Glasses and contact lens prescriptions with regular astigmatism have two different power numbers, one for each curve in order to focus light together instead of in two separate spots. Unfortunately, prescriptions can only be written for regular astigmatism. Irregular astigmatism is when the two different curves are not perpendicular to each other(blue and red lines, Figure below).

Due to this irregularity, contact lenses need to be personalized to match these curves.

The terms irregular and regular astigmatism are used to describe the curvature of the eye, not necessarily the health of the eye. The eye can have either type of astigmatism and still be considered healthy.

What Causes Irregular Astigmatism?

Not all causes of irregular astigmatism are known, but eye care experts have linked it to systemic and ocular diseases and conditions that cause the cornea to warp or thin, injuries from sports or work, and procedures such as LASIK and other types of vision correction surgery.

Causes of Irregular Astigmatism

1. Keratoconus: This condition thins and weakens the central cornea until the age of 30-40, causing it to bulge outwards. This thinning causes the cornea to go from being rounded and smooth to being pointed, creating a cone-like shape, and making the corneal surface uneven.
2. Pellucid marginal degeneration (PMD): This condition is very similar to keratoconus, except that it continues throughout life. Specifically, the lower third of the cornea continues to thin and weaken. This creates a protrusion at the bottom, resembling a belly shape.
3. Keratoglobus: This condition is also similar to keratoconus, except that the cornea thins and weakens in the periphery creating a very rounded bulging ‘globe’ appearance.
4. Trauma to the eye: A deep cut or tear, stitches, or warpage from a previous contact lens fitting too tight can change the curves on the cornea.

   

5. Corneal surgery: Corneal transplant/partial corneal tissue replacements and refractive surgeries can lead to complicationsof changing curves on a cornea.

A list of procedures are as follows:

-DALK (Deep anterior lamellar keratoplasty)

-PKP (Penetrating keratoplasty)

-RK (Radial keratotomy)

-PRK (Photorefractive keratectomy)

-LASIK (Laser in-situ keratomileusis)

As mentioned above, pellucid marginal degeneration (PMD) is a corneal disease and a rare form of corneal ectasia (a group of disorders that cause the cornea to thin and protrude outward).

Normally, a healthy and thick cornea enables the eyes to see clearly. However, in PMD, the cornea thins over time. This thinning typically occurs in the lower part of the cornea, extending from the 4 o'clock to 8 o'clock position. Above the thinned area, the cornea protrudes outward. These changes in the cornea can lead to astigmatism and a gradual decline in visual acuity.

PMD usually affects both eyes, although it may occur in only one eye in some cases. This condition does not cause scarring in the eyes, allowing the cornea to remain transparent. For reference, the term "pellucid" means clear or transparent.

To date, experts have not yet determined the exact cause or risk factors for developing pellucid marginal degeneration (PMD). However, PMD is more commonly diagnosed when one is in their 20s, 30s, or 40s. While it can also be diagnosed when an individual is in their 50s or older, such cases are rare. So far, there is no scientific evidence suggesting that PMD is hereditary. However, some patients diagnosed with PMD have been recorded to have family members with moderate to severe astigmatism. eResearch by Navid Ajamin -- winter 2025

Treatment

Eye glasses

Most patients can be treated non-surgically with eye glasses, or contact lenses.

Contact lenses

Early stages of PMD may also be managed with soft contact lenses. Success has been shown with the use of rigid contact lenses combined with over-refraction. Patients wearing contacts report increased problems with glare and contrast sensitivity, but it is not clear if this is due to the corneal disease, or the contact lenses themselves.

New studies show that the use of a "GP" or Scleral contact lens has shown promise for most patients that exhibit Pellucid Marginal Degeneration. Most of these lenses are in the range of 15.5mm to 18.0mm in diameter.

Regardless of the lens size, it is thought that the larger the GP lens, regardless of the fact that it is a rigid lens, will in most cases be more comfortable then standard rigid corneal lenses, and at times more comfortable than soft lenses.

The highlight to the scleral design and the correction of eye disorders such as Pellucid Marginal Degeneration is that vision with these types of lenses is exceptional when fit correctly.

Intacs

The use of intacs implants has been tested as a treatment for PMD, with slight improvement in visual acuity noted after eleven months,and intacs have been used with keratoconus with success.

Collagen Cross Linking

There is evidence suggesting corneal collagen cross-linking may be beneficial for patients with pellucid marginal degeneration.

