Scleral Lenses for Pellucid Marginal Degeneration

Pellucid marginal degeneration (PMD) is a corneal condition that, like keratoconus, causes progressive thinning and irregular curvature — but in a distinctive band low down near the edge of the cornea, producing a characteristic pattern of irregular, often against-the-rule astigmatism that’s particularly difficult to correct with glasses or standard soft lenses.

Understanding PMD

PMD typically presents somewhat later than keratoconus, often in the third to fifth decade of life, and progresses more slowly in many cases. Like keratoconus, it isn’t fully understood in terms of underlying cause, though it shares some overlapping features and is sometimes considered part of the same broader family of corneal ectatic disorders. The key distinguishing feature is the location: a band of thinning low down and near the edge (periphery) of the cornea, rather than the more central cone typical of keratoconus.

Why PMD is challenging to correct

Because the thinning in PMD sits inferior and peripheral rather than centred on the cornea, the resulting irregularity doesn’t behave like standard astigmatism. Glasses can address regular astigmatism but not this kind of localised distortion, and standard soft lenses simply flex with the irregular shape rather than correcting it.

How PMD is diagnosed

Corneal topography is central to diagnosing PMD, since the characteristic pattern — often described as a distinctive “crab claw” or “butterfly” shape on a topography map — is one of the more recognisable signatures in corneal imaging. This pattern helps distinguish PMD from keratoconus and from other causes of irregular astigmatism, which matters because the specific pattern of irregularity affects how a scleral lens needs to be designed.

How scleral lenses help with PMD

A scleral lens vaults entirely over the irregular area, resting on the sclera rather than adapting to the distorted corneal shape beneath it. Because PMD’s irregularity is often asymmetric, corneal topography is particularly important here — it maps the specific pattern and location of thinning so the lens can be designed to clear it properly, including the peripheral zone where PMD is most pronounced.

PMD can sometimes be mistaken for other forms of corneal ectasia without detailed corneal topography, which is why mapping the exact location and pattern of thinning is so important.

What to expect from fitting

PMD fittings are often a little more involved than standard keratoconus fittings, because of the peripheral, asymmetric nature of the irregularity — the lens landing zone needs particular attention to avoid uneven pressure over the thinned area. This typically means careful review of topography data and, often, more than one diagnostic lens to reach the right design.

• Corneal and scleral topography, with particular attention to the peripheral pattern characteristic of PMD
• Anterior OCT imaging to confirm clearance specifically over the thinned peripheral zone
• Diagnostic lens fitting, often requiring more than one iteration given the asymmetric shape involved
• A final lens designed around the confirmed measurements, followed by fine-tuning follow-up appointments

Living with scleral lenses for PMD

Once a stable fit is achieved, most PMD patients describe outcomes similar to keratoconus patients — significantly improved, more stable vision compared with glasses, and a level of comfort that corneal RGP lenses often couldn’t match given the specific shape of PMD irregularity. Because PMD’s peripheral pattern is more prone to lens decentration with smaller lens designs, many PMD patients find scleral lenses particularly well suited compared with the alternatives they may have previously tried.

Why PMD is sometimes diagnosed later than keratoconus

Because PMD tends to present later in life and progress more slowly than keratoconus, and because it’s less commonly encountered even among optometrists and ophthalmologists, it’s sometimes not identified until later in the process of investigating unexplained vision changes or difficulty with standard lens fitting. This isn’t unusual, and it doesn’t reflect a missed diagnosis so much as the genuinely lower profile of the condition compared with keratoconus.

What to bring to your assessment

If you’ve already been diagnosed with PMD elsewhere, bringing any topography scans, clinic letters or previous lens fitting history is genuinely useful, giving us a starting point for comparison rather than beginning entirely from scratch. If you’re coming in with undiagnosed but persistent vision or fitting difficulties, a full topography assessment as part of your visit will help establish whether PMD, keratoconus or another condition is the underlying cause.

PMD and surgical considerations

For some patients with more advanced PMD, surgical options may be discussed by an ophthalmologist alongside or instead of contact lens correction, depending on severity and individual circumstances. Scleral lenses and surgical management aren’t mutually exclusive considerations, and many PMD patients are successfully managed with scleral lenses alone without ever needing surgical intervention, particularly when the condition is caught and managed proactively.

Long-term outlook with PMD

Like keratoconus, PMD’s progression varies significantly between individuals, and many patients find their condition remains stable for extended periods once appropriately managed. Regular monitoring, including repeat topography at routine reviews, helps track whether your PMD remains stable or is changing, informing whether your current scleral lens design continues to be the right fit or needs updating.

If you’ve struggled to get a diagnosis

Because PMD is less commonly encountered than keratoconus, some patients describe a longer or more circuitous route to diagnosis, having been told their vision problems were simply astigmatism or that lenses “should” fit better than they did. If this describes your experience, a thorough topography-based assessment can usually clarify what’s actually happening with your corneal shape, whether that turns out to be PMD or something else.

Why an accurate diagnosis matters for lens design

Getting the specific diagnosis right — PMD rather than keratoconus, or vice versa — matters practically, not just academically, because the two conditions call for meaningfully different lens design approaches given their different patterns of corneal irregularity. A scleral lens designed around an assumed keratoconus pattern when the underlying condition is actually PMD is less likely to achieve a stable, comfortable fit on the first attempt, which is part of why detailed topography is so central to getting this right from the outset.

If you have unexplained lens fitting difficulty or vision problems that don’t quite match a standard astigmatism or keratoconus picture, it’s worth requesting a thorough topography-based assessment specifically to check whether PMD might be the explanation. Getting this clarity, even if the answer turns out to be a different condition entirely, is usually worth the appointment on its own, and rules out uncertainty that can otherwise linger for years without a clear answer. Booking that first assessment is a small step that can resolve a question that’s sometimes gone unanswered for a considerable time.

Scleral Lenses & PMD FAQs

Is PMD the same as keratoconus?

No — both cause corneal thinning and irregular astigmatism, but PMD affects a distinct, usually inferior peripheral band of the cornea, giving a different pattern of irregularity that needs its own careful assessment.

Can scleral lenses stop PMD from progressing?

Scleral lenses correct vision by masking the irregular surface — they don’t treat the underlying thinning. If your PMD is progressing, your eye care team may also discuss other management options alongside lens fitting.

Do PMD patients need a different scleral lens design from keratoconus patients?

Often yes, particularly around the landing zone and peripheral design, since PMD’s asymmetric, low, peripheral pattern is different from the more central cone typical of keratoconus.

Is PMD rarer than keratoconus?

Yes, PMD is considerably less commonly diagnosed than keratoconus, which is part of why it’s less well understood by patients and why specialist assessment, including topography, matters so much in reaching an accurate diagnosis.

Can PMD affect both eyes?

PMD can affect one or both eyes, and when both eyes are involved, the severity and exact pattern often differs between them — each eye is assessed and fitted individually based on its own topography.

Book a Scleral Lens Assessment

Call the practice on 0141 943 3300 to arrange a scleral lens consultation.