Case Report 7 – Unilateral Keratoconus

Unilateral keratoconus and contact lens fitting

  • 24 yo male
  • Sniper in army – shoots off right shoulder and sights with right eye.
  • Unilateral keratoconic (20% of keratoconus)
  • Corneal Specialist referral – good time to riboflavin?
  • Monocular fit = flip lid / applanation tonometry.  The sensitivity test.
  • OD: +2.00 / -2.25 x 88     (6/5=)
  • OS: +0.50DS

Plan A

  • SDTCL 8.7/14.0/+1.75/-1.75 x 90 (BLPVFA)
    • Rotationally unstable

Plan B

  • RGP CL Correction
    • 7.40/14.0/C edge/-3.50 Boston XO (6/6+)
    • Rotationally symmetric RGP
    • No need for Front Surface Toric or asymmetric periphery

Case Report 7 – Highly Asymmetric / Apparently Unilateral Keratoconus and RGP Contact Lens Correction

Clinical Presentation

24-year-old male serving in the Army as a sniper.

He shoots from the right shoulder and sights predominantly with the right eye, making the optical performance of the right eye particularly important occupationally.

The original case was described as:

Unilateral keratoconus

In 2026, I would use that terminology more cautiously.

A more appropriate initial description is:

Highly asymmetric keratoconus – clinically manifest OD with an apparently normal fellow eye requiring multimodal assessment and longitudinal surveillance.

Spectacle Refraction

OD: +2.00 / −2.25 × 88 — VA 6/5=

OS: +0.50 DS

There is an immediate teaching point here.

The right eye has keratoconus yet achieves approximately 6/5 high-contrast spectacle acuity.

Therefore:

Keratoconus does not automatically mean poor Snellen acuity.

Early or relatively regular keratoconus may retain excellent high-contrast acuity despite abnormal corneal shape.

The patient may nevertheless experience differences in:

  • contrast sensitivity;
  • ghosting;
  • night vision;
  • glare;
  • monocular diplopia;
  • visual stability; and
  • mesopic visual quality.

For a patient whose occupation involves precision sighting, these outcomes may matter considerably more than whether he can read another high-contrast Snellen letter.

Is Keratoconus Really Unilateral?

This is the major 2026 update to this case.

The original page stated:

“Unilateral keratoconic – 20% of keratoconus.”

That figure should no longer be used.

Keratoconus is generally regarded as a bilateral but frequently highly asymmetric ectatic disorder.

Historically, an eye could appear completely normal by slit lamp and Placido-disc topography while the fellow eye had obvious keratoconus.

Modern imaging has changed that interpretation.

Today we can assess:

  • anterior curvature;
  • posterior elevation;
  • full pachymetric distribution;
  • pachymetric progression;
  • epithelial thickness;
  • anterior and posterior corneal radii;
  • corneal biomechanics; and
  • combined tomographic-biomechanical indices.

Many supposedly “normal” fellow eyes demonstrate subtle abnormalities when examined using these modalities.

How Rare Is a Truly Normal Fellow Eye?

A 2023 study of 458 patients with keratoconus identified only 34 tomographically regular fellow eyes – 7.4%.

More importantly, when those apparently normal eyes underwent biomechanical assessment:

  • only 38% had a regular TBI; and
  • only 21% had both regular TBI and CBI values.

The authors concluded that highly asymmetric ectasia with a genuinely regular fellow eye is uncommon and that biomechanical analysis may reveal abnormalities before conventional tomography becomes diagnostic. [2]

Another study examining topographically and tomographically normal fellow eyes found that the Tomographic Biomechanical Index (TBI) had the strongest ability among the tested indices to distinguish these eyes from normal control corneas. [3]

Teaching Point

Do not call the fellow eye:

“normal”

simply because its anterior topography looks normal.

A better description is:

No current clinical or tomographic evidence of ectasia.

Then continue surveillance.

Can True Unilateral Keratoconus Exist?

Probably – but it appears to be uncommon.