Surgery

Corneal transplant surgery may be difficult due to the peripheral thinning of the cornea, even with large and off-center grafts. Therefore, surgery is usually reserved for patients that do not tolerate contact lenses.

Several different surgical approaches may be taken, and no one approach is currently established as the standard.

Examples of surgical procedures used for PMD include: wedge resection, lamellar crescentic resection, penetrating keratoplasty, lamellar keratoplasty, epikeratoplasty and intracorneal segments.

Transplantation of the entire thickness of the cornea (penetrating keratoplasty) may be performed if there is enough normal tissue present. However, if there is not enough normal tissue present, then attaching the graft is difficult.

Due to the thinning of the cornea, PMD patients are poor candidates for procedures such as LASIK and photorefractive keratectomy.

Epidemiology

The incidence and prevalence of PMD are unknown, and no studies have yet investigated its prevalence or incidence. However, it is generally agreed that PMD is a very rare condition. Some uncertainty regarding the incidence of PMD may be attributed to its confusion with keratoconus. PMD is not linked to race or age, although most cases present early in life, between 20 and 40 years of age. While PMD is usually considered to affect men and women equally, some studies suggest that it may affect men more frequently.

Several diseases have been observed in patients with PMD. However, no causal relationships have been established between the any of the associated diseases and the pathogenesis of PMD. Such diseases include: chronic open-angle glaucoma, retinitis pigmentosa, retinal lattice degeneration, scleroderma, kerato-conjunctivitis, eczema, and hyperthyroidism.

Prognosis

Visual function declines as a result of the irregular corneal shape, resulting in astigmatism, and causing a distortion in vision. Deterioration can become severe over time.

Reference:

• researchgate.net/figure/Corneal-thickness-map-of-a-true-case-of-pellucid-marginal-degeneration-The-pachymetric_fig2_255706001
• drbrendancronin.com.au/blog/pellucid-marginal-degeneration-vs-keratoconus-understanding-the-similarities-and-differences
• siloamhospitals.com/en/informasi-siloam/artikel/understanding-pellucid-marginal-degeneration
• oogziekenhuis.me/Keratoconus/Pellucide_marginale_degeneratie.html
• clinevada.com/irregular-astigmatism

What is the difference between keratoconus and keratoglobus topography?

Keratoglobus is a special corneal thinning disease that differs from keratoconus in one important way: instead of a single point of thinning of the cornea (which results in a bulging and the development of a cone), the cornea in people with keratoglobus is thin everywhere in topography.

How common is keratoglobus?

Keratoglobus is a very rare condition that causes corneal thinning primarily at the edges of the cornea.

The distinction between the 2 similar conditions was first made by Cavara in 1950. The cornea in keratoglobus is diffusely thinned, often more markedly in the peripheral cornea, whereas in keratoconus the thinning is most prominent in the central cornea.

What is early keratoconus?

With keratoconus, the cornea thins and bulges into an irregular cone shape, resulting in vision loss. Keratoconus generally begins at puberty and progresses into the mid-30s. There is no way to predict how quickly the disease will progress, or if it will progress at all.

What are the risk factors for keratoglobus?

Keratoglobus is thought to be caused by a structural weakness in the cornea, leading to a gradual thinning and bulging of the cornea over time. This weakening may be due to a variety of factors, including genetic predisposition, collagen disorders, or excessive eye rubbing.

Is keratoglobus progressive?

Keratoglobus is a bilateral condition that is usually present at birth. It is usually minimally progressive. Keratoglobus has been associated with systemic connective tissue disorders such as Ehlers-Danlos syndrome, Marfan syndrome, and Rubenstein-Taybi Syndrome.

Understanding the Distinctions Between Keratoconus and Keratoglobus.

Two conditions that often confuse people due to their similarity in names and symptoms are keratoconus and keratoglobus. While both conditions affect the cornea, they have distinct features and require different approaches for diagnosis and management.

