ANATOMY OF CHOROID

In this article we discuss in detail about the anatomy of the Choroid. In our previous posts,we studied in detail about the anatomy of iris and Anatomy of the ciliary body. Taking it a step further we shall discuss the posterior most part of the uveal tract ; Choroid .If you are an audio visual learner you can visit my video on the same here. Also you may refer to the text below as a read along notes for the same.

Table of Contents

What is Choroid?

The Choroid is the posterior part of the vascular coat of the eyeball.
It is thicker posteriorly (0.22mm) than anteriorly (0.1mm).
It is smooth inner surface in contact with the retinal pigment epithelium of the retina.
It has a rough outer surface in contact with the sclera.

Layers Of Choroid

From outwards to inwards , choroid has 4 layers :-

Suprachoroidal lamina (LAMINA FUSCA)
Choroidal stroma ( Haller’s and Sattler layers)
Choriocapillaris layer
Bruch’s membrane

Let us discuss about each of these layers one by one.

Suprachoroidal Lamina / Lamina Fusca

Suprachoroidal lamina is the outermost layer of the choroid. It is a made by the condensation of the choroidal stroma.
It is continuous anteriorly with supraciliary lamina of the ciliary body
A potential space between the suprachoroidal lamina and the sclera is called the suprachoroidal space.

CLINICAL NUGGET:

IMPORTANCE OF SUPRACHOROIDAL SPACE

The suprachoroidal space is a potential space for drug delivery.
Targeted drug delivery can be achieved using this mode of drug delivery.
The surgical approach includes an ab-externo incision through the sclera (a.k.a. sclerotomy) .
This is followed by insertion of a catheter or canula in order to reach the area to be treated(targeted area).
Similarly the Suprachoroidal space can be accessed from within the eye( ab-interno access) for the placement of micro stent like iStent suprachoroidal bypass system and many more.

CLINICAL NUGGET:

Choroidal Effusion & Suprachoroidal Hemorrhage

Suprachoroidal space is essentially nonexistent.
There is a close apposition of the choroid to the sclera.
The space is kept empty and dry by a balance between the intraocular pressure (IOP) and the hydrostatic pressure of the vessels present in the suprachoroidal space (long and short posterior ciliary arteries)
Decrease in the intraocular pressure (HYPOTONY) is often considered to be the cause.
Imbalance in ocular fluid dynamics leads to fluid accumulation in this potential space. This is known as the choroidal Effusion/ uveal effusion
Sometimes the vessels can bleed into the suprachoroidal space .This is known as the Suprachoroidal Hemorrhage.
Suprachoroidal Hemorrhage can occur intraoperatively during cataract surgery and in the postoperative period following a glaucoma surgery. The causative factor in both being the hypotony.

Choroidal Stroma

The choroidal stroma consists of loose framework of collagen , elastin, reticular tissue with the pigment cells and other cells
Major part of the choroidal stroma is formed by the vessels.
OUTER HALLERS LAYER : Consists of larger diameter vessels
INNER SATLER LAYER : It consists of intermediate size vessels.
INNERMOST CHORIOCAPILLARY LAYER : Consists of the smallest size vessels in the choroid.
The innermost vessels are mainly the arterioles and the outermost vessels near the suprachoroidal lamina are the veins

CLINICAL NUGGET:

CHOROIDAL THINNING

With increasing age and longer axial length (pathological myopia); reduction in total choroidal thickness is seen.
Choroidal thinning involves thinning of Sattler’s layer and Haller’s layer rather than thinning of the choriocapillaris.

CHOROIDAL THICKENING (PACHYCHOROID)

A new spectrum of disorders involve focal or diffuse increase in the choroidal thickness.(Pachychoroid spectrum)
It consists of dilated choroidal vessels mainly in the Haller’s layer (PACHYVESSELS).
These vessels cause thinning of the underlying choriocapillaris and Sattler’s Layer.
For more on pachychoroid disease and OCT findings , visit this video.

Choriocapillaries Layer

This layer has rich capillary network
Arranged in a single layer
Consists of the smallest of the vessels in the choroid.
The choriocapillaries contain fenestrated endothelium, surrounded by a basement membrane and the pericytes.
These capillaries have an unusually wide calibre.
A single choriocapillary can permit multiple erythrocytes to pass through it simultaneously.
Choriocapillaris are divided into non overlapping lobules homologous to that of the liver
The density of choriocapillaries is highest in the macular area, where the choriocapillaris is thickest (reaching approximately 10 μm),
The density and thickness of the choriocapillaries gradually decreases towards the periphery.

Peculiar Features of Choroidal Blood flow

The choroid receives 65-70% of total ocular blood flow.
10% is received by the ciliary body and 1% by the iris .
But rate of oxygen extraction by the choroid is only 3% per ml of blood flowing through choroid.
However, the Retina receives only 5% or less , but rate of oxygen extraction is 38%. This is because of the high oxygen consumption of the photoreceptors present in the retina.

Choroid – A heat sink.

Due to the large volume of blood flowing through the choroid , the choroid acts a heat sink of the eye
This is useful when photocoagulation is done on retina as it can dissipate the heat well!

CLINICAL NUGGET: SLOWER BLOOD FLOW IN CHORIOCAPILLARIES

Erythrocytes travel approximately 5 times more slowly through the choriocapillaris compared to the retinal vasculature.Three factors responsible for this are :-

Large lumina of the choriocapillaris
Large blood volume in the choroid,
Lobular configuration of the choriocapillaris, through which erythrocytes traverse via numerous parallel route.

This optimizes the transport of oxygen from the choriocapillaris to the photoreceptors, which consume the highest amount of oxygen per gram of tissue in the entire body

Bruch's Membrane

Bruch’s membrane is the innermost layer of the choroid.
It is also known as the Basal Lamina/ Lamina vitrea
It is situated between the RPE and the choriocapillaris.
On electron microscopy, it consists of 5 layers:-

Basement membrane of the RPE
Inner collagenous layer
Elastic layer
Outer collagenous layer
Basement membrane of the choriocapillaris

The elastic layer of the bruchs membrane gives biomechanical properties to the bruchs membrane.
Also it has an antiangiogenic function.
The elastic layer of bruchs membrane is absent in macula.
This is one of the reason why macula is more predisposed to developing neovascularization.

CLINICAL NUGGET:

DRUSEN

Drusen are small, round, yellow white accumulations of extracellular material , that build up between the Bruchs membrane and the RPE; mostly in the post equatorial retina
Histologically, this material corresponds to proteins and lipid deposits.

Conclusion

So that was the detailed anatomy of the posterior part of the uveal tract. In the next post we shall be discussing about the blood supply of the eye in detail. If you are an audio visual learner you can check out our video on the choroidal anatomy here.