Scleral lenses can provide a major improvement in vision for people with keratoconus, irregular corneas, corneal grafts and other complex corneal conditions.
But sometimes patients are surprised to find that even with a well-fitting scleral lens, their vision is still not perfectly crisp.
They may describe:
- ghosting around letters;
- shadows or double images from one eye;
- halos around lights;
- glare when driving at night;
- reduced contrast;
- distorted vision despite reading 6/6 on the eye chart; or
- vision that is “clear” but still does not feel sharp.
This does not necessarily mean the scleral lens has failed.
The reason is that not all visual distortion comes from the basic prescription or the front surface of the cornea.
What Does a Scleral Lens Actually Correct?
In keratoconus, the front surface of the cornea becomes irregular.
A scleral lens vaults over this irregular surface and is filled with preservative-free saline before insertion.
The fluid reservoir between the lens and the cornea helps create a much smoother optical system and can neutralise a large amount of the irregularity of the anterior corneal surface.
For many patients, this produces a dramatic improvement in vision.
However, the eye is a complex optical system, and a standard scleral lens may not remove every source of distortion.
Why Can Vision Still Look Distorted?
Most people are familiar with:
- short-sightedness;
- long-sightedness; and
- astigmatism.
These are called lower-order refractive errors and can usually be corrected with ordinary spectacle or contact-lens prescriptions.
Keratoconus can also produce more complicated optical distortions called higher-order aberrations, or HOAs.
One of the most common is coma.
Instead of a point of light appearing as a clean point, it may look stretched, smeared or have a tail.
Patients often describe this as:
“I can read the letters, but every letter has a shadow beside it.”
A scleral lens can reduce many of these aberrations substantially, but some may remain.
How Can I Be 6/6 but Still Have Poor Vision?
The standard letter chart mainly measures whether you can identify letters of a particular size.
It does not fully measure:
- glare;
- halos;
- ghosting;
- contrast sensitivity;
- night vision;
- image quality; or
- subtle distortion.
It is therefore possible for somebody to achieve 6/6 visual acuity but still be dissatisfied with the quality of their vision.
For patients with keratoconus, this distinction can be very important.
Could the Scleral Lens Prescription Simply Be Wrong?
Sometimes, yes.
After a scleral lens is fitted, there may still be a small amount of residual:
- short-sightedness;
- long-sightedness; or
- astigmatism.
We can test this using an over-refraction, where additional lenses are placed in front of the scleral lens.
If the vision becomes clearly sharper, changing the power of the scleral lens may solve the problem.
But if the patient continues to see ghosting or distortion despite an accurate over-refraction, we need to look further.
What About Residual Astigmatism?
Not all astigmatism comes from the front surface of the cornea.
Residual astigmatism may come from:
- the posterior surface of the cornea;
- the natural lens inside the eye;
- scleral-lens flexure; or
- the way the lens sits on the eye.
In suitable cases, a front-surface toric correction can be incorporated into the scleral lens.
The key question is whether the remaining blur behaves like ordinary astigmatism or whether higher-order aberrations are also limiting the visual quality.
Lens Centration and Stability Matter
A scleral lens does not always sit perfectly centred over the visual axis.
The shape of the sclera can cause the lens to settle slightly:
- downwards;
- sideways; or
- with some rotation.
For many patients, this causes no significant problem.
But when the corneal optics are highly irregular, small changes in lens position can affect the quality of vision.
This becomes even more important when more customised optics are being considered.
Before trying to correct higher-order aberrations, we first want the scleral lens itself to be stable, predictable and appropriately centred.
Can the Scleral Lens Flex?
Although scleral lenses are rigid, some lenses can undergo a small amount of flexure once they are on the eye.
This may create unwanted residual astigmatism.
Factors that can influence flexure include:
- lens thickness;
- diameter;
- material;
- scleral landing-zone design; and
- the underlying shape of the eye.
If flexure is contributing to blur, changing the lens design may improve the vision.
Can Too Much Clearance Affect Vision?
A scleral lens needs enough clearance to safely vault the cornea.
But more clearance is not automatically better.
Excessive clearance can potentially influence:
- lens stability;
- oxygen transmission;
- fogging;
- overall optical quality; and
- lens performance during the day.
This is one reason why scleral lens fitting involves more than simply checking that the lens does not touch the cornea.
Why Does My Vision Start Clear and Become Foggy Later?
If your vision is good when the lens is inserted but becomes hazier several hours later, the problem may not be higher-order aberrations at all.
One possibility is midday fogging.
Cells, lipids and other debris can accumulate in the fluid reservoir beneath the scleral lens.
Patients often describe this as:
- milky vision;
- hazy vision;
- fogging;
- reduced contrast; or
- vision that improves after removing, cleaning and reinserting the lens.
When this occurs, we may assess:
- the scleral landing zone;
- tear exchange;
- ocular-surface inflammation;
- eyelid health;
- filling solution; and
- how the lens settles over time.
The Front Surface of the Lens Can Also Cause Blur
Sometimes the lens fit underneath the lens is excellent, but the front surface does not stay evenly wet.