Di Felici, Elahi and Gatinel reported patients with unilateral keratoconus whose fellow eyes remained normal across both tomographic and biomechanical testing during longitudinal follow-up. [4]

Therefore the modern position is more nuanced:

Do not say:

“Keratoconus can never be unilateral.”

But equally, do not assume:

“One normal topography means unilateral disease.”

The diagnosis of true unilateral keratoconus should require a high standard of evidence and longitudinal follow-up.

What Would I Measure in the Fellow Eye in 2026?

A contemporary fellow-eye assessment should include:

Tomography

Evaluate:

  • anterior elevation;
  • posterior elevation;
  • K1 / K2;
  • Kmax;
  • thinnest pachymetry;
  • location of the thinnest point;
  • pachymetric progression;
  • BAD-D;
  • anterior radius of curvature;
  • posterior radius of curvature; and
  • Belin ABCD staging.

Biomechanics

Where available:

  • Corvis Biomechanical Index – CBI
  • Tomographic Biomechanical Index – TBI
  • stiffness parameter at first applanation – SP-A1
  • stress-strain index – SSI
  • biomechanical E-stage where applicable.

Epithelial Thickness

Anterior-segment OCT epithelial mapping can be particularly useful because epithelial remodelling may partially mask underlying stromal irregularity.

A relatively regular anterior surface can therefore coexist with early structural disease beneath it.

The principle is:

Topography tells us shape. Tomography tells us structure. Biomechanics tells us how the cornea behaves under load.

No single parameter should be interpreted in isolation.

Age Matters – He Is Only 24

The patient was 24 years old when this case was recorded.

That is important.

Keratoconus progression generally becomes less likely with increasing age, but 24 is not old in keratoconus terms.

A 2025 study comparing patients aged 19–24 years with those aged 25–30 found markedly higher progression rates in the younger group using several tomographic progression criteria. [5]

This does not mean that every 24-year-old with keratoconus should immediately undergo CXL.

It does mean that:

A 24-year-old with established keratoconus deserves careful serial surveillance and a relatively low threshold for corneal-specialist review if objective progression is demonstrated.

“Good Time to Riboflavin?” – Update the Terminology

The original case asked:

“Corneal Specialist referral – good time to riboflavin?”

The contemporary term is:

Corneal cross-linking – CXL

Riboflavin is the photosensitising agent used as part of the treatment; it is not the treatment by itself.

CXL combines riboflavin with controlled ultraviolet-A exposure to induce additional biomechanical cross-links within the corneal stroma.

The primary purpose of CXL is:

to reduce the risk of further ectatic progression.

It is not primarily a refractive procedure.

A patient may still require spectacles or specialty contact lenses following successful CXL.

When Should CXL Be Considered?

The 2026 Global Consensus continues to emphasise progression and risk of progression when considering CXL.

Progression should not be declared from one noisy Kmax measurement.

Serial assessment should consider concordant change in multiple parameters such as:

  • anterior curvature;
  • posterior curvature;
  • pachymetry;
  • ABCD parameters;
  • refraction;
  • visual acuity; and
  • biomechanical findings where appropriate.

The REDCAKE study demonstrated why single progression criteria can produce substantially different progression rates and why combinations of parameters provide a more robust clinical assessment. [6]

In This Patient

Because he is:

  • young;
  • occupationally visually demanding;
  • already manifestly keratoconic in the sighting eye; and
  • dependent upon high-quality monocular visual performance,

I would have a relatively low threshold for corneal-specialist review if reproducible progression were identified.

Do We Cross-Link the Apparently Normal Fellow Eye?

Not simply because the other eye has keratoconus.

A fellow eye without clinical, tomographic or biomechanical evidence of ectasia should generally be observed rather than treated prophylactically without justification.

However, observation must be active.

Active surveillance means:

  • repeat tomography;
  • repeat refraction;
  • repeat BCVA;
  • comparison of posterior elevation;
  • comparison of pachymetric progression;
  • biomechanics where appropriate; and
  • careful documentation of modifiable risk factors.