1. Definition and Prevalence: Keratoconus is a relatively common eye disorder where the cornea, the transparent front layer of the eye, thins and bulges into a cone shape. It affects approximately 1 in 84 individuals and usually begins during adolescence or early adulthood. On the other hand, keratoglobus is a rarer condition characterized by a more generalized thinning and globular protrusion of the cornea. Although it is less prevalent than keratoconus, its exact occurrence is not well-documented due to its scarcity.
2. Corneal Shape: The primary distinction between the two conditions lies in the shape of the cornea. In keratoconus, the cornea thins and protrudes asymmetrically in a cone shape, leading to distorted and blurred vision. This irregular curvature causes light to scatter as it enters the eye, resulting in visual impairment. Conversely, keratoglobus induces a more uniform thinning of the cornea, causing it to bulge outward like a globe. This results in a more significant visual distortion than keratoconus and may even lead to high myopia and astigmatism.
3. Progression and Age of Onset: Keratoconus typically begins in the teenage years or early adulthood and may progress over a span of 10 to 20 years before stabilizing. In some cases, the cornea's cone shape may remain relatively stable for prolonged periods. Keratoglobus, on the other hand, tends to manifest at a younger age, often during infancy or early childhood. Its progression is usually slower and may stabilize in early adulthood.
4. Treatment Options: The treatment approaches for keratoconus and keratoglobus are similar in some aspects but may differ based on the severity and progression of each condition. In the early stages of keratoconus and mild keratoglobus, eyeglasses or soft contact lenses might help correct vision. As the conditions progress, specilaised hard contact lenses are often prescribed to provide better visual acuity by compensating for the irregular corneal shape.

For both conditions, collagen cross-linking, CAIRS and other minimally invasive keratoconus surgery (MIKS) and other traditional types of corneal transplantation may be used. Topography guided laser treatments are usually not applicable for keratoglobus.

In conclusion, while keratoconus and keratoglobus both impact the cornea and cause visual disturbances, they are distinct conditions with varying characteristics and management strategies. Early diagnosis and appropriate treatment can significantly improve the quality of life for individuals affected by these conditions, emphasizing the importance of regular eye check-ups and timely intervention.

Reference:

• drbrendancronin.com.au/blog/z3gpu0uyvy4ear1px1pyi2dyjksgn1
• link.springer.com/rwe/10.1007/978-3-540-69000-9_931
• reviewofoptometry.com

شرح بیماری

کراتوکونوس Keratoconus یا قوز قرنیه، بیماری پیشرونده‌ای است که معمولا" در سنین نوجوانی یا اوایل دهه 20 زندگی بروز می‌کند. در این بیماری قرنیه نازک شده و شکل آن تغییر می‌کند. قرنیه به طور طبیعی شکلی گرد یا کروی دارد ولی در کراتوکونوس قرنیه برآمده و مخروطی شکل می‌شود. این مسئله بر روی انکسار نور هنگام ورود به چشم تأثیر گذاشته و سبب کاهش وضوح بینایی و تاری دید می‌شود. کراتوکونوس ممکن است در یک یا هر دو چشم رخ دهد ولی در 90% موارد در هر دو چشم دیده می‌شود.

علائم و نشانه‌ها

Keratoconus (KC) is a disorder of the eye that results in progressive thinning of the cornea. This may result in blurry vision, double vision, nearsightedness, irregular astigmatism, and light sensitivity leading to poor quality-of-life. Usually both eyes are affected. In more severe cases a scarring or a circle may be seen within the cornea.

Keratoconus is a vision disorder that occurs when the normally round cornea (the front part of the eye) becomes thin and irregular (cone) shaped. This abnormal shape prevents the light entering the eye from being focused correctly on the retina and causes distortion of vision.

While the cause is unknown, it is believed to occur due to a combination of genetic, environmental, and hormonal factors. Patients with a parent, sibling, or child who has keratoconus have 15 to 67 times higher risk in developing corneal ectasia compared to patients with no affected relatives. Proposed environmental factors include rubbing the eyes and allergies. The underlying mechanism involves changes of the cornea to a cone shape. Diagnosis is most often by topography. Topography measures the curvature of the cornea and creates a colored "map" of the cornea. Keratoconus causes very distinctive changes in the appearance of these maps, which allows doctors to make the diagnosis.

تشخیص کراتوکونوس ممکن است به دلیل بروز و پیشرفت آهسته آن مشکل باشد. این بیماری ممکن است با نزدیک بینی و آستیگماتیسم همراه باشد. در نتیجه ممکن است موجب عدم وضوح و تاری دید شود. بیمار همچنین ممکن است دچار هاله بینایی و حساسیت به نور باشد. بیماران معمولا" در هر بار مراجعه به چشم پزشک نسخه‌شان تغییر می‌کند.

علل کراتوکونوس

علت کراتوکونوس مشخص نیست. این بیماری ممکن است ژنتیکی باشد زیرا بیماران متعددی در یک خانواده دیده می‌شوند. و ممکن است بصورت اتوزوم مغلوب یا غالب به ارث برسد.

