Aqueous humour

The GSH reductase inhibitor l,3-bis(2-chloroethyl)-l-nitrosourea (BCNU) also promotes corneal swelling in the isolated cornea. The addition of GSH prevents the action of BCNU as the cornea needs a constant supply of NADPH for maintaining adequate concentrations of reduced glutathione for the detoxification of hydrogen peroxide. It has been shown that hydrogen peroxide and BCNU primarily affect the permeability of the endothelial cells rather than the active processes transporting sodium and chloride ions across the membrane (Riley, 1985). 

Exposure to UVB radiation has a profound effect on the corneal endothelium. Following exposure to UV radiation, the cornea swells, the extent and duration of which is directly related to the magnitude and duration of the exposure (Riley etal., 1987). At very high in vivo exposures, these authors reported a decrease in ascorbate concentration and an increase in protein content, which they suggested resulted from a breakdown of the blood-aqueous barrier. They concluded that UV radiation may cause or promote changes in the endothelium associated with ageing. 

The cornea is avascular and is thus immunologically privileged to some extent. Inflammatory cells and antibodies can only enter the cornea from the corneoscleral limbus or from the aqueous. However, if inflammation occurs, the almost inevitable result is opacification of the cornea with visual deterioration, which is frequently permanent. 

The most potent inducers of corneal inflammation are infection and trauma (Plate 1). Infective agents include 

The intraocular pressure depends primarily on the rate of secretion of aqueous humour. The most notable con-stititutional difference between aqueous humour and blood plasma lies in their protein contents. Protein concentration in the aqueous is 5-15 mg/100 ml, and that of the plasma is 1000-fold greater (6-7 g/100 ml). However, all the plasma proteins are present in aqueous humour - albeit at very low concentrations, therefore it is 

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Florfenicol concentrations in the brain, cerebrospinal fluid (CSF), and aqueous humor were one-fourth to one-half the corresponding semm concentrations. Concentrations in these tissues and fluids did not decrease as rapidly, maintaining a low, but fairly constant value. Because the brain, CSF, and aqueous humour are separated from the blood by specialized barriers, florfenicol can seemingly only cross these barriers to a limited extent. 

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

Since the cornea derives its eneigy from the breakdown of glycogen and glucose, it requires oxygen, which it derives from the atmosphere, the tears and from the aqueous humour. A high concentration of lactate is built up, which is eventually removed by passage into the aqueous humour. 

Hydrogen peroxide is present in normal aqueous (approximately 30 /imol/1) whilst mean concentrations of around 70 tmol/l have been reported in aqueous from patients with cataracts, supporting a role for oxidative damage in the formation of cataracts (Spector and Garner, 1981). Diamine oxidase is one of the few enzymes to have been detected in bovine aqueous humour (albeit in trace quantities). It has been su ested that the hydrogen peroxide present in aqueous may be the product of the oxidative deamination of diamine substrates. This hypothesis is still unproven, since diamine oxidase substrates have not been identified in aqueous humour. 

GSHPx, CAT and SOD, which normally protect cells from free-radical damage have not been detected in aqueous humour. It has therefore been su ested that damage by free radicals and hydrogen peroxide to the anterior segment is prevented by a non-enzymatic extracellular oxidoreduction system involving a constant supply of reduced glutathione to the aqueous fluid from the ciliary epithelium, cornea and lens (Riley, 1983). 

Acetazolamide is a carbonic anhydrase inhibitor that reduces aqueous humour production and is therefore indicated in glaucoma to reduce the intraocular pressure. Salbutamol is a selective, short-acting beta2-agonist used as a bronchodilator in asthma. Tolbutamide is a short-acting sulphonylurea used in type 2 (non-insulin dependent) diabetes mellitus. Chlorpromazine is an aliphatic neuroleptic antipsychotic drug used in schizophrenia. Zafirlukast is a leukotriene-receptor antagonist that is indicated in the prophylaxis of asthma but should not be used to relieve acute severe asthma. 

Acetazolamide is a carbonic anhydrase inhibitor, used primarily in glaucoma to reduce aqueous humour production. Acetazolamide may cause blood disorders including agranulocytosis (deficiency of neutrophils) as a side-effect. 

Acetazolamide is a carbonic anhydrase inhibitor, which reduces intraocular pressure by reducing aqueous humour production. It is used in the treatment of glaucoma. Acetazolamide is administered systemically. Recently newer carbonic anhydrase inhibitors have been developed, which are available as topical agents (for example, dorzolamide). 

Acetazolamide is a carbonic anhydrase inhibitor that is administered orally for the treatment of glaucoma. Topical carbonic anhydrase inhibitors include dorzolamide and brinzolamide. Carbonic anhydrase inhibitors reduce the production of aqueous humour, thereby reducing intraocular pressure. They can be used alone or in addition to beta-blocker therapy in glaucoma patients. 

Brubaker, R. 1984. The physiology of the aqueous humour formation. In Glaucoma Applied Pharmacology in Medical Treatment, eds. S.M. Drance and A.H. Neufield, 35. Orlando, FL Grune Stratton. 

