Blood-aqueous barrier

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. 

Kinsey, V.E. (1947). Transfer of ascorbic acid and related compounds across the blood-aqueous barrier. Am. J. Ophthal. 30, 1262-1266. 

Concomitant ocular conditions When an intraocular inflammatory process is present, breakdown of the blood-aqueous barrier from anticholinesterase therapy requires abstention from, or cautious use of, these drugs. Use with great caution where there is a history of quiescent uveitis. 

Cystoid macular edema has been reported in four other patients shortly after they started to use latanoprost (24) and other reports have appeared (25-29). A possible explanation is enhanced disruption of the blood-aqueous barrier induced by latanoprost (28). 

Miyake K, Ota I, Maekubo K, Ichihashi S, Miyake S. Latanoprost accelerates disruption of the blood-aqueous barrier and the incidence of angiographic cystoid macular edema in early postoperative pseudophakias. Arch Ophthalmol 1999 117(l) 34-40. 

The eye globe is a hollow structure filled anteriorly by the aqueous humor and posteriorly by the vitreous. The aqueous humor (Figure 24.1) is continuously formed by the ciliary body. It is secreted into the posterior chamber from which it passes through the pupil into the anterior chamber and is drained at the anterior chamber periphery. Due to the blood aqueous barrier formed by zonulae occludentes of the nonpigmentary ciliary epithelium, macromolecules such as proteins can pass to the aqueous in very small quantities, regardless of their plasma concentrations. In humans, the aqueous humor protein level is around 20 mg/100 mL, less than 0.5% of the normal plasma total protein levels (Table 24.1) [22]. In the human eye, the rate of aqueous formation is around 2.5 pL/min, whereas in the case of rabbit eye it is... 

Blood-Ocular Barriers The blood-ocular barriers can be divided into the blood-aqueous barrier and the blood-retinal barrier. 

The major function of the ciliary body is aqueous humor production. Aqueous is composed of a clear ultrafiltrate of blood plasma devoid of large proteins, together with some substances actively transported across the blood-aqueous barrier. 

Similar to CME, a number of cases have been reported alleging an association between latanoprost therapy and the development of anterior uveitis. Although prostaglandins play an important role in the development of vascular permeability associated with uveitis, disruption of the blood-aqueous barrier associated with latanoprost can be small, transient, and sometimes reversible despite continued latanoprost therapy. 

Pilocarpine therapy should be avoided in certain patients (Box 10-10).This drug is contraindicated in patients with cataract, especially nuclear sclerotic and posterior subcapsular cataract, because the drug can affect vision and may accelerate the formation of lens opacities. Pilocarpine is generally contraindicated in patients younger than 40 years of age because of the intolerable accommodative spasm and refractive changes. Because breakdown of the blood-aqueous barrier occurs with the use of pilocarpine and other miotics, particularly in the presence of neovascular and uveitic glaucoma, pilocarpine should be avoided in these patients. 

If pupillary capture persists, secondary complications can occur, including pupillary block glaucoma, iris chafing, iris sphincter erosion, and disruption of the blood-aqueous barrier with secondary inflammation... 

The eye (Fig. 9.32) has two barrier systems a blood-aqueous barrier and a blood-vitreous barrier. The former is composed of the ciliary epithelium, the epithelium of the posterior surface of the iris, and blood vessels within the iris. Solutes and drugs enter the aqueous humour at the ciliary epithelium and at blood vessels. Many substances are transported out of the vitreous humour at the retinal surface. Solutes also leave the vitreous humour by diffusing to the aqueous humour of the posterior chamber. 

Krootila K, Uusitalo H, Lehtosalo JI, et al. 1986. Effect of topical chemical irritation on the blood-aqueous barrier of the right eye. Ophthalmic Res 18 248-252. 

Smith EB, Castaneda FA. 1970. Effect of UDMH on blood coagulation, the blood-aqueous barrier and the cornea. Aerosp Med 1240-1243. 

Although patients with pseudoexfoliation syndrome generally showed higher blood-aqueous barrier permeability than the control subjects, the patients implanted with heparin surface-modified intraocular lenses showed decreased permeability compared with the control subjects after surgery (95). Correspondingly, eyes with exfoliation syndrome exhibited a reduced incidence of posterior capsule opacification after implantation of heparin surface-modified intraocular lenses (96). 

Consequently, frequent instillation of eye drops is required, resulting in pulsed administration and patient noncompliance. Clearly, the main prerequisite for absorption of drugs into the eye is good corneal penetration and prolonged contact time with the corneal epithelium. Many intrinsic barriers, such as the cornea barrier, blood-aqueous barrier (BAB), and blood-retinal barrier (BRB), restrict ocular drug delivery (Figure 51.2). The barriers related to topical drug administration route are as follows. 

Ross, E.J. Influence of insulin on permeability of blood-aqueous barrier to glucose. J. Physiol. (Lond.) 116, 414-423 (1952)... 

Oral/Systemic Patient compliance and noninvasive route of administration Blood-aqueous barrier, blood-retinal barrier, high dosing causes toxicity, bioavailability <2%... 

IgM synthesis and filtration through the blood-aqueous barrier in cataract patients. Current Eye Res. 9/Suppl 45(1990). 

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