Electrofisiología Visual Evaluación en Ceguera y Baja Visión Parte 1

Electrophysiology of Vision: Evaluating Blindness and Low Vision

Introduction to Visual Electrophysiology

• Iván Plaza Rosales introduces the topic of visual electrophysiology, focusing on its potential in assessing blindness and low vision.

• The presentation outlines the contents, defining ocular electrophysiology as a set of tests based on the bioelectric characteristics of various retinal structures, including photoreceptors.

Techniques in Visual Electrophysiology

Electroretinogram (ERG)

• The primary goal of these tests is to evaluate the integrity of the visual pathway. The first technique discussed is the electroretinogram (ERG), which measures the functionality of photoreceptors and ganglion cells in the retina.

Visual Evoked Potential (VEP)

• A second type of electrodiagnostic test is introduced: the visual evoked potential (VEP). This test measures electrical activity generated in the occipital cortex in response to stimuli such as light or specific patterns.

Electrooculogram (EOG)

• Finally, the electrooculogram (EOG) assesses retinal pigment epithelium functionality and indirectly measures eye movement through extraocular muscle activity. This technique has been clinically applied since around 1950.

Understanding Electrooculography

Mechanism and Functionality

• The EOG measures an electric potential difference between two ocular structures: the cornea and retinal pigment epithelium. This dipole generates a field that allows for indirect measurement of eye displacement.

Measurement Process

• A dark adaptation period is required before testing, typically lasting about 10 minutes to achieve maximum potential difference reduction.

Eye Movement Analysis

• Movements are analyzed based on whether they deviate left or right from a baseline position. Physiological origins relate to light activation at photoreceptors leading to changes in conductance.

Graphical Representation of Eye Movements

• Graphical data illustrates horizontal and vertical eye movements over time with voltage differences indicating directionality—positive for rightward movement and negative for leftward movement.

Additional Techniques: ERG Components

Overview

• Other evaluations include ERG, which focuses on responses from photoreceptors and ganglion cells.

Types of Electrodes Used

• Various electrode types are utilized for ERG measurements, including contact lenses like Burian Allen's bipolar lens that do not require additional reference electrodes.

Components Measured by ERG

Signal Interpretation

• Different components within ERG signals include P1, P2, P3 waves associated with distinct cellular layers within the retina—P2 primarily linked to bipolar cells.

Full Field vs Patterned Responses

• (Full-field ERG provides a comprehensive assessment under high-intensity light conditions while patterned responses focus on localized retinal areas using phase-inverted stimuli.

Visual Evoked Potentials Explained

Transitioning Focus from Retina to Cortex

• (Visual evoked potentials assess cortical responses rather than just retinal activity by measuring electrical changes due to sensory stimuli at Broadmann area 17.

Key Characteristics

• VEP reflects postsynaptic potentials generated by pyramidal cells; it offers real-time evaluation with high temporal resolution but lower spatial accuracy compared to other methods.

Summary Insights into Techniques Discussed

1. Electrooculogram - Measures retinal pigment epithelium functionality; indirectly assesses eye movements through extraocular muscles.

2. Electroretinogram - Evaluates photoreceptor function; utilizes various electrode types for accurate readings across different cell layers.

3. Visual Evoked Potential - Assesses cortical response; characterized by multiple components reflecting neural processing related to visual stimuli.

4. Each method contributes uniquely towards understanding visual pathways' integrity essential for diagnosing conditions like blindness or low vision.

5. These techniques collectively enhance clinical assessments regarding visual impairments through detailed analysis at both retinal and cortical levels.