Keratoconus is a progressive eye condition where the cornea thins and bulges outward into a cone-like shape, leading to distorted vision. It is typically known as a bilateral condition, meaning it affects both eyes. However, many patients and even some clinicians ask: Can keratoconus affect only one eye?
The answer is more complex than a simple yes or no. While unilateral keratoconus (or monocular keratoconus) is sometimes reported, most cases that appear one-sided eventually reveal subtle or delayed changes in the other eye. This makes careful monitoring and screening the second eye critical.
For this one, we’ll explore what it means to have keratoconus in one eye only, the differences between asymmetrical keratoconus and truly unilateral cases, how doctors detect subtle changes, and the best strategies for treatment and long-term care.
What Is Unilateral Keratoconus?
Unilateral keratoconus refers to cases where one eye shows the characteristic signs of the condition, while the other eye appears completely healthy. Some studies report this atypical presentation in less than 5% of patients.
It is important to distinguish unilateral keratoconus from asymmetrical keratoconus. In asymmetrical cases, both eyes are affected but at different severities—one eye may show advanced thinning while the other is only mildly involved. Many so-called “one-sided keratoconus” patients actually fall into this category once more advanced diagnostic tools are used.
Most doctors also use terms like subclinical keratoconus or keratoconus suspects when the “unaffected” eye shows very early or borderline signs. This highlights why early detection in the fellow eye is so important: even if vision seems normal, the disease may already be developing quietly.
How Rare Is Keratoconus in One Eye Only?
Older research suggested keratoconus was rare overall, affecting about 1 in 2,000 people. Recent population studies, however, show higher prevalence, around 1% in some regions. Within those numbers, true unilateral keratoconus appears very uncommon.
- In one classic study, only 3 out of 164 patients had no detectable signs in the fellow eye after careful imaging.
- Long-term follow-up research revealed that around 35% of patients with one apparently normal eye eventually developed keratoconus in that eye too, often within the first 6–10 years.
- Case reports do exist of patients who remain stable in one eye for over a decade, but these are exceptional.
This evidence suggests that while unilateral keratoconus is possible, most cases represent early or hidden changes in the second eye rather than a truly one-sided disease.
Why Does It Sometimes Affect Only One Eye?
There are several theories as to why keratoconus may appear monocular or strongly asymmetrical:
- Genetic predisposition with uneven expression – Both eyes share the same DNA, but one eye may be more biomechanically vulnerable.
- Environmental and behavioral factors – Eye rubbing, particularly if done more on one side, can cause localized damage. This is a well-documented risk factor.
- Allergies and inflammation – Conditions that cause frequent rubbing or irritation may disproportionately affect one eye.
- UV exposure or trauma – Uneven exposure to environmental stressors can trigger asymmetric disease.
- Biomechanical differences – Advanced imaging reveals that even when one eye looks normal, subtle weaknesses may already exist in the corneal structure.
Symptoms of One-Sided Keratoconus
Patients with unilateral keratoconus often notice:
- Worse vision in one eye – blurry or distorted images.
- Visual imbalance – a noticeable difference in clarity between the two eyes.
- Depth perception issues – difficulty judging distances due to impaired binocular vision.
- Compensatory vision – relying heavily on the unaffected eye, which can cause strain and fatigue.
Because the “good” eye often compensates, patients may not seek help until the affected eye becomes significantly impaired. This is why regular eye exams are crucial for early detection in the fellow eye.
Diagnosis: How Do Doctors Confirm It’s in One Eye Only?
Diagnosing unilateral keratoconus is challenging. The goal is not only to confirm disease in the affected eye but also to rule out subtle changes in the supposedly healthy eye.
Key diagnostic tools include:
- Corneal Topography – creates a map of corneal curvature to detect early irregularities.
- Corneal Tomography – provides 3D imaging of corneal thickness and elevation.
- Pachymetry – measures corneal thickness, as thinning is a hallmark of keratoconus.
- Biomechanical analysis – newer tools assess corneal stiffness to reveal early weakness.
Eye specialists carefully monitor for bilateral asymmetry and corneal stability over time. A patient may be classified as a keratoconus suspect if borderline changes are found, even when vision is still normal.
Fan-out query: How often should patients with unilateral keratoconus undergo topography scans?
Answer: Typically every 6–12 months, depending on age, severity, and risk factors like eye rubbing or family history.
Progression and Long-Term Outlook
Studies show that many patients with unilateral keratoconus eventually develop changes in the fellow eye.
- Around one-third of patients see progression within 15–17 years.
- Conversion is most likely within the first 6 years after diagnosis.
- Some rare cases remain unilateral for more than 10 years, especially if risk factors are minimized.
