Aqueous humor

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. 

Vision is vital for human activities, and eyes are very sensitive to a number of toxic insults induced by chemical compounds. The most serious outcome is permanent eye damage which may be so severe as to cause loss of vision. The eye consists of the cornea and conjunctiva, the choroid, the iris, and the ciliary body. It also contains the retina, which is of neural origin, and the optic nerve. The retina contains photoreceptors, a highly specific light-sensitive type of neural tissue. The eye also contains the lens and a small cerebrospinal fluid system, the aqueous humor system, that is important for the maintenance of the steady state of hydration of the lens and thus the transparency of the eye. 

The eye has its own hydraulic system, and disturbances in it may cau.se serious damage to the eye. The normal eye pressure is 22 mm Hg, but when the pressure increases to 28-30 mm Hg, the optic nerve is squeezed and becomes hypoxic. This increase in the eye pressure may be due to acids or alkali causing inflammation in the anterior chamber of the eye, blocking the outflow of aqueous humor back into the systemic circulation. 

Augen-hornhaut,/. cornea, -kammerwasser, n. aqueous humor, -licht, n. eyesight, -lid, n. e3 elid. -linse, /. crystalline lens eye lens. e3 epiece. -marmor, m. eye-spotted marble, -mass, n. measure by eye. -merk, n. object in view, aim. -nerv, m. optic nerve, -nichts,... 

FIGURE 24-1. In the normal eye, aqueous humor flows through the ciliary body into the posterior chamber, through the pupil into the anterior chamber, and out through the trabecular meshwork to the canal of Schlemm into the venous drculation. 

Carbonic anhydrase is an enzyme that produces free hydrogen ions, which are then exchanged for sodium ions in the kidney tubules. Carbonic anhydrase inhibitors inhibit the action of the enzyme carbonic anhydrase This effect results in the excretion of sodium, potassium, bicarbonate, and water. Carbonic anhydrase inhibitors also decrease the production of aqueous humor in the eye, which in turn decreases intraocular pressure (IOP) (ie, the pressure within the eye). 

Glaucoma is an increase in the IOP that, if left untreated, can result in blindness. Normally the eye is filled with aqueous humor in an amount that is carefully regulated to maintain the shape of the eyeball. In glaucoma, aqueous humor is increased, which causes the IOP to rise and can, without treatment, damage the retina... 

Brimonidine tartrate is an alph -adrenergic receptor agonist used to lower IOP in patients with open-angle glaucoma or ocular hypertension. This drug acts to reduce aqueous humor production and increase the outflow of aqueous humor. 

The (3-adrenergic blocking drugs decrease the rate of production of aqueous humor and thereby lower the IOP. These drugs are used to treat glaucoma... 

Except for dorzolamide and brinzolamide, carbonic anhy-drase inhibitors are administered systemically. Carbonic anhydrase is an enzyme found in many tissues of the body, including the eye Inhibition of carbonic anhydrase in Hie eye decreases aqueous humor secretion, resulting in a decrease of IOE These drugs are used in the treatment of elevated IOP seen in open-angle glaucoma... 

These drugs dilate the pupil (mydriasis), constrict superficial blood vessels of the sclera, and decrease the formation of aqueous humor. Depending on the specific drug and strength, these dru may be used before eye surgery in the treatment of glaucoma, for relief of minor eye irritation, and to dilate the pupil for examination of the eye... 

An alternative approach was reported (49) in which tobramycin was directly incorporated into a porcine scleral shield with a 72-hr dissolution time by immersion in a solution of the drug. After implantation in rabbit eyes for up to 8 hr, the shields were removed and the concentration of antibiotic in the corneas and aqueous humor... 

Current therapy is directed at altering the flow of aqueous humor, which is the major determinant of intraocular pressure. 

After the transport of aqueous humor into the posterior chamber, it flows through the pupil into the anterior chamber where it provides oxygen and nutrition to the avascular lens and cornea. Aqueous humor then exits the anterior chamber through the trabecular meshwork and drains into Schlemm s canal which drains aqueous humor into the episcleral venous system. 

Eighty percent of aqueous humor drains through the trabecular meshwork, which is a lattice of connective tissue that surrounds the anterior chamber. The size of the trabecular meshwork can be altered by the contraction or the relaxation of the ciliary muscle. Stimulation of muscarinic receptors on the ciliary muscle causes contraction, which in turn causes the pores of the trabecular meshwork to open, increasing aqueous humor outflow. 

A second pathway, uveoscleral outflow, comprises the other 20% of aqueous humor drainage. In the uveoscleral pathway, aqueous humor exits the anterior chamber through the iris root and through spaces in the ciliary muscles, which then drains into suprachoroidal space. Uveoscleral outflow can be pharmacologically modulated by adrenoceptors, prostanoid receptors, and prostamide receptors.4,10,12-15... 

FIGURE 58-2. Anterior chamber of the eye and aqueous humor flow. (From Lesar TS, Fiscella RG, Edward D. Glaucoma. In DiPiro JT, Talbert RL, Yee GC, et a I, (eds.) Pharmacotherapy A Pathophysiologic Approach. 6th ed. New York McGraw-Hill 2005 1715, with permission.)... 

IOP at or below a pressure at which further optic nerve damage is unlikely to occur and educate and involve the patient in the management of their disease. Current therapy is directed at altering the flow of aqueous humor, which is the major determinant of IOP. 

Laser trabeculoplasty uses laser energy aimed at the trabecular meshwork to improve the outflow of aqueous humor and can be used as first-line therapy, especially in patients unable or unwilling to adhere to medical therapy. Laser trabeculoplasty may be used as bridge therapy between medical and surgical intervention, and approximately 50% of laser trabeculoplasty procedures will fail at 10 years. Trabeculectomy is the removal of a portion of the trabecular meshwork to improve aqueous... 

Carbonic anhydrase inhibitors decrease aqueous humor production by inhibition of the carbonic anhydrase isoenzyme II located in the ciliary body. In the eye, carbonic anhydrase catalyzes the conversion of water and carbon dioxide to bicarbonate and hydrogen ion, which is a significant step in aqueous humor production. Carbonic anhydrase inhibitors are available in systemic and topical preparations.10,13,14... 

Pilocarpine directly stimulates the muscarinic (M3) receptors of the ciliary body which causes contraction of the ciliary muscle. This results in widening of the spaces in the trabecular meshwork, which causes an increase in aqueous humor outflow and reduces IOP by 20% to 30%. 

The five layers of the cornea contain no blood vessels but are nourished by tears, oxygen, and aqueous humor. Minor corneal abrasions heal quickly. Moderate abrasions take 24 to 72 hours to heal. Deep scratches may scar the cornea and require corneal transplant if vision is impaired. Do not use eye patches to treat corneal abrasion, as they decrease oxygen delivery, increase pain, and increase the chance of infection.3... 

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