The vitreous humour is a large chamber (80% of the volume of the eye) located between the lens and retina of the human eye; it consists of water, collagen, proteins and sugar whilst possessing a gelatinous consistency. (Vision Eye Institute, 2017). Its uniqueness stems from its plasticity, whilst being completely transparent. The three main components of the vitreous humour are water, collagen and hyaluronic acid. Water takes up 99% of the vitreous humour, and this water content is replenished every 15minutes, and is highly influenced by the systemic circulation (Cristiansson et al.
,1954). Collagen in the vitreous humour is insoluble; the type II is the largest component in the vitreous humour (Ayad et al., 1984). While, hyaluronic acid is a molecule with high viscosity. It’s able to absorb water very well and is negatively charged (Larra±eta et al.,2018). These mucopolysaccharides are produced by the hyalocytes. Some forms of albumins are also found in the vitreous humour, as well as some sugars, ascorbic acids and amino acids.
In addition, the vitreous humour contains a small number of electrolytes that play a vital role of electrical and osmotic homeostasis. The only metabolic part of the vitreous humour are the hyalocytes, which form the hyaluronic acid (Forrester,2016). The vitreous humour is limited anteriorly by the lens and ciliary body, while posteriorly it is enveloped by the retina. It is the largest connective tissue in the body and occupies approximately two-thirds of the globe volume with a weight of four grams, giving a volume of four millilitres (Rakesh, 2016).The vitreous body is divided into three parts: the hyaloid layer; the cortical vitreous and the medullary vitreous. The differences are in location and histology (Dahal, 2018). The hyaloid vitreous is a condensation of the vitreous when it encounters the neighbouring structures; it is a pseudo-membrane. The collagen fibres in this connective tissue are located parallel to the boundaries in this layer. The hyaloid vitreous is divided into the anterior and posterior parts by the vitreous base. Anterior hyaloid membrane is located towards the anterior part of the globe, and starts 11.5mm anterior to the ora serrata (Khurana, 2007). The lens is located over this membrane and the surrounding area of contact forms a condensed ligament. The space is called Berger’s space and the ligament is known as the hyaloid capsular ligament of wieger. The Berger’s space is continuous with the Cloquet’s canal (Kirby, 1931). Meanwhile, the posterior hyaloid membrane extends from the vitreous base to the optic disc and is in close contact with the internal limiting membrane of the retina (Sebag, 2013). Cortical vitreous is a peripheral zone of main mass with a width of approximately 100 јm. This zone contains condensed fibrillary vitreous, with the main components being type two collagen with hyaluronate; whilst also possessing the properties: high viscosity; tensile strength and elasticity. Also, the cortical vitreous is the metabolic centre containing hyalocytes (the vitreous cells), which synthesis mucopolysaccharides and have phagocytotic action. (American Academy of Ophthalmology, 2018).Lastly, the medullary vitreous is a major central part, and has similarities to the cortical vitreous, but it is less fibrillary; instead it contains more water and is cell free. The medullary vitreous also contains the Cloquet’s canal, which is serves as a perivascular sheath surrounding the hyaloid artery in the eye. It runs from Berger’s space to the optic disc and is 1-2mm wide. Vitreous condensation in the walls is prominent (Kagemann et al., 2006).The vitreous is strongly attached to the peripheral structures at four locations, which include: the ora serrata (vitreous base); optic disc; post capsule of lens and the foveal region. The vitreous base has a width of 4mm, and overlines the ora serrata (Dahal, 2018). The physiochemical properties of the vitreous humor include: the refractive index being 1.3349 with 90% light transmission (Schepens et al.,1987) ; it has high plasticity, meaning that it is able to mould against any shape (Dahal, 2018); it is a viscoelastic substance due to the presence of hyaluronic acid; it is also stable due to the interaction between the hyaluronic acid and collagen (Balazs, 1965). The vitreous humour also undergoes expansion and contraction based on the sodium concentration, which regulates the water content by osmosis. Other solutes are unable to exert this effect because of the selective permeability into the vitreous humour (Duke-Elder, 1929). As well as this, the temperature of the vitreous humour is 34µ’C. The blood vitreous barrier is a result of the tight junctions of the internal limiting membrane; the basal lamina of the internal limiting membrane; and the tightly packed cortical vitreous. This barrier physically inhibits drugs from travelling across it. The vitreous humour undergoes changes as humans age; the cross links in the collagen break and the hyaluronic acid increases, making the vitreous humour absorb more water. As a result, the stability of the gel weakens (Bishop, 2004). The vitreous humour becomes more liquid and the collagen fibres begin to clump together, which are sometimes presented as floaters’. The unstable gel gradually starts to separate from its posterior attachments. This degenerative process is called Posterior Vitreous Detachment. The liquified vitreous may also lead to retinal detachment. There are other forms of vitreous problems, such as the developmental remnants of hyaloid system in Cloquet’s canal. Other degenerative changes include: Asteroid hyalosis; Synchysis scintillans and Amyloid degeneration. Asteroid hyalosis is common in the elderly, and diabetics. Calcium containing crystals are seen