Latanoprost (Figure 3.19) is a recently introduced prostaglandin analogue which increases the outflow of aqueous humour from the eye. It is thus used to reduce intraocular pressure in the treatment of the eye disease glaucoma. 

That which proves that the dark Abyss, the Chaos, or the World s First Matter, was an aqueous and humid mass, is that, besides the reasons which we have brought forward, we have a palpable instance under our eyes. The property of water is to run, to flow, so long as heat animates and holds it in its fluid state. The continuity of bodies, the adhesion of their parts, is due to the aqueous humour. It is the ciment which unites and binds the elementary parts of bodies. So long as it is not separated from them entirely, they preserve the solidity of their mass. But if fire warms these bodies beyond the degree necessary for their preservation in their state of actual being, it drives away, rarefies this humour, makes it evaporate, and the body is reduced to powder, because the bond which united its parts no longer exists. 

The igneous spirit, the vivifying principle gives life and vigour to the Mixts, but this Fire would soon consume them, if its activity was not tempered by the aqueous humour which binds them together. This moisture circulates continually in all things. It makes a revolution in the Universe, by means of which some of the Mixts are formed, or nourished, or even increased in volume, while its evaporation and absence causes others to perish. 

Bours J. The protein distribution of bovine, human and rabbit aqueous humour and the difference in composition before and after disruption of the blood/aqueous humour barrier. Lens Eye Toxic Res 1990 7(3—4) 491—503. 

Primary open-angle glaucoma is a very common form (also known as chronic simple or wide-angle glaucoma). It results from obstruction in the trabecular meshwork which acts as the drainage system for the aqueous humour. 

Primary angle closure glaucoma (also known as acute close-angle or narrow-angle glaucoma) results from blockage of aqueous humour flow into the anterior chamber. The condition develops very quickly with a sudden increase in eye pressure. The eyes become very painful and red. 

It may be necessary to combine these or add another agent such as oral acetazolamide and/or dorzolamide eye drops, which are both carbonic anhydrase inhibitors that reduce aqueous humour production. 

Q6 Pilocarpine eyedrops are suitable. In severe conditions, in addition to the eyedrops, intravenous acetazolamide and intravenous hypertonic mannitol (an osmotic agent) may be used to reduce pressure. Acetazolamide prevents the actions of carbonic anhydrase in the ciliary body and inhibits bicarbonate synthesis. This causes reduction in sodium transport and aqueous humour formation since there is a link between bicarbonate and sodium transport. 

The ciliary epithelium in the posterior chamber secretes aqueous humour. The aqueous humour flows in between the cornea and iris. After filtration through the trabecular meshwork, it returns to the venous circulation via the canal of Schlemm. 

Patients with closed-angle glaucoma have a small angle between the iris and cornea which can also completely close and therefore flow of aqueous humour will be prevented. This results in a rise in IOP, which must be reduced quickly to prevent any damage to the retina. 

Q8 Normal IOP is 15 mmHg (range is 10-20 mmHg). It is maintained by a balance between the secretion of aqueous humour by the ciliary body and its flow into the canal of Schlemm via the trabecular meshwork. Glaucoma is present when IOP rises to >21 mmHg. 

Sympathomimetic agents, such as brimonidine tartrate, apraclonidine, adrenaline and dipivefrine hydrochloride, which is a prodrug for adrenaline), act on a-adrenoreceptors to induce dilation of the veins to reduce IOP. They also induce mydriasis (dilation of the pupils). Adrenaline may reduce the rate of formation of the aqueous humour, which in turn reduces the IOP it may also increase the outflow through the trabecular meshwork. Stimulation of alpha-2-adrenoreceptor (c -adrenoreceptor) by drugs such as brimonidine and apraclonidine on the adrenergic neurons supplying the ciliary body can also result in reduction of secretion of aqueous humour. 

Carbonic anhydrase inhibitors, such as acetazolamide and dichlor-phenamide, act as diuretics to increase excretion of water by inhibiting carbonic anhydrase activity. This in turn leads to a reduction in the level of bicarbonate in aqueous humour. 

Qll Yes. Laser surgery can be used to form a hole in the iris to permit increased flow of aqueous humour. 

Acetazolamide, dichlorphenamide, methazolamide and ethoxzolamide facilitate the excretion of hydrogen ions and the recovery of bicarbonate, They are never used as diuretics but are employed to reduce the production of aqueous humour in they eye (in glaucoma). 

IPRATROPIUM BRONCHODILATORS -SALBUTAMOL A few reports of acute angle closure glaucoma when nebulized ipratropium and salbutamol were coadministered Ipratropium dilates the pupil, which i drainage of aqueous humour, while salbutamol t production of aqueous humour Warn patients to prevent the solution to mist or enter the eye. Extreme caution in co-administering these bronchodilators by the nebulized route in patients with a history of acute closed-angle glaucoma... 

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