Introduction
The vitreous is a transparent extracellular gel consisting of collagen, soluble proteins, hyaluronic acid and water. Its total volume is approximately 4.0 ml. The few cells normally present in the gel are located predominantly in the cortex and include hyalocytes, astrocytes and glial cells. The vitreous provides structural support to the globe while allowing a clear and optically uniform path to the retina. Once liquefied or surgically removed it does not re-form. Vitreous opacities can be caused by a variety of pathological processes primarily involving other ocular sites; apart from vitreous haemorrhage, the conditions discussed below are those in which the vitreous gel is the primary site of pathology.
Muscae volitantes
Muscae volitantes (Latin for ‘hovering flies’), commonly referred to as ‘floaters’, is an almost ubiquitous entoptic phenomenon of fly-, cobweb- or thread-like lesions best seen against a pale background. It is thought to predominantly represent tiny embryological remnants in the vitreous gel. A sudden exacerbation can occur due to vitreous haemorrhage or, more commonly, a change in the conformation of the gel, such as a posterior vitreous detachment (see Fig. 16.22 ).
Vitreous haemorrhage
Vitreous haemorrhage is a common condition with many causes ( Table 17.1 ). Symptoms vary according to severity. Mild haemorrhage ( Fig. 17.1A ) causes sudden onset floaters and diffuse blurring of vision, but may not affect visual acuity, whilst a dense bleed ( Fig. 17.1B ) may result in very severe visual loss. B-scan ultrasonography in unclotted vitreous haemorrhage generally shows a uniform appearance, and once cellular aggregates develop, small particulate echoes become visible ( Fig. 17.1C ); ultrasonography is critical in the evaluation of eyes with dense vitreous haemorrhage to exclude an underlying retinal tear or detachment ( Fig. 17.1D ). Treatment is dictated by severity and cause, but an increasingly low threshold is being adopted for early vitrectomy (see Ch. 16 ) in cases of dense haemorrhage.
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