floating in the vitreous. Intervention is only required in cases of visual impairment. Meanwhile, Synchysis scintillans are cholesterol crystals in the vitreous (occasionally they are seen in the anterior chambers of the vitreous). Moreover, Amyloid degeneration is a systemic inherited disease, which leads to deposition of amyloid in collagen containing tissues, and Persistent hyperplastic vitreous is a developmental error (Griff, 2017). The vitreous humour also causes problems whilst it is detached. It may form a band causing vitreomacular traction, which may cause disruption of the retinal anatomy (Bakri et al., 2016), thus compromising vision. These bands are seen in the epiretinal membrane, formed by migrating glial cells; post-inflammatory conditions; post haemorrhage or diabetes (Vision Eye Institute, 2017). What happens if the vitreous humour gets damaged?Posterior vitreous detachment (PVD) occurs when the posterior vitreous cortex separates from the internal limiting membrane of the retina (Remington, 2012). The deterioration of the vitreous humour tends to occur with advancing age- those that are above the age of 50 years old are most commonly affected. Eye trauma; myopia (near-sightedness) and cataract surgery are other factors that may cause vitreous detachment (University of Michigan Health System). During PVD, the vitreous weakens, becomes more fluid and shrinks causing the vitreous to separate from the retina and thus begin to float freely in the eye. These strands cast small shadows in the eye, which are known as floaters that can be accompanied by flashes of light in the peripheral vision. Floaters are barely distinguishable for the majority of the time, however they become more visible the closer they are to the retina. Abnormally numerous or large floaters that interfere with vision usually indicate a more serious condition, requiring immediate medical treatment (National Eye Institute, 2009). Moreover, asteroid hyalosis is a degenerative condition of the vitreous humour that is characterised by the appearance of white particles in the eye. This occurs when deposits of calcium and phosphate adhere to the vitreous. Asteroid hyalosis is not considered serious unless it causes disturbances to one’s eyesight which is rare. While, amyloidosis of the vitreous arises due to systemic neuropathy, and it is portrayed as sheet-like’ deposits of grey and off-white material in the vitreous, while its symptoms involve floaters and blurry vision (Griff, 2017).As well as the diseases of the vitreous humour that can lead to blindness, there are many diseases associated with the human eye that can cause an individual’s perception of an image to alter, and in the worst scenario can lead to blindness. Table 1.1 to show the different eye conditions/diseases and their treatment.As explored earlier, it has been illustrated that the vitreous humour occupies a large portion of the eye and so damage to the vitreous humour can cause eye sight problems, such as the loss of vision or blurred vision. In cases where the damage to the vitreous is significant, it is necessary for doctors to remove the vitreous humour from the patients’ eye and replace the fluid with a substitute in order to solve or improve the problem. The removal of the vitreous is called a vitrectomy and is a surgical procedure conducted to improve the condition of the eye. However, there are instances where a vitrectomy in conducted to obtain better access to the retina of the eye. In doing so, a variety of repairs can occur, for example there will be more access to the macula which is located at the back of the retina and provides central vision, and so with the removal of the vitreous, the macular hole would be easier to fix (Ellis, 2017). The vitreous humour cannot regenerate, and so the cavity left when the vitreous is taken out must be filled with a substitute during and after the vitrectomy (Donati, 2014).Commercial substitutes for the vitreous humourAir is a substitute that doctors use to replace the vitreous humour in vitrectomy; it was the first gas to be injected into the eye and was first used by Ohm in 1911 to repair retinal detachment (Alovisi,2017). It is colourless and inert and has some advantages for its use in vitrectomy as it is readily available and inexpensive. On the other hand, it only remains in the eye for a short period of time, a few days, before it is replaced by the aqueous humour; thus, reducing its tamponade effect (Alovisi,2017) . Tamponade effect refers to the blockage of a cavity. Air is also easily absorbed by red blood cells and diffuses into the blood circulation quickly (Alovisi,2017), which reduces its reliability as a substitute for the vitreous humour. It also has a low refractive index (used as a measure of how light travels through a material), meaning that air causes complete reflection of light and decreases optical function, and it is also an inconvenience as patients are instructed to remain face down after the surgery. Air is mostly used as an emergency option; other gases include sulfur hexafluoride (SF6) and perfluoro propane (C3F8) which are other options that are becoming more common to use for vitrectomy (Alovisi,2017).As well as this, perfluorocarbon liquid (PFCL) is a substitute used in vitrectomy because it demonstrates physical properties that surgeons deem as useful in the procedure. For example, it is transparent, has high specific gravity and is immiscible with water (Yu et al., 2014). While, silicone oil is another liquid used as a substitute in vitrectomy; it is a hydrophobic polymer and has specific properties that make it useful as a substitute. Such as, its similar refractive index to the vitreous humour; its stability; lubricating properties; transparency; low toxicity and ease of removal.