This means that while unilateral keratoconus may stay stable, most patients should expect ongoing monitoring and the possibility of asymmetrical keratoconus developing.
Treatment Options: Focused on the Affected Eye
The treatment plan depends on the severity and progression of the disease.
- Glasses or contact lenses – For mild cases, lenses correct the irregular vision. Some patients only need a contact lens in one eye only.
- Specialty contact lens fitting – Rigid gas permeable, scleral, or hybrid lenses may be prescribed for better vision.
- CXL on one eye – Corneal cross-linking strengthens the affected cornea to halt progression. Often performed only on the keratoconic eye.
- Observation of the fellow eye – Careful monitoring of the healthy eye to check for early detection of subclinical changes.
- Advanced surgical options – For severe cases, procedures such as intracorneal ring segments (ICRS) or corneal transplantation may be required.
Fan-out query: Should the healthy eye undergo preventive cross-linking?
Answer: This remains controversial. Some specialists consider it if imaging shows early weakness, but most recommend observation of the fellow eye until definite signs appear.
Conversational FAQs
Yes, but it is very rare. Most patients initially diagnosed with monocular keratoconus eventually show signs of asymmetrical keratoconus.
Unilateral keratoconus means only one eye shows disease. Asymmetrical keratoconus means both eyes are affected, but one is much more severe.
Not always. Studies show that about 1 in 3 “healthy” eyes develop keratoconus over time.
Yes. Many patients with unilateral disease need a specialty contact lens fitting only in the affected eye.
Most specialists recommend CXL on one eye first, while continuing close monitoring of the other.
Typically every 6–12 months, but more often if you’re young or have risk factors like frequent eye rubbing.
Conclusion
While unilateral keratoconus is rare, it carries important implications for diagnosis, treatment, and lifelong monitoring. In most cases, what seems like one-sided keratoconus is actually asymmetrical keratoconus, with the second eye showing signs later on.
The key is regular screening, early detection, and a tailored single eye treatment plan when necessary.
For patients in the Philippines, having access to advanced diagnostic tools and expert care makes a critical difference. Shinagawa Lasik & Aesthetics, one of the world’s leading providers of LASIK and eye care solutions, is committed to guiding patients through every stage of their vision journey. With expertise in corneal health and advanced imaging, Shinagawa ensures that both eyes—whether affected or seemingly healthy—receive the best possible care.
For inquiries, questions, and appointments, call our Patient Care Lines:
🖥 Talk to our Consultants via Livechat: https://direct.lc.chat/6329011/
📱 Instagram: https://instagram.com/shinagawa_ph/
Citations and Resources
Shinagawa LASIK & Aesthetics strives to provide accurate and reliable information regarding LASIK procedures and eye health. We utilize primary sources to support our content, including peer-reviewed scientific studies, data from reputable medical organizations, and expert opinions. We also reference established publications and research where appropriate.
Our commitment to evidence-based information ensures that you receive trustworthy and up-to-date details to make informed decisions about your eye care.
Resources Used in This Article
- PubMed. “Unilateral keratoconus. Incidence and quantitative topographic analysis, https://pubmed.ncbi.nlm.nih.gov/9307634/“
- Slack Journals. “Incidence and Characteristics of Unilateral Keratoconus Classified on Corneal Topography, https://journals.healio.com/doi/full/10.3928/1081597X-20110426-01“
- PubMed. “Crosslinking vs. Observation in Fellow Eyes of Keratoconus Patients, https://pubmed.ncbi.nlm.nih.gov/35692966/“
- PubMed. “Extremely asymmetric ectasia: Tomographically unilateral keratoconus, https://pmc.ncbi.nlm.nih.gov/articles/PMC12235680/“
- PubMed. “Diagnosis and management of keratoconus in the paediatric age group: a review of current evidence, https://pmc.ncbi.nlm.nih.gov/articles/PMC10698037/“
- Philippine Journal of Ophthalmology. “Clinical Profile of Keratoconus Patients at the Philippine General Hospital, https://paojournal.com/index.php/pjo/article/view/17/422“
- National Library of Medicine. “Keratoconus, https://www.ncbi.nlm.nih.gov/books/NBK470435/“
- BMC Ophthalmology. “Fourteen years follow-up of a stable unilateral Keratoconus: unique case report of clinical, tomographical and biomechanical stability, https://bmcophthalmol.biomedcentral.com/articles/10.1186/s12886-022-02412-z“
- PubMed. “Topographic Evaluation of Unilateral Keratoconus Patients, https://pmc.ncbi.nlm.nih.gov/articles/PMC6624469/“
Fill out the form below to schedule your free, no-obligation consultation.