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PubMed This is a summary of 2 peer-reviewed journal articles Updated
Ophthalmology

Confirming the Diagnosis: Tests and Subtypes

At a Glance

Macular corneal dystrophy (MCD) is diagnosed using a slit-lamp exam and AS-OCT imaging to identify grayish-white flecks across the full thickness of the cornea. The condition is categorized into three subtypes and is caused by a CHST6 genetic mutation.

Diagnosing Macular Corneal Dystrophy (MCD) requires a combination of clinical observation, high-tech imaging, and sometimes biochemical or genetic testing. Because it is much rarer than other dystrophies, specialists look for specific “hallmark” signs to distinguish it from similar conditions.

The Diagnostic Process

Your ophthalmologist uses several tools to confirm a diagnosis of MCD and rule out other possibilities:

  • Slit-Lamp Examination: This is the standard eye exam using a high-intensity light and a microscope. In MCD, the doctor looks for irregular, grayish-white flecks (macules) with fuzzy edges. A key diagnostic sign is that these flecks extend all the way to the limbus—the border where the clear cornea meets the white of the eye—and involve the entire thickness of the cornea [1][1].
  • Anterior Segment OCT (AS-OCT): This non-invasive imaging test uses light waves to take cross-sectional pictures of your cornea. In patients with MCD, the OCT shows hyperreflectivity (bright areas) throughout the corneal layers, helping the doctor see exactly how deep the “haze” goes [1].
  • Histopathology: If corneal tissue is removed (such as during a transplant), it is examined under a microscope. Pathologists use special dyes like Alcian blue or colloidal iron to stain the tissue. These dyes turn a deep blue when they encounter the abnormal sugar molecules (glycosaminoglycans) that build up in MCD [1][1].

How MCD Differs from Other Dystrophies

MCD is often confused with other “stromal” dystrophies, but the way it looks under a microscope and during an exam is unique:

  • vs. Granular Corneal Dystrophy (GCD): GCD typically has clear spaces between its “snowflake” spots and does not reach the very edge of the cornea [1].
  • vs. Lattice Corneal Dystrophy (LCD): LCD is characterized by branching, lattice-like lines and involves a different protein called amyloid [1].

Understanding the Three Subtypes

Specialists further categorize MCD into three subtypes (I, IA, and II) based on the presence of antigenic keratan sulfate (a specific sugar-protein marker) in the blood and eye tissue [1]. While knowing your specific subtype does not currently change the surgical options or treatment plan your doctor will recommend, this classification helps researchers understand the underlying biochemistry and develop targeted future therapies.

Subtype Keratan Sulfate in Blood (Serum) Keratan Sulfate in Cornea
Type I Absent (None detected) Absent
Type IA Absent Present only inside specific cells (keratocytes)
Type II Present (Near-normal levels) Present

Table data sources: [1][1]

Reading Your Lab Reports

If you see terms like CHST6 mutation on your report, it refers to the genetic “blueprint” error that causes the disease [2]. Mentions of descemet membrane thickening or corneal guttae (tiny bumps on the back layer of the cornea) are specific findings that confirm the disease is affecting the deepest layers of your eye, a common occurrence in MCD [1].

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Common questions in this guide

How is macular corneal dystrophy diagnosed?
Ophthalmologists diagnose MCD using a slit-lamp exam to look for grayish-white flecks that extend across the entire cornea. They may also use an imaging test called AS-OCT to see exactly how deep the haze goes into the corneal layers.
What is the difference between MCD subtypes I, IA, and II?
These subtypes are categorized based on whether a specific sugar-protein marker called keratan sulfate is found in your blood or corneal tissue. While your subtype does not currently change your treatment plan, it helps researchers understand the biochemistry of the disease.
What does a CHST6 mutation mean on my lab report?
A CHST6 mutation refers to the specific genetic error that causes macular corneal dystrophy. This genetic blueprint error leads to the abnormal buildup of sugar molecules in your eye.
How does MCD look different from other corneal dystrophies during an exam?
Unlike other dystrophies, the cloudy spots in MCD extend all the way to the edge of the cornea and involve its entire thickness. There are also no clear spaces between the spots, which helps doctors tell it apart from granular corneal dystrophy.
What do descemet membrane thickening and corneal guttae mean?
These terms describe physical changes to the deeper layers of your cornea. Corneal guttae are tiny bumps on the back layer of the cornea, and descemet membrane thickening confirms that the disease is affecting the deepest parts of your eye.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Based on my slit-lamp exam, do the opacities in my cornea extend all the way to the limbus?
  2. 2.What did the AS-OCT imaging reveal about the depth of the haze in my corneal layers?
  3. 3.Which subtype of MCD (I, IA, or II) do my lab results suggest, and how does that affect my understanding of the condition?
  4. 4.If a biopsy or pathology was performed, did the report mention Alcian blue or colloidal iron staining?
  5. 5.How does the absence or presence of keratan sulfate in my blood relate to my specific CHST6 mutation?

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References

References (2)
  1. 1

    IC3D Classification of Corneal Dystrophies-Edition 3.

    Weiss JS, Rapuano CJ, Seitz B, et al.

    Cornea 2024; (43(4)):466-527 doi:10.1097/ICO.0000000000003420.

    PMID: 38359414
  2. 2

    Macular Corneal Dystrophy: An Updated Review.

    Singh S, Das S, Kannabiran C, et al.

    Current eye research 2021; (46(6)):765-770 doi:10.1080/02713683.2020.1849727.

    PMID: 33171054

This page is for educational purposes only and does not replace professional medical advice. Always discuss your eye exam findings and laboratory test results with your ophthalmologist.

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