If objective disease or progression emerges, the treatment discussion changes.

Eye Rubbing Matters

Every keratoconus assessment should include a specific history of:

  • habitual eye rubbing;
  • forceful rubbing;
  • unilateral rubbing;
  • ocular itch;
  • allergy;
  • atopy;
  • sleeping habits; and
  • occupational/environmental irritation.

Eye rubbing has a strong epidemiological association with keratoconus.

A systematic review and meta-analysis reported a pooled association between eye rubbing and keratoconus, although the evidence is observational and does not prove a simple causal relationship. [7]

A 2025 review also identifies eye rubbing among the important potentially modifiable non-genetic risk factors for disease development and progression. [8]

Practical Advice

Patients with keratoconus should be counselled to avoid:

vigorous, repetitive or prolonged eye rubbing.

Any underlying ocular itch, allergy, blepharitis or ocular-surface disease that encourages rubbing should be actively managed.

Contact Lens Management

The contact-lens problem in this case was interesting because the patient’s spectacle acuity was already extremely good.

The reason for fitting a contact lens was therefore not simply:

“Improve Snellen VA.”

The objective was to provide a stable, practical optical correction suitable for his visual and occupational requirements.

Plan A – Soft Toric Contact Lens

Historical Fit

OD: 8.7 / 14.0 / +1.75 / −1.75 × 90

The lens was recorded as:

Rotationally unstable

This is especially important when correcting significant cylinder.

A toric soft lens depends on a stabilisation system to maintain the cylindrical axis in the intended orientation.

If the lens rotates unpredictably:

  • the cylinder axis changes;
  • residual astigmatism varies;
  • vision fluctuates;
  • the patient may experience blur immediately after blinking; and
  • precision visual performance becomes inconsistent.

For a patient whose occupation involves precision monocular sighting, inconsistent lens rotation may be unacceptable even if average Snellen acuity appears reasonable.

Why Is Rotation So Important?

A toric contact lens only provides its intended cylindrical correction when its axis is correctly aligned.

As the lens rotates away from the intended position, increasing residual astigmatism is induced.

The higher the prescribed cylinder, the more clinically important even relatively modest rotational instability becomes.

Therefore:

Stable orientation can be more important than the acuity measured during one lucky blink.

This is a useful distinction between a chart-room endpoint and real-world visual performance.

Plan B – Rigid Gas-Permeable Lens

Historical Fit

OD: 7.40 / 14.0 / C edge / −3.50 D
Material: Boston XO
VA: 6/6+

The historical case recorded this as a:

Rotationally symmetric RGP

with:

No need for front-surface toric or asymmetric periphery.

Figure 1. Original fluorescein image from Case Report 7. The image can be used to discuss centration, movement, apical relationship, mid-peripheral alignment and edge clearance rather than judging the fit from visual acuity alone.

Why Does a Spherical RGP Work?

A rigid lens does not simply reproduce the patient’s spectacle cylinder.

The rigid anterior lens surface creates a regular optical interface, while the post-lens tear layer masks much of the irregularity of the anterior cornea.

This can substantially neutralise:

  • regular corneal astigmatism;
  • irregular astigmatism;
  • anterior corneal coma; and
  • other anterior-surface higher-order aberrations.

Consequently a spherical rigid lens can often provide excellent vision in a keratoconic eye without incorporating the spectacle cylinder into the lens itself.

This remains one of the fundamental optical principles of keratoconus contact-lens management. [9]

Why Doesn’t the RGP Need to Stay at a Particular Axis?

A spherical rigid lens is rotationally symmetric.

Rotate a spherical optical surface through 10°, 50° or 180° and its power remains the same.

This is fundamentally different from a toric soft lens.

Provided that:

  • the RGP centres adequately;
  • movement is appropriate;
  • the tear lens provides the required corneal correction; and
  • residual astigmatism is clinically insignificant,

rotation of the lens itself does not alter its optical axis.

That explains why the rigid lens solved the instability problem encountered with the soft toric lens.

When Would a Front-Surface Toric Be Required?

