Keratoconus and myopia: what’s the link?

Keratoconus and myopia are often considered distinct ocular conditions; however, studies suggest there may be a link between the two, potentially affecting their management and treatment strategies in clinical practice. This connection underscores the need for comprehensive diagnostic approaches and individualized care plans, especially for patients presenting with both conditions. Here, we’ll be discussing the association between keratoconus and myopia, the importance of screening for and ruling out keratoconus in myopic patients, and how keratoconus can affect myopia management.

In keratoconus, the cornea undergoes progressive thinning and structural changes, leading to its characteristic conical shape and irregular astigmatism.¹ In myopia, ocular structural changes are primarily (though not exclusively) related to axial elongation of the globe.² A study which followed 50 keratoconic eyes and compared them with 50 emmetropic eyes, to investigate the potential relationship between keratoconus and axial myopia, found that there was a significant correlation. Those in the keratoconus group appeared to have greater axial length (23.97 mm compared to 23.21 mm), correlating with a myopic subjective refraction.³ Another study also found that keratoconic eyes had greater axial length on average compared to emmetropic eyes (24.40mm vs 23.24 mm).⁴ 

This increased axial length is largely due to an increase in the posterior segment length of the eye³ rather than an increase in the anterior segment, as might be presumed given keratoconus is an anterior ocular condition. It is thought that the anterior segment changes of keratoconus may influence the growth patterns of the internal structures of the eye, thereby potentially inducing progressive myopia.⁵  

Fortunately, not all children with myopia have or are at risk of keratoconus. However, it is an important condition to exclude in myopic patients, even if they do not present with high astigmatism. A 2019 study that evaluated children with myopia between -6.00 and -13.50D (mean around -8.25D) was conducted to explore the presence of corneal abnormalities in this group, with the aim of enabling the early identification of keratoconus. 174 eyes were included in the study, and of those 9.2% were found to have keratoconus of varying severity. One third of the keratoconic cases identified amongst the high myopes presented with low levels of astigmatism, and the average astigmatism (around 4D) was no different between keratoconics and controls.⁶ 

The age of onset for paediatric keratoconus can be anywhere between 6 to 17 years of age, with males tending to have more advanced keratoconus.⁷ In children younger than 15 years old, keratoconus tends to be more severe when first diagnosed, and its progression is more frequent and rapid.⁸ This necessitates prompt detection to mitigate the risk of significant vision loss through proactive referral to ophthalmology for treatment considerations.⁹ Given the link between keratoconus and myopia, children (especially those with high myopia) should undergo comprehensive anterior segment examination that includes a baseline corneal topography to investigate for corneal abnormalities.⁶

When examining pediatric patients, a full comprehensive eye examination should include a thorough ocular health assessment – this includes the cornea. Initial myopia workups should involve a focus on ruling out keratoconus as a differential diagnosis and subtle, early signs should be proactively sought. Some investigations are:

• Corneal Topography (anterior): One of the most crucial steps in the early detection of keratoconus is conducting corneal topography as a primary examination. This is because it can detect subtle changes indicative of the disease, such as shape and symmetry changes, including the characteristic cone-shaped distortion,¹⁰ even before the appearance of clinical or biomicroscopic signs.¹¹
• Axial length: While not a direct measurement of the cornea, axial length is invaluable when distinguishing between progressive myopia and keratoconus, as it helps to determine whether the cause of changes in refractive error is due to axial elongation or corneal steepening, or both. Therefore, incorporating axial length assessment alongside corneal topography can provide important insights regarding detection and management of keratoconus.⁴
• Visual Acuity: A decrease in visual acuity that is not fully correctable with spectacle refraction may indicate keratoconus.¹⁰
• Retinoscopy: This can reveal irregular astigmatism, which is often an early sign of keratoconus and manifests as a scissor reflex.¹⁰
• Slit-Lamp Examination: Under slit-lamp examination, you might observe corneal thinning, increased visibility of corneal nerves, Fleischer's ring (an iron deposit ring around the cone), or Vogt's striae (fine vertical lines in the stroma of the cornea).¹⁰
• Corneal thickness maps and posterior corneal topographical maps are valuable for those involved in keratoconus co-management,¹⁰ but require advanced instrumentation not always available in primary eyecare. 

When evaluating corneal topography for keratoconus, careful attention should be paid to irregularities in corneal shape, particularly asymmetric steepening, which often manifests in the inferior cornea.¹² Elevation maps (see Figure 1) provide a visual representation of the corneal surface profile, highlighting focal protrusions or irregularities characteristic of keratoconus.

