Physiology
The choroid contains many blood vessels. These have a total of two functions. The first important task is to feed the outer layer of the retina.
These are mainly the photoreceptors, which receive and transmit the light impulses. The retina also consists of several layers. The inner layers are supplied with blood by a specific blood vessel, namely the branches of the central retinal artery.
It has been observed that although the choroid has a very high blood flow due to the strong network formation by blood vessels, the oxygen uptake from the red blood cells is relatively low. This is an indication of the second important function of the choroid, namely temperature regulation. During the process of processing and transmitting the light stimuli incident on the sensory cells (photoreceptors), heat is generated which is dissipated through the blood vessels. In this way the temperature in the eye is adjusted and kept stable.
Diseases of the choroid
Since the choroid contains no pain fibres, pain only occurs when diseases of the choroid spread to neighbouring areas supplied with pain fibres or when there is an increase in pressure. However, there may be visual disturbances, the severity of which depends on the location of a disease at the back of the eye. Tumours often remain undetected for a long time.
An inflammation of the choroid skin (chorioditis) usually occurs as a result of an allergic reaction (immunological disease). However, it can also be triggered by foreign bodies that enter the eye from outside or by germs from other centres of inflammation in the face and skull. The reason for this is the good blood circulation of the choroid, which not only supplies it with nutrients, but can also carry pathogens and germs into the choroid in the case of an existing infection.
Possible pathogens can be bacteria, viruses or fungi. Immunocompromised persons are considered to be at risk because the body’s own defence system cannot kill the germs sufficiently. Since the choroid itself does not contain any nerve fibres, pain only manifests itself when adjacent structures such as the sclera or retina are affected.
Pressure pain occurs, usually as a result of increased intraocular pressure. In addition, affected persons suffer from visual disturbances, opacities and veil formation as well as a general decrease in visual performance as a result of inflammation of the neighbouring retina. In most cases, a conspicuously reddened eye is visible from the outside.
The ophthalmologist first carries out an eye test to determine whether there are already visual field failures. Then the eye is examined with a slit lamp to assess the anterior and inner parts of the eye. In order to be able to see the back of the eye, consisting of the retina and underlying eyes, the pupil must be dilated.
A tonoscopy is performed to determine a possibly increased intraocular pressure. In the case of chorioditis, quick action should be taken, as otherwise it can lead to permanent visual disturbances or, in the worst case, blindness. Immediate therapy consists of tablets containing cortisone to combat the focus of inflammation.
In addition, pressure-reducing medication is given to protect the adjacent structures, such as the optic nerve head, from the increased pressure. A choroidal inflammation can have different individual courses in the course of the disease and its severity. The exact therapy should therefore be determined by an ophthalmologist.
A coloboma (Greek for “the mutilated”) is a congenital or acquired cleft formation in the eye. In the congenital variant, the embryonic development of the eye results in an insufficient or faulty closure of the ocular goblet cleft during the 4th to 15th week of pregnancy. The causes of these embryological malformations are still the subject of current research.
Mutations in the so-called PAX genes, which assume many regulatory functions in embryonic development, are being discussed. Acquired choroidal colobomas are usually caused by external violent influences (e.g. blow to the eye, accident, etc.) or complications during eye surgery.
The choroidal hemangioma is a vascular tumour (haemangioma) localised in the choroid of the eye (choroidea). Due to the numerous branches into many small vessels and capillaries, the tumour is also highly branched and cavernous, as it follows the course of the vessels. People between the ages of 10 and 40 are particularly frequently affected.
The choroidal hemangioma is usually benign and shows no symptoms. Only when the surrounding tissue of the capillaries (exudative stage) is affected do visual disorders such as clouded or distorted vision occur. To make a diagnosis, an ultrasound or fluorescence angiography is performed to show the spread and size of the tumour.