A spherical RGP neutralises predominantly anterior corneal astigmatism.

It cannot automatically correct significant:

  • lenticular astigmatism;
  • posterior corneal astigmatism; or
  • other residual internal cylinder.

If the spherical rigid lens fits well but the over-refraction repeatedly demonstrates clinically significant residual cylinder, a front-surface toric design may be required.

In This Historical Case

The patient achieved approximately:

6/6+

with the rotationally symmetric design.

There was therefore no obvious clinical reason to add front-surface toricity.

Teaching Principle

Do not add complexity unless the over-refraction demonstrates that complexity is required.

Why Was an Asymmetric Periphery Not Required?

Keratoconic corneas are not rotationally symmetric.

That does not automatically mean that every keratoconus RGP requires asymmetric peripheral geometry.

If a rotationally symmetric design provides:

  • good centration;
  • appropriate movement;
  • an acceptable fluorescein pattern;
  • adequate edge clearance;
  • good comfort;
  • stable optics; and
  • satisfactory corneal physiology,

there is no reason to add asymmetric peripheral geometry merely because tomography looks asymmetric.

Modern corneal GP, hybrid and scleral designs allow extensive customisation, but customisation should solve a demonstrated fitting problem.

The Historical Lens Diameter

The original record lists:

7.40 / 14.0 / C edge / −3.50 Boston XO

These parameters should be regarded as the historical lens data recorded for this patient, not a current fitting recommendation.

A 14.0-mm diameter would be unusually large for what many clinicians currently describe as a conventional corneal GP.

Rather than attempting to retrospectively reinterpret the old record, students should concentrate on the principle:

Evaluate the lens that is actually on the eye – centration, movement, clearance/bearing, edge relationship, comfort and physiology – rather than prescribing from a historical number.

Fluorescein Assessment – What Should the Student Look For?

Do not simply ask:

“Is there green under the lens?”

Systematically assess:

Centration

  • Does the lens remain appropriately centred?
  • Is there excessive inferior, superior or lateral displacement?
  • Does the visual axis remain within the useful optic zone?

Apical Relationship

  • excessive apical bearing?
  • apical alignment?
  • excessive pooling?

Modern keratoconus fitting generally aims to avoid significant chronic mechanical trauma to the cone apex.

Mid-Peripheral Relationship

Look for excessive bearing or pooling surrounding the cone.

Edge Clearance

Assess:

  • edge lift;
  • tear exchange;
  • bubble formation;
  • lid interaction; and
  • excessive stand-off.

Movement

The lens should move sufficiently to permit appropriate tear exchange without becoming unstable.

“Monocular Fit – The Sensitivity Test”

The original notes included:

“Monocular fit = flip lid / applanation tonometry. The sensitivity test.”

This should be reframed for students in 2026.

A patient wearing a specialty lens in only one eye provides an interesting opportunity to compare:

  • comfort;
  • corneal staining;
  • conjunctival response;
  • tear-film behaviour; and
  • subjective sensation

between the two eyes.

However, applanation tonometry is not a formal corneal-sensitivity test.

If corneal sensitivity genuinely needs to be quantified, use a validated method such as:

  • Cochet–Bonnet esthesiometry; or
  • non-contact corneal esthesiometry where available.

Likewise, everting the upper eyelid remains important if a unilateral RGP wearer develops acute discomfort because a subtarsal foreign body can mimic or exacerbate contact-lens symptoms.

Occupational Vision – The Sniper Is the Teaching Point

This patient’s occupation makes the case more interesting than a simple RGP fit.

The affected eye is also his sighting eye.

High-contrast Snellen acuity alone is therefore an incomplete outcome measure.

I would also consider:

  • monocular contrast sensitivity;
  • mesopic acuity;
  • glare;
  • ghosting;
  • stability following blink;
  • lens decentration in different gaze positions;
  • wind/dust tolerance;
  • comfort over prolonged wear;
  • recovery following lens displacement; and
  • binocular function when transitioning between sighting and general visual tasks.