In Figure 1, we see a screenshot of a corneal topography output from the Topcon MYAH showing a patient with keratoconus. Note the indices with the red text, which together highlight the finding of keratoconus. These include:

• Apical keratometry (AK): this is measurement of the curvature of the cornea at its apex. Normal values are below 48D, suspicious values are 48D to 50D and values above 50D are considered abnormally high.¹² The AK in the Figure 1 example is 59.35D indicating a very steep AK which is highly indicative of keratoconus. 
• Apical curvature gradient (AGC): This measures how much the corneal power changes per unit of length compared to the central corneal power. If the value is higher than 2 D/mm, it may suggest keratoconus; between 1.5 and 2 is concerning, while less than 1.5 is considered normal.¹² Again, the example below the AGC is 7.52D/mm – much higher than what is considered normal. 
• Vertical asymmetry index (SI): this is a numerical measurement used to assess the asymmetry of the corneal surface between the superior and inferior hemispheres. It quantifies the difference in curvature between the upper and lower parts of the cornea. 
• A positive SI value suggests higher inferior corneal curvature, whereas a negative SI value indicates higher superior corneal curvature. SI values exceeding 1.8 D are indicative of clinical keratoconus, and above 1.4 suspected keratoconus.¹² In the example below, SI is 6.16D indicating the inferior cornea is much more curved than superior.  
• Keratoconus probability index (Kpi): provides a percentage measurement of the likelihood of keratoconus. In this example it is 100%.

In managing myopia associated with keratoconus, the focus shifts to addressing the irregular corneal surface and preventing further progression of the disease. Traditional methods of correcting myopia, like regular spectacles or standard contact lenses, might not be effective due to the irregular shape of the cornea in keratoconus patients. Instead, specialized contact lenses, such as rigid gas-permeable lenses, hybrid lenses, or scleral lenses, are often used to improve vision by creating a smooth refractive surface over the irregular cornea.¹³

Corneal collagen cross-linking (CXL) is a key treatment that should also be considered. This procedure strengthens the cornea by creating new links between collagen fibres, which helps to stabilize the cornea and can slow or halt the progression of keratoconus.⁹ This treatment is crucial for management of keratoconus patients as it can stabilize the worsening of myopia and irregular astigmatism associated with the progression of corneal ectasia. Proactive CXL has a high success rate of halting keratoconus progression, giving better long-term acuity outcomes and reducing the likelihood of more invasive procedures like corneal transplant. In the young, progressive myope that has received cross-linking treatment for keratoconus, it is still important to still manage myopia. Post-crosslinking, the cornea undergoes significant structural changes, necessitating careful consideration in selecting optical treatments.¹⁴ There is scarce literature on the effects of fitting orthokeratology post-crosslinking, however given the exertion of stress orthokeratology necessitates on the cornea it is most likely inadvisable when considering the need for corneal healing and stability post-procedure. Management with myopia controlling spectacle lenses, atropine or soft contact lenses may be more suitable, although again the evidence base is limited. 

Aside from optical and surgical interventions, it is also important to address eye-rubbing. Frequent eye rubbing, often linked with ocular allergies, is a key contributing factor in both the development and progression of keratoconus.^15-16 Thus, it's vital to not only educate myopic patients on avoiding eye rubbing but also effectively manage any underlying ocular allergies. Research indicates that proper control of ocular allergies in keratoconus can significantly reduce the risk of disease progression and the occurrence of acute hydrops.^17-18

In addition to these measures, managing dry eye syndrome is also important for keratoconus patients, particularly those who wear contact lenses. Dry eye is commonly found in keratoconus patients regardless of the type of contact lenses they use.¹⁹

Corneal evaluation plays an important role in clinical practice, and those with myopia warrant particular attention due to the increased possibility of concurrent conditions like keratoconus. Identifying keratoconus at its earliest stage—even before myopia develops—is vital for timely intervention. This proactive approach towards all paediatric patients facilitates early referral for treatments like corneal cross-linking, which can halt the progression of keratoconus and ideally mitigate its impact on myopia. Therefore, it is important to leverage the latest advancements in corneal imaging and diagnostic technologies to enhance our capacity for early intervention. Understanding the complex relationship between keratoconus and myopia allows us to tailor our approaches more effectively and ensure enhanced patient outcomes through appropriately changing management plans.