If the tear film breaks up over the lens, vision can fluctuate between blinks.
Possible contributing factors include:
- deposits on the lens;
- tear-film abnormalities;
- cosmetics or facial oils;
- inappropriate cleaning products;
- lens age; or
- underlying dry-eye disease.
A well-fitting scleral lens can still give poor vision if the optical surface over the front of the lens is unstable.
What About Corneal Scarring?
A scleral lens can correct irregular refraction, but it cannot make opaque corneal tissue transparent.
If there is significant central or paracentral corneal scarring, some patients may continue to experience:
- reduced contrast;
- glare;
- light scatter;
- halos; or
- reduced best-corrected vision.
In these situations, the limitation may not be the contact lens itself.
Could Higher-Order Aberration Correction Help?
In selected patients, yes.
If the scleral lens is:
- fitting appropriately;
- stable;
- reasonably well centred; and
- giving an accurate conventional prescription,
but the patient still has significant ghosting or distortion, we may consider a higher-order-aberration-correcting scleral lens design.
Rather than correcting only sphere and astigmatism, these designs attempt to reduce more complex optical distortions such as coma.
This can be useful in selected patients with keratoconus who achieve good visual acuity but continue to experience poor visual quality.
However, HOA correction is not automatically the answer for everyone.
The lens needs to remain sufficiently stable for the customised optical correction to remain aligned with the eye.
Could a Specialised Soft Lens Actually Be Better?
Sometimes, yes.
This is one of the more interesting options in patients whose vision remains distorted despite a technically good scleral lens.
A rigid scleral lens works by creating a smooth optical surface in front of the irregular cornea and neutralising much of the front corneal irregularity.
However, the anterior and posterior surfaces of the cornea can both contribute to the overall pattern of higher-order aberrations.
In some eyes, these aberrations may partially interact or compensate for each other.
A specialised soft keratoconus lens such as KeraSoft IC behaves differently.
Rather than vaulting completely over the cornea, the soft lens more closely follows the corneal shape.
This can preserve the relationship between the aberrations from the front and back of the cornea to a greater extent than a rigid tear-lens system.
For some patients, particularly where the aberrations are acting in a similar or favourable direction, this can result in surprisingly good visual quality.
A specialised soft lens will not be better for every patient, and it may not neutralise corneal irregularity as strongly as a rigid or scleral lens.
But when a patient has persistent ghosting despite an otherwise good scleral fit, trialling a different optical strategy can sometimes make more sense than simply making the scleral lens more complicated.
So Does Persistent Distortion Mean I Need a More Advanced Scleral Lens?
Not necessarily.
The solution depends on where the remaining distortion is coming from.
Options may include:
- adjusting the scleral lens prescription;
- correcting residual astigmatism;
- reducing lens flexure;
- improving lens centration or stability;
- changing corneal clearance;
- managing midday fogging;
- improving front-surface wettability;
- treating ocular-surface disease;
- considering an HOA-correcting scleral lens; or
- trialling a specialised soft keratoconus lens such as KeraSoft IC.
The goal is not simply to use the most technologically advanced lens.
The goal is to find the optical system that gives the patient the best real-world visual quality.
When Should Persistent Distortion Be Investigated?
Further assessment may be worthwhile if you continue to notice:
- significant monocular ghosting;
- halos around lights;
- poor night-driving vision;
- reduced contrast;
- distorted letters despite good eye-chart acuity;
- one eye performing much worse than the other;
- vision that changes substantially throughout the day; or
- vision that remains disappointing despite a comfortable scleral lens.
Sometimes the solution is relatively simple.
In other cases, the reason for the distortion only becomes apparent after assessing the cornea, lens fit and optical quality together.
How We Assess Persistent Distortion at Westmead Eyes
For patients with complex corneas, assessment may include:
- corneal topography and tomography;
- assessment of scleral lens clearance;
- lens centration and rotation;
- over-refraction;
- assessment of residual astigmatism;
- checking for lens flexure;
- evaluation of lens-surface wettability;
- investigation of midday fogging;
- examination of corneal scarring and ocular health; and
- assessment of whether more customised optical correction may be beneficial.
The important question is not simply:
“Does the scleral lens fit?”
It is:
“Why is this patient still experiencing poor visual quality?”
Complex Scleral Lens and Keratoconus Assessment in Sydney
At Westmead Eyes, we regularly assess patients with:
- keratoconus;
- previous corneal cross-linking;
- corneal grafts;
- highly irregular corneas;
- unsuccessful previous RGP or scleral lens fittings; and
- persistent ghosting or distortion despite apparently good contact-lens vision.
Sometimes the solution is a power change.
Sometimes the scleral lens fit needs refinement.
Sometimes an HOA-correcting lens design may be considered.
And in selected patients, a specialised soft keratoconus lens such as KeraSoft IC may provide a different (and occasionally better) optical result.
If you already wear scleral lenses but continue to experience ghosting, halos or distorted vision, you can book a Keratoconus/ Specialty Contact Lens Assessment to investigate whether the limitation is coming from the prescription, lens fit, ocular surface or the optics of the eye itself.