A spectacle acuity of 6/5 and contact-lens acuity of 6/6+ does not mean the contact lens has failed.

If the rigid lens provides:

more stable, predictable and functional optics under the required occupational conditions,

it may still be the preferable correction.

Treat the task, not just the chart.

Contact Lenses Do Not Treat Keratoconus

A contact lens may dramatically improve vision.

It does not stabilise the ectatic disease.

Likewise:

Contact lens success does not mean the keratoconus is stable.

This patient still requires serial structural assessment irrespective of how well he sees through the RGP.

This is especially important because a rigid lens can provide excellent acuity while underlying ectasia continues to change.

Contact Lens Removal Before Tomography

Rigid lens wear can temporarily alter:

  • corneal curvature;
  • refraction;
  • pachymetric measurements; and
  • higher-order aberrations.

Therefore serial keratoconus imaging should be undertaken after an appropriate period without rigid-lens wear when clinically necessary to obtain reproducible baseline measurements.

The required cessation period varies with:

  • lens design;
  • wearing history;
  • corneal response; and
  • magnitude of lens-induced shape change.

Do not compare serial tomography obtained under substantially different lens-wear conditions and assume that every difference represents true progression.

What Would I Do if This Patient Presented in 2026?

1. Confirm the Diagnosis OD

Perform comprehensive tomography rather than relying solely on anterior topography.

Document:

  • anterior surface;
  • posterior surface;
  • pachymetric progression;
  • thinnest point;
  • BAD-D;
  • ABCD stage; and
  • epithelial profile where available.

2. Interrogate the Fellow Eye

Do not simply document:

“OS normal.”

Assess:

  • tomography;
  • posterior elevation;
  • pachymetry;
  • BAD-D;
  • ABCD;
  • CBI;
  • TBI;
  • SSI; and
  • epithelial thickness mapping where appropriate.

3. Ask Specifically About Rubbing and Allergy

Determine whether there is:

  • unilateral rubbing;
  • occupational irritation;
  • allergy;
  • habitual nocturnal rubbing;
  • ocular itch; or
  • another modifiable trigger.

4. Establish Progression

Compare properly acquired serial examinations.

Do not rely on Kmax alone.

5. Refer for CXL if Appropriate

At 24, objectively progressive disease warrants timely corneal-specialist review.

6. Optimise the Optical Correction

If spectacles provide 6/5 but are occupationally unsuitable and the soft toric rotates unpredictably, a rigid optical system is a logical alternative.

7. Measure Functional Vision

Particularly in the dominant sighting eye.

Assess more than high-contrast VA.

Clinical Take-Home Points

  1. Keratoconus is usually bilateral but often markedly asymmetric.
  2. The historical claim that 20% of keratoconus is unilateral should be retired.
  3. True unilateral keratoconus can occur, but it is uncommon and requires careful multimodal and longitudinal confirmation.
  4. A normal anterior topography does not prove that the fellow eye is normal.
  5. Tomography and biomechanics are complementary. TBI can reveal abnormalities in apparently normal fellow eyes.
  6. A 24-year-old remains at meaningful risk of progression.
  7. CXL treats progression; contact lenses treat optical disability.
  8. Do not diagnose progression from Kmax alone.
  9. Ask about eye rubbing and manage the reason the patient rubs.
  10. Keratoconus can coexist with 6/5 spectacle acuity. Snellen acuity does not describe the entire optical system.
  11. Soft toric rotation matters when significant cylinder is being corrected.
  12. A spherical RGP is rotationally symmetric and therefore avoids toric-axis instability.
  13. A front-surface toric is only required when clinically significant residual cylinder remains.
  14. Do not add asymmetric peripheral geometry unless the fit demonstrates a reason to do so.
  15. A specialty lens can provide excellent vision while the underlying keratoconus continues to progress.
  16. For a precision occupational task, visual stability, contrast and mesopic performance may matter more than one Snellen line.

