Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 91

As an ultrafiltrate of plasma, the compositions of AH and plasma are similar, with a few notable exceptions: a low protein concentration, high ascorbate and lactate concentrations, and reduced amounts of urea, glucose, and nonprotein nitrogen occur within AH versus plasma (Figure 2.6). Thus, breakdown of the BAB modifies the composition of the AH, primarily by the addition of proteins and prostaglandins, and increases light scattering. The resultant Tyndall effects makes a slit‐lamp beam evident within the anterior chamber, an observation clinically known as “aqueous flare.” With the addition of proteins, the aqueous composition closely approximates that of plasma and is termed plasmoid aqueous. Plasmoid aqueous in domestic animals forms fibrin clots easily due to high concentrations of the glycoprotein fibrinogen. Unless treated pharmacologically, these fibrin clots can cause numerous complications, including anterior and/or posterior synechiae or adhesions between the iris and the cornea and/or lens.

Figure 2.6 AH drainage occurs via the traditional and uveoscleral outflow pathways in the iridocorneal angle of the dog. The ciliary body epithelium produces AH, which flows from the posterior chamber, through the pupil, and into the anterior chamber. Then, AH drains through the pectinate ligament to enter the TM. In the traditional outflow pathway, AH enters the AAP to drain anteriorly to the episcleral and conjunctival veins or posteriorly into the scleral venous plexus (SVP) and vortex veins. With uveoscleral outflow, AH flows through the ciliary muscle interstitium to the supraciliary and suprachoroidal spaces to diffuse out the sclera.

In addition to protein and ascorbate, other organic compounds constitute the AH, and their concentrations vary relative to plasma. In most mammalian species, the concentration of amino acids in the AH is higher than that in the plasma, suggesting that active transport of amino acids is occurring across the ciliary epithelium. In the dog, however, amino acid concentrations are less in AH than in plasma. In this species, the vitreous may act as a “sink” for some of the amino acids, thus causing the deficiency.

The major cations in the AH are sodium, potassium, calcium, and magnesium, with sodium comprising 95% of the total cation concentration. Sodium enters the AH via active transport, with a net flow of water into the posterior chamber. The major anions in AH are chloride, bicarbonate, phosphate, ascorbate, and lactate. The chloride and bicarbonate ions enter with sodium, but their concentrations vary among species.