Asteroid hyalosis is a degenerative eye disease characterized by the appearance of asteroid bodies in the vitreous humor of the eye. Visual impairment due to asteroid hyalosis is usually minimal.
The disease is related to the following processes: degenerative and has an incidence of about 9 / 100.000.
Asteroid hyalosis (AH) is a degenerative disease affecting the vitreous humor of the eye.
The vitreous body of the eye consists of the vitreous stroma, a delicate net of fibers that confers a certain stability to the vitreous body, and the vitreous humor, a physiologically transparent filling substance. The main function of the vitreous body is to maintain the form of the eye. The vitreous humor itself is composed of water and glycosaminoglycans like hyaluronic acid .
Degenerative alterations of the vitreous humor are partially triggered by reactive oxidative species and result in glycosaminoglycan breakdown and liquefaction of the vitreous body  . It is not yet known if similar processes trigger the appearance of asteroid bodies in the vitreous humor, i.e. if they provoke AH.
AH does usually affect one eye, not both. In patients suffering from AH, small, light opacities can be observed in ophthalmologic examinations. Because they reflect the incoming light, they appear like bright stars in front of the dark ocular fundus, the "sky". This phenomenon accounts for the name of the disease. AH does not generally interfere with vision and thus, treatment is required only in severe cases. If necessary, a vitrectomy is the method of choice to remove asteroid bodies.
The etiology of AH is not completely understood. Evidence has been provided to support the hypothesis that diabetes mellitus, hypertension, hypercholesterolemia, and atherosclerosis predispose for this condition. However, not many studies could confirm these findings.
The causes of AH are not yet understood. The disease may also be detected in other mammals, particularly in dogs. In this species, AH has been related to neoplasms of the ciliary body. This link could, however, not yet be confirmed in men.
While some scientists defend the hypothesis that pathologic systemic conditions may trigger AH, this theory is not uniformly accepted. In this context, the risk for AH may be increased in patients suffering from diabetes mellitus, hypertension, hypercholesterolemia or atherosclerosis.
An inverse relation to posterior vitreous detachment has been found in a large, retrospective study.
To a certain degree, old age seems to be another factor that predisposes for AH.
AH is a rather common disease of the eye and prevalence has been estimated to range between 1 and 2% in Caucasians. It has been proposed that AH prevalence increases with age and indeed, the vast majority of AH patients are older than 60 years. Due to the fact that AH is a degenerative eye disease, this hypothesis is rather unsurprising. Researchers who examined nearly 5000 individuals in the United States reported a more than 10-fold increase in prevalence between patients who had an average age of 50 years and those who were 80 years old. Here, observed prevalence was 0.2% and 2.9%, respectively . An Australian study reported an even lower prevalence in people aged 50 years . These values could be confirmed in a third, retrospective study conducted in the United States. More than 10,000 autopsy reports were analyzed and an overall AH prevalence of 2% was determined . Of note, this large sample was found to be representative of the population of the United States.
With regards to gender distribution, there does not seem to be any consensus. Different studies reported men to be affected more frequently than women or vice versa. The occurrence of the disease is mostly unilateral.
The asteroid bodies consist of hydroxyapatite, a calcium phosphate mineral. Because little is known about the causes of hydroxyapatite precipitates in the vitreous humor, conclusive information regarding the pathogenesis of AH is not available.
There is a considerable number of studies associating AH with diabetes mellitus          , hypertension   and hypercholesterolemia   . Also, increased serum calcium levels have been proposed as a possible trigger of AH  .
The main argument against systemic causes of AH is the fact that these should trigger bilateral AH, but approximately 90% of all patients present with unilateral AH. In order to explain this apparent contradiction, unilateral and bilateral percentages have been determined for the diabetic subpopulation of AH patients. However, only non-significant trends towards a higher share of bilateral cases could be detected . Indeed, one of the above mentioned, large studies regarding AH found less than 3% of diabetic AH patients to present bilateral AH .
Prognosis of this condition is good. AH is generally not associated with visual impairment. If such is the case in severe AH, vitrectomy may serve as a remedy.
AH is more commonly diagnosed in patients aged 60 years and older. Both men and women may be affected. Although most patients show unilateral AH, bilateral findings do not rule out this degenerative condition. Patients usually do not present with visual impairments and only in severe cases of AH visual acuity may slightly be affected .
Patients may report cognitive phenomena that correspond to the Pulfrich effect. Asteroid bodies may reflect light and may alter stimulation of retinal photoreceptors and signal transduction to the cortex in terms of time. Fractions of seconds are sufficient to create the impression that one eye is seeing an object shortly before or after the other one does.
Ophthalmoscopic examination reveals the presence of several- round to oval, opaque but highly refractile particles in the vitreous humor . The number of these so-called asteroid bodies augments over the course of the disease. Initially, asteroid bodies concentrate in the anterior portions of the vitreous humor. They are generally of white-yellowish or yellowish color. If eye movement provokes movement of the vitreous body, asteroid bodies seem to change their position. Because they are suspended in the vitreous humor rather than floating freely, they return to their initial position if the eye is moved accordingly.