Treatment

Eyeglasses or soft contact lenses may be used to correct the mild nearsightedness and astigmatism that is caused by the early stages for keratoconus. Corneal cross-linking surgery is indicated early after diagnosis to stabilize the structure of the cornea and slow progression. As the disorder progresses and cornea continues to thin and change shape, rigid gas permeable contact lenses can be prescribed to correct vision adequately. In most cases, this is adequate. The contact lenses must be carefully fitted, and frequent checkups and lens changes may be needed to achieve and maintain good vision. In a few cases, a corneal transplant is necessary. However, even after a corneal transplant, eyeglasses or contact lenses are often still needed to correct vision.

Corneal cross-linking surgery halts or slows progression and is indicated early after diagnosis, or if later in the course of the disease there is still an increase in the glasses prescription. Intacs intracorneal removable inserts can stretch the cornea to stabilize astigmatism and near-sighted refractive errors. Once the progression has stabilized from life or cross-linking, an implantable Collamer lens surgery can correct higher prescriptions so that glasses or conventional soft contact lenses can be utilized.

درمان

در انواع خفیف بیماری ، عینک یا لنز نرم (Soft Contact Lenses) ممکن است کمک کننده باشد ولی با پیشرفت بیماری و نازکتر شدن و تغییر شکل بیشتر قرنیه این درمانها دیگر چندان کارساز نخواهد بود.

درمان بعدی بیماری استفاده از لنزهای سخت دارای قابلیت نفوذ گاز (Rigid gas permeable contact lenses) است. این لنزها قابلیت بیشتری برای اصلاح آستیگماتیسم نامنظم ناشی از کراتوکونوس دارند. قرار گیری مناسب لنز بر روی قرنیه مبتلا به کراتوکونوس کاری ظریف و زمان گیر است. به همین دلیل مراجعات مکرر برای تنظیم لنز و اصلاح نسخه امری قابل انتظار است. این فرایند با نازکتر شدن و تغییر شکل بیشتر قرنیه نیاز به تکرار داشته و نسخه بیمار باید مجددا" تغییر داده شود.

در صورتی که قرنیه قادر به تحمل لنز سخت نبوده و یا لنز توانایی اصلاح دید بیمار را نداشته باشد قدم بعدی پیوند قرنیه است. بیمار ممکن است حتی بعد از پیوند قرنیه برای دید بهتر به عینک و یا لنز نیاز داشته باشد.

در حال حاضر در بیمارانی که قادر به تحمل لنزهای سخت نیستند و از طرفی انجام پیوند قرنیه بدلیل عوارض آن در آنها توصیه نمی‌شود از روشهای جدیدتری نظیر قرار دادن حلقه‌های داخل قرنیه مثل Intacs و Ferrara استفاده می‌شود. مطالعه بر روی این روش در چند سال اخیر در جریان بوده و نتایج نشان دهنده بهبود نسبی بیماران است. هر چند نشان داده نشده است که این روش مانع پیشرفت بیماری شود.

The classic symptom of keratoconus is the perception of multiple "ghost" images, known as monocular polyopia. This effect is most clearly seen with a high contrast field, such as a point of light on a dark background. Instead of seeing just one point, a person with keratoconus sees many images of the point, spread out in a chaotic pattern. This pattern does not typically change from day to day, but over time, it often takes on new forms. People also commonly notice streaking and flaring distortion around light sources. Some even notice the images moving relative to one another in time with their heartbeat.

The predominant optical aberration of the eye in keratoconus is coma. The visual distortion experienced by the person comes from two sources, one being the irregular deformation of the surface of the cornea, and the other being scarring that occurs on its exposed highpoints. These factors act to form regions on the cornea that map an image to different locations on the retina. The effect can worsen in low light conditions, as the dark-adapted pupil dilates to expose more of the irregular surface of the cornea.

کراتوکونوس و جراحی عیوب انکساری

انجام هیچکدام از اعمال جراحی اصلاح عیوب انکساری نظیر لیزیک (LASIK) و لیزر (PRK) در بیماران مبتلا به کراتوکونوس بدلیل نازکی پیشرونده قرنیه امکان پذیر نیست.

Reference:

• en.wikipedia.org/wiki/Keratoconus
• aoa.org/healthy-eyes/eye-and-vision-conditions/keratoconus?sso=y