Affects: Dogs
Synchysis scintillans, also known as cholesterolosis bulbi, is a rare degenerative condition of the eye characterized by the accumulation of cholesterol crystals within the vitreous cavity. These crystals are characterized by freely mobile, highly refractile, multicolored particles that move with eye movements and settle inferiorly under gravity. This gives it a characteristic "snow-globe" effect on exam.
This condition is commonly associated with advanced ocular disease, but it has also been reported as an incidental finding in normal aging eyes. While historically described in younger population and affecting eyes bilaterally, it has also been reported to suggest it can occur across a wide age range and commonly affecting the eye unilaterally.
Synchysis scintillans is often asymptomatic and identified incidentally during examination. Diagnosis is clinical, based on distinct appearance, but histological analysis can confirm birefringent cholesterol crystals.
Pathophysiology: The pathophysiology of synchysis scintillans is not completely understood, but several mechanisms have been proposed.
One widely accepted proposed mechanism involves the breakdown of intraocular red blood cell products. The lipid rich-cell membrane releases intraocular cholesterol during degradation, creating a lipid-rich environment that favors cholesterol crystallization. This mechanism is thought to occur in chronic vitreous hemorrhages or hyphemas. Repeated hemorrhages, impaired vitreous blood absroption, or retinal neovasculrization may further promote crystal formation.
Cholesterol crystals can also arise independently of hemorrhage in the setting of degenerative ocular conditions. Chronic retinal detachment is an implicated source of crystals. The lipid-rich subretinal fluid may pass through retinal defects or degenerative retinal tissue into the vitreous cavity, where cholesterol then crystallizes.
Changes in the ocular barrier systems and intraocular fluid composition may contribute to crystal formation. Trauma, inflammation, or degenerative disease can disrupt the blood-aqueous of blood-retinal barriers, allowing cholesterol, serum lipids, and red blood cells to enter intraocular structures. This facilitates a lipid rich environment and subsequent crystal fromation. Degenerative vitreous changes may impair the ability to maintain normal levels of cholesterol within the intraocular vitreous solution, allowing precipitation of crystals. Fluid composition can also be compromised in the setting of a blood absorption insufficiency.
Diagnosis: Synchysis scintillans is usually a clinical diagnosis and based on its characteristic ophthalmic findings. Slit-lamp or fundoscopic examination is used for visualization. On examination, the affected eye will reveal refractive multicolored crystals, larger than other intraocular particulate matter. The crystals freely move in suspension within the vitreous cavity and settle inferiorly under gravity at rest, creating a snow-globe effect.
A definitive diagnosis can be made through histology, which will demonstrate birefringent cholesterol crystals under polarized light and positive staining with lipid-specific stains.
The main differential diagnosis is asteroid hyalosis, another degenerative vitreous condition. Where as synchysis scintillans crystals are composed of cholesterol, asteroid hyalosis is composed of calcium-lipid deposits. Asteroid hyalosis is typically seen in older adults and usually not associated with significant underlying ocular disease. Asteroid hyalosis ophthalmic findings reveal particulate matter suspended within the vitreous gel, moving with it and returning to their original position. Cholesterol crystals in synchysis scintillans, however, move freely and settle with gravity.