Shu Yi Teh

Introduction

Asteroid hyalosis (AH) is a benign vitreous opacification in which calcium-lipid complexes are suspended throughout the collagen fibrils inside the vitreous cavity (1). These complexes are named asteroid bodies due to their reflective appearance that resemble the stars (asteroids) in the night sky.

Epidemiology

The occurrence of AH is found to be approximately 1 in 200 persons, with increasing prevalence as age increases. There is no established racial predisposition but AH appears to be more common in males than in females. AH is also not inherited, and familial cases are seldomly reported (2).

General Pathology

The mechanism by which asteroid bodies form remains unknown, but element mapping through electron spectroscopic imaging has shown an even distribution of calcium, phosphorus, and oxygen. Asteroid bodies exhibit similar structural and elemental characteristics with hydroxyapatite, and contribute in regulating the biomineralisation process with proteoglycans and their glycosaminoglycan side chains (3).

Histology

Asteroid bodies are spherical structures that show positive staining with alcian blue and with dyes for neutral fats, phospholipids, and calcium. They also display metachromatic staining and birefringence. In some cases, asteroid bodies are surrounded by foreign body giant cells, but the condition is not typically associated with vitreous inflammation (4).

Pathophysiology

Asteroid hyalosis has been associated with several systemic diseases but the pathophysiology of AH is still not fully understood. Major studies have concluded that AH is primarily associated with aging. There were also others that suggested correlations between AH with diabetes mellitus, hypertension, hypercholesterolemia or lipidaemia, or increased serum calcium levels. Although the link to diabetes mellites is most often cited, it also seemed to be the most controversial. This is because AH predominantly occurs unilaterally, but the association with systemic diseases like diabetes mellitus would have yield AH to be bilateral, as both eyes are equally exposed to hyperglycaemia. There were also suggestions that AH formation in diabetics may be secondary to retinal vascular changes. The relationship between DM and AH is further complicated with data indicating that AH may halt the process of vitreous collapse or contraction in proliferative diabetic retinopathy (5).

Evaluation

Asteroid hyalosis mostly presents asymptomatically and is commonly diagnosed during a routine eye examination (6).

Asteroid bodies are visualised through ophthalmoscopy to be small round refractile opacities associated with vitreous strands, sized between 3 to 100 microns (7). They are suspended in the vitreous cavity and do not settle with gravity. They move in accordance with the vitreous body and return to their initial position when the movement stops. Reflected light from the particles may confound the examiner’s view during ophthalmoscopy examination, but direct ophthalmoscopy may provide an improved view of the fundus in AH.

Fundus fluorescein angiography (FFA) is warranted for cases of AH in diabetic retinopathy eyes, especially in suspicion of neovascularisations (8). The advent of wide-angle FFA is also useful for other peripheral lesions such as malignancies to be detected (9).

Optical coherence tomography (OCT) allows for definitive diagnosis via identification of specific retinal anatomical abnormality. Diagnosis may be assisted by FA or fundoscopy, but appropriate disease management should be ultimately guided by OCT (10). This is because the 830-nm wavelength light beam used in OCT is less susceptible to distortion caused by asteroid bodies and other media opacities. Though dense asteroids may absorb or reflect the light beam, OCT technicians can minimise these image artifacts with proper positioning of patient, sufficient corneal lubrication, adjustment of dioptre focus, and with aid of polarisation function.

B scan may also demonstrate the presence of moderate to high amplitude point spikes in the vitreous cavity, separated by a clear zone from the retinal surface, to help in the diagnosis of AH (11).

In the context of a difficult fundoscopy, such as dense asteroid hyalosis obscuring the fundus causing limited view, care should be taken not to miss other significant fundus pathology. Aside from FFA to detect neovascularization in eyes with AH and diabetes mellitus (8), OCT could screen for vitreoretinal surface anomaly, and B scan could reveal any mass lesion that may be obscured (11). Optos ultra-wide field imaging can also be help in the diagnosing and managing retinal and choroidal lesions (9).

Management

Since asteroid hyalosis is mostly asymptomatic, observation is usually sufficient (6). There is only minimal scattering of light in AH due to the smooth surface of asteroid bodies. However, visual disturbance may occur when asteroid bodies are sufficiently close to the macula, or when the particles concentrate behind the lens due to complete posterior vitreous detachment. Although AH is generally a benign vitreous opacification that usually require no active intervention, these unusual cases may need vitrectomy.

Patients with AH requiring phacoemulsification may also pose certain issues of which the ophthalmologist must be aware (12). For example, confusing echoes from asteroid bodies may cause an inaccurately short axial length showed on scan. Therefore, it may be helpful to compare the axial length with that of the fellow eye. Reflections from asteroid bodies may also cause disturbances to the view of the posterior capsule. In addition, due to deposition risk of calcium and phosphorus content of asteroid bodies over the posterior lens surface, silicone intraocular lenses (IOLs) should be avoided. IOL exchange remains the preferred treatment of choice should such cases happen. Though successful removal of calcium deposits from the surface of silicon IOLs in the patients with AH via laser capsulotomy and pars plana vitrectomy have been reported as well (13-15).

References

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