May 22, 2024

Understanding Global Electroretinogram: A Comprehensive Guide To Retinal Function Testing

What is Global ERG?

Global electroretinography (ERG) is a diagnostic test that measures the retinal cells’ electrical response to light. It helps ophthalmologists check whether the retina and its cells are functioning normally. An ERG examines the entire retina rather than specific parts of it. For this reason, it is also called “full-field ERG” or “full-field electroretinography.”

How is a Global ERG Performed?

During a global ERG test, contact lens type electrodes are placed on the surface of the eye after applying anesthetic eye drops. Reference electrodes are placed on the skin near the eye or earlobe. The other electrode is placed on the forehead to measure brain waves. The patient sits in a dimly lit testing room in front of special light-emitting goggles or contact lenses. Brief flashes of light are presented and the retinal response is recorded through the electrodes. The test usually takes around 30-45 minutes to perform on each eye separately Global Electroretinogram evaluates both rod and cone cell function by using different intensities and durations of light stimuli.

What does a Global ERG Test?

A global ERG test provides valuable information about the overall function of the retina’s light-sensitive cells called photoreceptors. It can detect problems in the rod and cone photoreceptors as well as in the inner retinal layers. The specific parameters measured through global ERG include:

Rod response: It assesses rod photoreceptor function which is important for night vision and peripheral vision. Abnormal rod response can indicate rod-cone dystrophies.

Combined rod-cone response: This evaluates the combined function of rods and cones. Abnormalities here may point to various inherited retinal diseases.

Oscillatory potentials: These waveforms arise from amacrine and bipolar cells. Problems with oscillatory potentials can indicate toxicity, mitochondrial disorders or optic neuropathies.

Cone response: It examines cone photoreceptor function which is crucial for central high-acuity color vision. Cone disorders cause visual impairment in bright light.

30Hz flicker response: This evaluates the integrity of cone pathway. Defects suggest conditions like cone dystrophy or cone-rod dystrophy.

Thus, a global ERG provides a functional overview of the entire retina which aids in diagnosing various retinal diseases.

Global ERG Applications

Some common applications of global electroretinogram include:

Evaluating hereditary retinal disorders: Inherited retinal dystrophies like retinitis pigmentosa, cone-rod dystrophy, Leber’s congenital amaurosis affect photoreceptor cells. ERG can help detect such genetic disorders.

Diagnosing retinal detachment: ERG is useful to confirm clinical suspicion of retinal detachment, especially rhegmatogenous retinal detachment involving the macula.

Monitoring treatment response: Global ERG proves helpful in monitoring patients on treatment for conditions like diabetic retinopathy, uveitis, retinal toxicity. It provides an objective measure of therapeutic effectiveness.

Detecting toxicity-induced retinal damage: Prolonged intake of certain medications like hydroxychloroquine, tamoxifen can damage retinal cells. ERG screening aids in identifying toxicity at an early stage.

Investigating unexplained vision loss: When routine eye exams and imaging fail to identify the cause for decreased vision, ERG evaluates the underlying retinal function which sometimes reveals the diagnosis.

Differentiating retinal from neurological pathologies: Global ERG makes it possible to distinguish conditions primarily affecting the retina from those of the visual pathway in the brain, like optic nerve disorders.

Predicting visual prognosis: The severity of ERG abnormalities often correlates with visual prognosis in many hereditary retinal degenerations. It helps counsel patients on disease progression.

Limitations of Global ERG

While very useful as a screening and diagnostic tool, global ERG has some limitations:

It cannot identify specific retinal regions affected since it evaluates the whole retina. Localized disorders may be missed.

Severe retinal diseases need to involve 30-50% of the retina before ERG detects abnormalities. Mild cases can be missed.

ERG readings depend on patient’s cooperation and ability to fixate. Young children may need anesthesia for the test.

Other ocular pathology like cataract, vitreous hemorrhage can affect ERG results by altering light stimulus.

ERG provides functional information but no anatomical details. It must be correlated with imaging tests.

Rarely, ERG may report false negatives especially in early stages of certain hereditary retinal degenerations.

A global ERG is a valuable investigative tool for retinal disorders when findings are interpreted carefully along with patient history and other test results. It provides objective evaluation of overall retinal function non-invasively.

1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it.