Phototransduction - Diagnosis and Treatment

Introduction

The eye is a complex structure similar to a camera capable of "collecting" and detecting light. The retina is a thin, sensitive membrane that covers the inner surface of the back of the eye. The retina is an essential part of the eye for receiving the light entering the eye; hence it is highly specialized in terms of the organization and structure of the cells. It comprises of several layers, one of which has specialized cells known as photoreceptors. Human eyes have two types of photoreceptor cells: rods and cones. Cones are mainly found in the macula, a small central region of the retina, and rods are spread out across the entire retina. Visual information processing may initiate in the retina with light detection. Rhodopsin, a type of protein and most abundant cells found in the rod cells, helps to mediate to detect black and white color, sense motion, and work effectively in dim light. Cones cells work best in medium and bright light and are responsible for the center and color vision change.

What Is Phototransduction?

Phototransduction is an action or process of visual processing that occurs when light (photon) is converted to an electric signal in the retina of the eye. Rhodopsin is structurally categorized as a chromoprotein. A colored chemical is generated from vitamin A called 11-cis-retinaldehyde, and the colorless protein opsin makes up this protein. When exposed to light, a photoreceptor membrane potential hyperpolarizes, reducing the amount of neurotransmitter released onto downstream neurons. Thus, phototransduction allows the photoreceptor to translate a light stimulus into a chemical output.

What Happens in Phototransduction?

Phototransduction comprises three primary metabolic processes, which are

1. When Light Enters the Eye, the Opsin Molecules in the Photoreceptors Are Activated.

Rhodopsin changes shape when 11-cis-retinal is isomerized to the all-trans form during photon absorption. The name of this activated rhodopsin is metarhodopsin II. All-trans-retinol serves as the precursor for 11-cis-retinal (vitamin A). Since humans, an essential vitamin for vision, cannot synthesize vitamin A.

2. The Level of Intracellular CGmp Decreases as a Result of Activated Rhodopsin.

Cyclic guanosine monophosphate (cGMP), an enzyme that degrades cGMP, regulates the amount of cGMP in the cytoplasm. The activity of this enzyme could be stronger in the dark. Metarhodopsin II promotes the movement of cGMP phosphodiesterase when the photoreceptor is exposed to light through the G protein transducin. After interacting with activated rhodopsin, GDP-bound inactive transducin will convert GDP to GTP. Active transduction that is coupled to GTP will make cGMP phosphodiesterase more active. As a result, the cytoplasmic concentration of cGMP is reduced.

3. The Photoreceptor Becomes Hyperpolarized Following Light Exposure.

Reduced cGMP levels result in the closure of cGMP-gated ion channels, which causes membrane hyperpolarization.

What Diseases Are Caused by Defects in the Phototransduction Process?

When the photoreceptor cells are unable to work, usually due to the genetic abnormalities that cause certain retinal disorders, includes:

• Bradyopsia - Gene mutations cause the disorder bradyopsia, sometimes known as "slow vision. People with bradyopsia have problems adjusting to lighting changes, temporarily become blind, and cannot perceive the movement of objects in bright environments.

• Retinitis Pigmentosa - Retinitis pigmentosa is an inherited condition characterized by the accumulation of retinal pigments and photoreceptor cell degeneration.

• Congenital Stationary Night Blindness - A genetic condition that damages rod photoreceptors and affects vision in dim light.

What Are the Diagnostic Options Available for Visual Phototransduction Eye Abnormalities?

The diagnostic options available for visual transduction eye abnormalities includes:

• Eye Examinations - Eye professionals will do an eye check-up by dilating the pupils using special eye drops.

• Visual Acuity Test - The eye care professional asks the patient to read letters off a chart (known as a Snellen chart). Initially, the patientl reads with one eye covered, followed by the other eye, and finally, with both eyes. Next, the doctor can evaluate each eye's distance from 20/20 vision. The findings will provide the patient's eye care provider with information about their vision and any potential prescriptions they may need to rectify.

• Optical Coherence Tomography - The imaging method known as optical coherence tomography (OCT) employs a low-coherence light to produce two and three-dimensional images with micrometer resolution from a visual scattering medium. Optical coherence tomography scanner provides an automated, segmented representation of the vitreous and macula layers. This image illustrates the degree of vitreomacular traction's involvement and tension on the macula. This imaging technique helps identify and treat retinal diseases.

• Electroretinogram - The eye doctor places a gold foil film or a specific contact lens in the eye for an electroretinogram. Then they will measure how the retina reacts to sudden light flashes.

• Ocular Pressure Eye Test - Ocular pressure eye test, also known as tonometry, is a simple test used by a medical professional to measure the pressure inside the eyes.

• Ophthalmoscope - The doctor will place drops in the eyes to enlarge the pupil to see the retina. Doctors will look into the back of the eye with a hand-held instrument.

What Are the Treatments Available for Visual Phototransduction Eye Abnormalities?

There is no known treatment for visual phototransduction eye issues since these conditions are all genetic. However, researchers are making great efforts to develop new therapies. Usually, the goal of the treatment is to control the symptoms, signs, and any developing complications. Several options can prevent further vision loss and perhaps partially restore sight. Such as:

• Vitamin A Palmitate - High doses of vitamin A palmitate may decrease the progression of retinal diseases such as retinitis pigmentosa and congenital night blindness by a minor amount each year. Still, too many doses may lead to excessive toxicity. Follow the physician's advice and collaborate with them carefully.

• Treatment of Systemic Conditions - Some conditions like glaucoma, age-related macular degeneration, myopia, and optic neuritis may also develop phototransduction abnormalities.

• Gene Therapy - Healthy genes will be introduced into the retina using gene therapy.

• Wearing Sunglasses - Sunglasses shield the eyes from dangerous ultraviolet radiation that could speed up vision loss and reduce the sensitivity of the eyes to light.

Conclusion

Phototransduction is the process in the eye where light is converted into an electrical signal in the retina, with the help of a protein called rhodopsin, and it allows the photoreceptor to communicate a light stimulus to the brain through a chemical output. Those with phototransduction eye problems should undergo genetic testing and counseling and attend regular eye exams to protect their vision.