Treatment is only necessary in the exudative stage if there is a threat to vision. Choroidal atrophy refers to tissue loss due to the death of choroidal cells. This is usually the result of a degenerated tissue such as a tumour.
Depending on the location, size and spread of the atrophy, this can have considerable consequences for the eye. In the early stages, there are visual disturbances and an increased susceptibility to infections, since, among other things, the blood-retina barrier can be disrupted and thus germs can enter the retina unhindered. In cases of severe choroidal atrophy, complete blindness can occur.
Choroidal folds are usually the result of a space requirement in the eye socket, such as a tumour, calcifications or a congested pupil. This causes increased external pressure on the eyeball. This pressure gives way and the individual layers of the eye, consisting of the retina, choroid and sclera, fold.
If only the choroid is affected, this does not result in visual disturbances. However, there is a risk that the wrinkles will pinch off small blood vessels, resulting in a reduced supply of oxygen and nutrients. However, if the retina is also affected, the retinal folds cause visual field defects, which can be compensated for by the healthy eye in the case of unilateral disease.
The choroidal melanoma (malignant uveal melanoma) is a malignant tumour that can develop from the pigmented cells of the choroid, the so-called melanocytes, when they start to divide uncontrollably. It is the most common tumour of the eye, in Europe it affects one in 100,000 people. The peak age for the disease is between the sixtieth and seventieth year of life.
Since the degenerated melanocytes are full of the pigment melanin, most choroidal melanomas have dark pigmentation. Like most malignant tumours, choroidal melanomas tend to spread (in about 50% of cases). The scattering takes place via the bloodstream mostly into the liver.
If scattering is already present, the disease usually leads to death within a few months/years. Since the choroid, unlike most other parts of the body, does not contain lymph vessels, which are of great importance for the immune system, the degenerated cells often remain undetected by the body and are therefore not fought by the immune system. The complaints of a sick person mainly include visual disorders and double vision.
Ophthalmologists often discover choroidal melanomas by chance. The treatment options range from radiation and laser therapy to radiosurgery and the removal of the affected eye. The choroidal melanoma must be distinguished from choroidal metastases.
These are rather flat, greyish-brown tumours, which are usually spread by breast cancer or lung cancer. Furthermore there is the benign choroidal nevus as differential diagnosis. In contrast to the choroidal melanoma a choroidal nevus is a benign tumour.
It is usually more strongly pigmented, sharply limited and does not grow progressively. Choroidal nevi appear dark due to an accumulation of melanin (comparable to a birthmark on the skin). It lies below the retina and does not cause any visual disturbances.
Approximately 11% of the population are carriers of such a nevus, making it the most common tumour of the inner eye. Mostly it is congenital. Because there are no symptoms, it is often noticed by chance during an examination of the back of the eye.
Rarely, in about 5 of 10000 cases, such a nevus can develop into a choroidal melanoma. Certain factors such as the size, location, pigmentation or fluid accumulation of the tumor indicate an increased risk of degeneration. Therefore, a choroidal nevus should be checked regularly to see if it shows a tendency to grow.
A check-up appointment should be made every half year. If the findings are unclear, a tissue sample (biopsy) can provide clarity. This is obtained with a small needle.
In addition to the ocular fundus examination, the following methods are available to examine a nevus: fluorescein angiography, indocyanine green angiography, fundus autofluorescence and optical coherence tomography. If the physician looks through the pupil (ophthalmoscopy) during the eye examination with special equipment, it is difficult to assess the choroid directly, since the retina limits the view of the choroid for anatomical reasons. The so-called ophthalmoscopic image is important for the diagnosis and course of diseases.
Ultrasound examinations can also be used to detect pathological changes in the choroid. Fluorescence angiography describes a special form of imaging the blood vessels. It is an imaging procedure in which the blood flow at the back of the eye through a pupil dilated by medication is observed and assessed by administering a suitable dye. If a tumour of the choroid is suspected, shading may occur in the area of the tumour by a cold light source placed on the eye.