Peer-Reviewed Reading – QUT Master of Optometry

1. Gomes JAP, Hafezi F, Ambrósio R Jr, et al. Global Consensus on Keratoconus and Ectatic Diseases—Edition 2. Cornea. 2026;45(7):888–908. doi:10.1097/ICO.0000000000004170. PMID: 42228627.

2. Augustin VA, Son HS, Kovalchuk B, et al. Biomechanical Analysis of Tomographically Regular Keratoconus Fellow Eyes Using Corvis ST. Klin Monbl Augenheilkd. 2023;240(8):944–951. doi:10.1055/a-2130-7503. PMID: 37567232.

3. Asroui L, Dagher SA, Elsheikh A, Lopes BT, Roberts CJ, Assouad M, Awwad ST. Biomechanical Evaluation of Topographically and Tomographically Normal Fellow Eyes of Patients With Keratoconus. J Refract Surg. 2022;38(5):318–325. doi:10.3928/1081597X-20220225-01. PMID: 35536713.

4. Di Felici F, Elahi S, Gatinel D. Unilateral keratoconus with normal tomographic and biomechanical indices in the fellow eye: Three-year follow-up. J Fr Ophtalmol. 2023;46(10)–e392. doi:10.1016/j.jfo.2023.03.025. PMID: 37620200.

5. Kosekahya P, Flockerzi E, Munteanu C, Sideroudi H, Seitz B. Comparison of Keratoconus Progression Rate between Adolescents Aged 19–24 Years and Young Adults: Impact on Indication for Crosslinking. Curr Eye Res. 2025;50(6):572–578. doi:10.1080/02713683.2025.2470408. PMID: 39988435.

6. Koppen C, Jiménez-García M, Kreps EO, Ní Dhubhghaill S, Rozema JJ; REDCAKE Study Group. Definitions for Keratoconus Progression and Their Impact on Clinical Practice. Eye Contact Lens. 2024;50(1):1–9. doi:10.1097/ICL.0000000000001038. PMID: 37816249.

7. Sahebjada S, Al-Mahrouqi HH, Moshegov S, et al. Eye rubbing in the aetiology of keratoconus: a systematic review and meta-analysis. Graefes Arch Clin Exp Ophthalmol. 2021;259(8):2057–2067. doi:10.1007/s00417-021-05081-8. PMID: 33484296.

8. Cheung IMY, Angelo L, Gokul A, Ziaei M. Non-genetic risk factors for keratoconus and its progression. Clin Exp Optom. 2025;108(6):648–656. doi:10.1080/08164622.2024.2443454. PMID: 39762118.

9. Swartz G, Alam K, Gentle A, Downie LE. Impact of contact lens correction on wavefront aberrations and vision quality in keratoconus. Ophthalmic Physiol Opt. 2025;45(7):1811–1828. doi:10.1111/opo.70037. PMID: 41159562.

10. Thulasidas M, Teotia P. Evaluation of corneal topography and tomography in fellow eyes of unilateral keratoconus patients for early detection of subclinical keratoconus. Indian J Ophthalmol. 2020. PMID: 33120630.

Suggested Discussion Questions for Students

Question 1

The patient has clinically obvious keratoconus OD while OS appears normal on conventional topography.

What evidence would you require before confidently describing this as true unilateral keratoconus?

Question 2

The patient achieves 6/5= in spectacles through the keratoconic eye.

Why does excellent high-contrast acuity not exclude clinically meaningful keratoconus?

Question 3

The soft toric lens provides the correct nominal prescription but rotates unpredictably.

Why can a spherical RGP provide more stable vision even though the spectacle prescription contains −2.25 D of cylinder?

Question 4

The patient is a sniper and the keratoconic right eye is his sighting eye.

Which measures of visual function would you assess in addition to high-contrast Snellen acuity before declaring the contact lens successful?

Question 5

A 24-year-old has apparently stable keratoconus at today’s examination.

What evidence would you require before referring for CXL, and why should age influence the frequency of review?

Question 6

The fellow eye has normal BAD-D and ABCD parameters but an abnormal TBI.

How would this alter your diagnosis, counselling and follow-up strategy?