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Scanning Laser Polarimetry (SLP):

GDx uses a technique called Scanning Laser Polarimetry (SLP).

It measures birefringence, an optical feature resulting from microtubules (tiny tube-like structures) aligned parallel within the nerve fibers of the Retinal Nerve Fibre Layer (RNFL).

When polarized light goes through the Retinal Nerve Fibre Layer (RNFL), it undergoes a phase shift (called retardation), which increases with RNFL thickness.

A larger amount of retardation measured by SLP indicates the presence of more nerve fibers, as it is proportional to the number of microtubules.

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Over the years, the GDx has seen many improvements that enhance its accuracy, speed, and ease of use. Some of the key advancements include:

Variable Corneal Compensation (VCC):

The fifth-generation technology individually compensates for each patient's ocular birefringence, improving accuracy over previous fixed compensation methods.

Nerve Fiber Indicator (NFI):

Provides a single-number output for quick and accurate assessment of glaucoma risk.

Enhanced Corneal Compensation (ECC):

The latest GDx models, such as the GDx Pro 8000, they use ECC to reduce image noise and improve scan quality across diverse patient profiles.

Automatic Image Alignment:

Ensures precise registration of images across multiple visits, improving the accuracy of longitudinal assessments.

These upgrades make the GDx more effective for early glaucoma detection and ongoing patient management.

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The GDx or known as Glaucoma Diagnosis is a kind of scanning laser polarimeter that measures the thickness of nerve fibre layers and retinal spots using a diode laser in the near-infrared spectrum and polarised light is used in this process to penetrate the layer of retinal nerve fibres.

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The GDx is developed and manufactured by Carl Zeiss Meditec, a global leader in ophthalmic diagnostic technology.

Carl Zeiss Meditec is known for its precision optics, innovative technology, and high reliability in the field of ophthalmology and optometry.

Earlier versions of the GDx were developed by Laser Diagnostic Technologies Inc., but Carl Zeiss Meditec is now the primary manufacturer and developer of the latest GDx models such as GDx Pro 8000 and GDx VCC.

They are the only company that manufactures this GDx instrument, making them a trusted name in glaucoma diagnosis and retinal fiber layer (RNFL) analysis.

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How GDx technology measures Retinal Nerve Fibre Layer (RNFL) thickness?

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How GDx technology measures Retinal Nerve Fibre Layer (RNFL) thickness?

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1. GDx uses Scanning Laser Polarimetry (SLP) to measure the thickness of the RNFL by detecting changes in light.

2. RNFL:

Made up of ganglion cell axons supported by parallel microtubules.

These microtubules cause form birefringence, which changes the phase of light (called retardation).

A thicker RNFL means there are more microtubules, which leads to greater retardation.

The RNFL is thickest in the superior and inferior regions, creating a "double hump" pattern in TSNIT plots.

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3. A near-infrared laser (785 nm) is used to scan the retina. As the polarized light passes through the retinal nerve fibers, their birefringence causes a delay or shift in the light's phase (called retardation).

4. The amount of light delay (retardation) is directly related to the thickness of the RNFL. This delay is measured in microns, using a conversion where 1 degree of retardation equals about 7.4 microns.

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The GDx machine is essential to optometry especially for the early identification,diagnosis and tracking of glaucoma which is one of the world's leading causes of permanent blindness.

In addition,retinal nerve fibre layer (RNFL) damage is one of the first indications of glaucoma and can happen decades before a patient noticeably loses their vision. By using GDx,optometrist may assess (RNFL) thickness precisely and non-invasively which aids in the early detection of structural alterations in optic nerve.

Moreover, GDx offers imaging and quantitative data that allows optometrist to monitor development over time and modify treatment regimens as necessary.

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5. The GDx PRO uses Variable Corneal Compensation (VCC) to remove the effect of birefringence from the cornea and lens, so only the RNFL signal is measured.

6. The procedure involves two scans. The first scan evaluates the birefringence in the anterior segment of the eye. The second scan captures the retina image after compensating for this birefringence.

7. RNFL thickness is measured using a circular band 8 pixels wide and centered on the optic disc, with a diameter of about 3.2 mm.

8. The retardation values within this band are used to calculate several RNFL measurements, such as the TSNIT Average and the Superior/Inferior Averages.

9. The results are shown as a colour-coded map, where brighter colours mean a thicker RNFL, and also as a TSNIT graph that shows RNFL thickness around the optic disc.

10. The Nerve Fiber Indicator (NFI) gives an overall score of RNFL health using a machine learning system.

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What are the new features of the GDx PRO 8000 model ?

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What are the new features of the GDx PRO 8000 model ?

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The presence of Guided Progression Analysis (GPA) helps detect glaucoma progression with two mode features, namely fast mode and extended mode.

Fast mode : using three individual eye scans.

Extended mode : using an external cohort to compare two baseline scans with the last two follow-up scans.

Offers Retinal Nerve Fiber Layer Integrity (RNFL-I), investigating beyond mere thickness measurements.

Enhanced Corneal Compensation (ECC), ensures exceptional scan quality and measurement accuracy across a wide range of patient profiles.

New Nerve Fiber Indicator (NFI), an important parameter in distinguishing various conditions.

Portable with unique optical locking feature

Compact system design

Supports printing to virtually any Windows XP-compatible printer

Capable of exporting in a variety of formats, including PDF, TIFF and JPEG

A new automatic mapping to an IP address that is generated automatically

Provides full network browsing capabilities

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GDx showing an abnormal result in both the eyes

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Although modern digital lifestyles such as overused of smartphones or other technologies that has direct light exposure can cause ocular stress, there is no clinical researches or evidences that can proved this lifestyles lead to glaucoma. However, GDx is a technological advancements that can help to diagnose glaucoma patients by measuring the thickness of retinal nerve fiber layer on retina. This help optometrists to give advices for the patients to get early treatments and spread awareness of getting an early eye checkup at a young age.

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Comparison between GDx VCC (old model) and GDx PRO 8000 (latest model) 

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Comparison between GDx VCC (old model) and GDx PRO 8000 (latest model) 

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The design of the old GDX remains the same, with only a few updates and modifications.

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To measures RNFL thickness

• To detect changes in retinal nerve fiber layer by using polarized laser light. By detecting the amount of birefringence (a property that affects how light passes through the RNFL), GDx can quantify RNFL thickness accurately, which is essential for evaluating optic nerve health. It also can detect early Glaucoma by identify loss of nerve fiber before visual field problem appear. This early detection capability allows clinicians to diagnose glaucoma in its earliest stages, when preventative or slowing treatments are most effective.

Detection of Optic Nerve Head Abnormalities

• Identifies structural changes in the optic nerve head that may not be visible in routine fundus examinations. Any damage or abnormality at the optic nerve head often causes detectable changes in RNFL thickness or pattern.

In short, the GDx system plays an important role in modern glaucoma management by offering objective, quantitative data on RNFL thickness. Its ability to detect structural damage at an early stage often before vision loss occurs makes it an important tool in preserving patient vision.