Understanding The Choroid Coat Of The Eye

The choroid coat, or choroid, is a vascular membrane in the eye that lies between the retina and the sclera. It is made up of large, branched pigmented cells and is responsible for nourishing the retina and absorbing scattered light. The choroid is thickest at the back of the eye, measuring about 0.2 mm, and narrows to 0.1 mm at the peripheral part of the eye. It is an essential component of the eye, providing oxygen and blood supply to the retina, as well as potentially cooling and warming it.

The choroid coat is a highly vascular membrane in the eye

The choroid coat, or simply the choroid, is a highly vascular membrane in the eye. It is also referred to as the choroid membrane or choroid layer. The choroid coat is located between the retina and the sclera (the whites of the eye). It is continuous with the iris and lies directly behind it.

The choroid coat is a pigmented layer, containing large branched pigmented cells. This pigmentation is dark-coloured melanin, which serves to absorb light and limit reflections within the eye. This absorption of light minimises the scattering of light inside the eye, improving vision. The choroid coat is also thought to protect the choroidal blood vessels from light toxicity.

The choroid coat is made up of four layers:

• Haller's layer (large blood vessel layer)
• Sattler's layer (medium-size blood vessels)
• Choriocapillaris (capillaries)
• Bruch's membrane (innermost membrane)

The choroid coat functions to nourish the retina and provide oxygen and blood supply. It also helps to regulate the temperature of the retina by warming and cooling it.

It lies between the sclera and the retina

The choroid coat, or simply the choroid, is a vascular membrane in the eye that lies between the sclera and the retina. It is composed of large, branched pigmented cells and is continuous with the iris. The choroid coat functions to nourish the retina and absorb scattered light. Its dark pigmentation, caused by the presence of melanin, minimises the scattering of light inside the eye, thus improving vision.

The choroid is made up of four layers: Haller's layer, which contains large blood vessels; Sattler's layer, which contains medium-sized blood vessels; Choriocapillaris, which contains capillaries; and Bruch's membrane, which is the innermost layer of the choroid. The choroid is thickest at the back of the eye, measuring about 0.2 mm, and narrows to 0.1 mm in the peripheral parts of the eye.

The choroid coat is essential for eye health and function. It provides a significant supply of oxygen and blood to the retina, and it may also help to cool and warm the retina. Additionally, the choroid contains cells that secrete substances that promote the growth of the sclera. The choroid can change thickness, and these changes can move the retina forward and backward, bringing the photoreceptors into focus.

Maintaining the health of the choroid is crucial for good vision. When the choroid is infected or damaged, it can lead to severe vision loss or even total blindness. Various diseases and disorders can affect the choroid, including hemorrhagic choroidal detachment, choroidal rupture, choroidal nevi, choroidal dystrophies, and chorioretinitis.

It nourishes the retina and absorbs scattered light

The choroid coat, or choroid, is a vascular membrane in the eye that lies between the sclera and the retina. It is highly pigmented and contains large branched pigment cells. The choroid coat has several functions, including nourishing the retina and absorbing scattered light.

The choroid coat is an essential part of the eye, providing oxygen and nourishment to the retina and regulating temperature. It accounts for about 85% of ocular blood flow and helps maintain blood circulation in the eye. This ensures that the retina, macula, and optic nerve receive the necessary nutrients for proper functioning.

One of the key functions of the choroid coat is its ability to absorb scattered light. This is achieved through the presence of melanin, a pigmented molecule. Melanin absorbs stray light that may interfere with visual images, improving the sharpness and clarity of vision. By absorbing excess light, melanin may also protect against light toxicity.

In addition to its role in light absorption, the choroid coat is also involved in reducing internal reflection of light. This is particularly important in nocturnal animals, where the reflective material in the choroid coat, known as tapetum lucidum, enhances night vision by reflecting visible light back through the retina. This reflective effect causes the eye-shine often observed in animals at night.

Overall, the choroid coat plays a crucial role in maintaining retinal health and visual acuity by providing nourishment to the retina and absorbing scattered light through the presence of melanin.

The choroid coat is made up of four different layers

The choroid, also known as the choroidea or choroid coat, is a vascular membrane that lies between the sclera (the whites of the eyes) and the retina (the light-sensitive tissue at the back of the eye). It is a highly pigmented layer of tissue, containing large branched pigmented cells. The choroid is part of the uvea, which also includes the iris and the ciliary body.