While asteroid bodies may not interfere with the patient's vision, they often impede a thorough examination of the ocular fundus that may be required for other reasons. Fluorescein angiography may be helpful in such cases.
Asteroid bodies may alter readouts of autorefraction and A-scan ultrasonography and may also be noted in optical coherence tomography imaging.
AH is rarely associated with visual impairment but should be distinguished from several differential diagnoses, e.g. floaters, amyloidosis and synchysis scintillans. Floaters are of fibrillar or cellular nature. Amyloidosis is a rare condition characterized by the appearance of amyloid fibrils in the vitreous humor but also in other ocular structures and adnexa. Contrary to AH patients, individuals suffering from amyloidosis will show additional symptoms. As for synchysis scintillans, precipitates consist of cholesterol and are not attached to the vitreous stroma. They move much more freely than asteroid bodies in AH patients and sink to the bottom if eye movement is ceased  .
Ophthalmologists should also know that asteroid bodies may interfere with certain diagnostic and therapeutic measures that may be required for other reasons.
With regards to the interference with diagnostic and therapeutic measures, the presence of asteroid bodies should be taken into account when evaluating the results of autorefraction measurements, A-scan ultrasonography or optical coherence tomography images. Refraction, intraocular lens power, and axis length may not be measured correctly. Moreover, silicone lenses should not be implanted in eyes affected by AH because they might calcify. Another material needs to be chosen.
Due to the fact that AH has been associated with diabetes mellitus, hypertension, hypercholesterolemia and atherosclerosis, patients diagnosed with AH should be examined for these systemic conditions.
AH does not usually require treatment, unless in the advanced state of the disease. The mineral deposits inside the vitreous humor most often do not impair vision but interfere with ophthalmologic examinations of the ocular fundus. With regards to this issue, alternative diagnostic measures should be considered to carry out the intended exam.
In rare cases, AH may decrease visual acuity. Such patients as may benefit from a vitrectomy . This same procedure should also be undertaken if the above-mentioned alternative diagnostics do not suffice to depict the ocular fundus and a thorough examination is still necessary, e.g. in patients suffering from retinal tears or detachment or diabetic retinopathy. Of note, vitreous humor does not regenerate after vitrectomy. It will rather be replaced by an aqueous substance.
No preventive measures can be recommended. If theories regarding an association between systemic diseases and AH can be verified, prevention of such conditions may also help to prevent AH.
Asteroid hyalosis (AH) is a degenerative disease of the eye. Hallmark of this condition is multiple, small, round to oval bodies, which are called asteroid bodies, inside the vitreous humor. In order to understand the function of the latter, one has to know that the vitreous humor is the main component of the vitreous body. The vitreous body, in turn, occupies a very large part of the orbit and maintains the form of the eye. While cornea, iris, pupil and lens are located in front of the vitreous body, the retina is behind it. That means incoming light has to pass through the vitreous body and therefore through the vitreous humor to reach retinal photoreceptors. That's why the vitreous humor is transparent.
Somewhat surprisingly, though, AH is most certainly not associated with visual impairment. Only in rare, severe cases of AH may visual acuity be slightly reduced.
Even though AH is a very common condition, little is known about the causes of AH. The above-mentioned asteroid bodies consist of calcium phosphate minerals and it has been speculated that certain systemic diseases, e.g. diabetes mellitus, hypertension, hypercholesterolemia, and atherosclerosis predispose for AH. Diabetes mellitus, for instance, may evoke diabetic retinopathy and thereby trigger AH. However, such hypotheses have not yet been confirmed. The fact that AH frequently affects one eye only argues against a systemic cause.
Age seems to be an important factor in AH development. The majority of AH patients are aged 60 years and older.
Usually, asteroid bodies do not cause any symptoms.
The image of the eye of an AH patient resembles a sky full of stars. The aforementioned asteroid bodies are of yellowish color and reflect the light shown into the eye during an ophthalmologic examination. Thus, light spots appear in front of the dark ocular fundus. This phenomenon gave the disease its name.
Because AH patients are unable to see the asteroid bodies in their own eyes and because they usually don't cause any visual problems, AH is frequently an incidental finding. Asteroid bodies may impede the ophthalmologist to evaluate the patient's retina and certain measurements may be altered by the presence of these bodies.
A thorough examination of the eye will allow for the ophthalmologist to distinguish AH from differential diagnoses such as floaters, amyloidosis and synchysis scintillans.
Because AH does usually not cause any symptoms, no treatment is required. In those rare cases where AH does cause visual impairment, a surgical procedure called vitrectomy may be conducted. Here, the vitreous humor and its multiple calcium phosphate deposits will be removed. Although the vitreous humor itself does not regenerate, the interior of the eye will be filled up with a similar substance that doesn't interfere with vision.