keratoconus Patients: A Comparative Study through 2-Year Follow-Up. J Ophthalmol. 2018:4947983.doi: 10.1155/2018/4947983.
5. Kandel H, Pesudovs K, Watson SL. Measurement of Quality of Life in Keratoconus. Cornea. 2020;39(3):386-393. doi: 10.1097/ICO.0000000000002170.
6. American Academy of Ophthalmology. Diagnosis and Management of Keratoconus. https://www.aao.org/eyenet/article/diagnosis-and-management-of-keratoconus . Accessed August 21, 2023.
7. Godefrooij DA, de Wit GA, Uiterwaal CS, Imhof SM, Wisse RP. Age-specific Incidence and Prevalence of Keratoconus: A Nationwide Registration Study. Am J Ophthalmol. 2017;175:169-172. doi: 10.1016/j.ajo.2016.12.015.

DR. RESNICK is president and managing partner at Drs. Farkas, Kassalow, Resnick, and Associates, a contact lens and anterior segment specialty practice in New York. She is a diplomate in the Cornea, Contact Lens, and Refractive Technology section of the AAO, a diplomate of the ABO, and a fellow of the Scleral Lens Education Society.