The choroid is made up of four layers:

Bruch's Membrane

This is the innermost layer of the choroid, also known as the Lamina Basalis, Complexus Basalis, or Lamina Vita. Bruch's membrane is a thin layer of tissue that separates the choroid from the retinal pigmented epithelium (RPE).

Choriocapillaris

The choriocapillaris is a layer of capillaries (small blood vessels connecting arteries and veins). These capillaries are highly fenestrated, and they form a single layer that provides nutrients to the RPE and the outer third of the retina.

Sattler's Layer

Sattler's layer is made up of medium-sized blood vessels.

Haller's Layer

Haller's layer is the outermost layer of the choroid, consisting of larger-diameter blood vessels.

These layers of the choroid play a crucial role in supplying oxygen and nutrients to the outer layers of the retina, as well as regulating temperature and pressure within the eye.

Diseases and disorders of the choroid coat

The choroid is a highly vascular layer of the eye that lies between the retina and the sclera. It is crucial for maintaining ocular health as it provides oxygen and nourishment to the outer layers of the retina. The choroid is divided into four to five layers, depending on the source. The four layers are:

• Haller's layer (consisting of larger blood vessels)
• Sattler's layer (consisting of medium-sized blood vessels)
• Choriocapillaris (consisting of capillaries)
• Bruch's membrane (innermost layer of the choroid)

Some sources also include the suprachoroidal lamina or lamina fusca as the outermost layer.

Due to its extensive blood supply, the choroid is susceptible to various diseases and disorders. Here are some of the common diseases and disorders that can affect the choroid:

Hemorrhagic Choroidal Detachment

This is a rare condition caused by the rupture of choroidal vessels, resulting in a hemorrhage in the choroidal space. While it can occur spontaneously, it usually happens due to eye trauma or, rarely, during eye surgery. Treatment options include topical steroid eye drops, cycloplegic eye drops, and eye-pressure-lowering eye drops. In severe cases, surgery may be recommended.

Choroidal Rupture

Choroidal rupture is a complete break in the choroid, often caused by blunt eye trauma, such as a punch to the eye. Unfortunately, these ruptures often involve the centre of the retina (macula), leading to a loss of photoreceptors and central vision. If the rupture is not in the macula, central vision is retained.

Choroidal Nevi

Choroidal nevi are collections of pigmented or non-pigmented cells in the choroid. Most cases only require monitoring by an eye doctor, who will photograph the area and check for any changes. However, if the nevus has orange pigmentation, appears elevated, or has an unusual shape, aggressive treatment may be necessary as it could become a malignant choroidal melanoma.

Choroidal Dystrophies

Choroidal dystrophies are a group of inherited diseases that affect the choroid and can lead to severe vision loss. Examples include choroideremia, gyrate atrophy, central areolar choroidal dystrophy, diffuse choroidal atrophy, and pigmented paravenous retinochoroidal atrophy.

Choroidal Metastases

The choroid is the most common site for metastasis in the eye due to its extensive vascular supply. These metastases can originate from breast cancer, lung cancer, gastrointestinal cancer, or kidney cancer. Bilateral choroidal metastases are typically due to breast cancer, while unilateral metastasis is usually associated with lung cancer.

Chorioretinitis

Chorioretinitis is an inflammatory disease that often causes floating dark spots and blurry vision. It can be associated with uveitis and may be caused by an infection or an autoimmune-related inflammatory condition. Different treatments are recommended depending on the underlying cause.

Age-Related Macular Degeneration (AMD)

AMD is a common condition that affects the choroid, resulting in compromised choroidal circulation. It is characterised by the development of abnormal blood vessels in the choroid, which can leak fluid and blood, damaging the macula.

Central Serous Chorioretinopathy (CSC)

CSC is another disease that affects the choroid's circulation, leading to fluid accumulation under the retina. It is often associated with increased choroidal thickness and vascular engorgement.

Vogt-Koyanagi-Harada Disease

Vogt-Koyanagi-Harada disease is a rare autoimmune disorder that can affect the choroid, causing uveitis and other ocular complications.

Choroidal Tumours

The choroid can be susceptible to tumours due to its vascular nature. These tumours can be primary, such as choroidal melanomas, or metastatic, arising from cancers in other parts of the body.

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