A human model of allergic conjunctivitis

The refractive index of the human cornea: A review

The refractive index of the cornea and overlying tear film are key factors affecting refraction and overall optical properties of the eye. A figure of 1.376 is often quoted for the refractive index of the human cornea over the visible spectrum. In the 19th century estimates for the average refractive index of the human cornea ranged from 1.335 to 1.4391. Over the last two decades data obtained from either ex or in vivo corneas (under local anaesthesia with or without stromal resection) by contact Abbé refractometry show the refractive index of the cornea changes along its' depth undulating from around 1.400 at the epithelium to 1.380 at Bowman's layer, a low of 1.369 in the mid stroma and 1.373 at the endothelium. The mean refractive index of harvested tear samples is 1.337 rising to 1.482 for the overlying lipid layer. Contemporary measurements obtained in vivo by non-invasive methods reveal the average, or equivalent, refractive index of the tear film-cornea complex along the antero-posterior direction ranges from 1.423 to 1.436. Over the last 200 years calculations, with respect to the optics of the human eye, were based on values for the refractive index of the cornea obtained from invasive techniques. The refractive index of the cornea and overlying tear film appears to be higher than previously accepted and varies from case to case.

Allergic conjunctivitis

Allergic conjunctivitis is common, especially during the allergy season. Consultation with the allergist to perform skin tests or in vitro tests may be useful and confirmatory in the diagnosis of ocular allergy. If treatment is necessary, antihistamines, mast cell stabilizers, and nonsteroidal anti-inflammatory drugs are safe and reasonably effective. Corticosteroids are an order of magnitude more potent than noncorticosteroids; however, they have attendant side effects that are best monitored by the ophthalmologist. The development of "modified" corticosteroids has been a boon to the treatment of ocular allergy because these drugs may reduce potential side effects without sacrificing potency.

Methodologies for the Study of Ocular Surface Disease

The ability to obtain reliable results from clinical trials of therapies for ocular allergic disease and dry eye disease is often limited because of inadequate control of variables, such as environment, patient life style, compliance, and individual fluctuations that occur from one assessment visit to another. The controlled allergen challenge (CAC) model of allergic conjunctivitis allows signs and symptoms of the disease to be elicited in a physiologically accurate and reproducible manner. The rigid criteria for subject selection, the controlled allergic reaction, and the standardized and quantified grading systems allow for a reproducible baseline from which statistically and clinically significant differences between formulations can be assessed. Similarly, the controlled adverse environment (CAE) model for dry eye mimics the environmental stimuli that lead to ocular surface drying. Preselected subjects have a reproducible, homogeneous baseline reaction from which the effects of various treatments can be significantly evaluated and compared. CAC and CAE provide accurate means to study highly variable and individual ocular surface disease.

Comparative efficacy of olopatadine 0.1% ophthalmic solution versus levocabastine 0.05% ophthalmic suspension using the conjunctival allergen challenge model

OBJECTIVE

To compare the efficacy of olopatadine and levocabastine in reducing ocular allergic itching and vascular hyperemia (redness) induced by conjunctival allergen challenge.

RESEARCH DESIGN AND METHODS

The study was a randomized, double-masked, contralateral study using the conjunctival allergen challenge (CAC) model. At Visit 1, subjects with a positive allergen skin test and a history of allergic conjunctivitis were administered increasing concentrations of allergen until at least a moderate grade 2 ocular itching and conjunctival redness response was obtained in both eyes. Allergic signs were graded on standardized 0-4 scales. Subjects who reacted positively were re-challenged at Visit 2 with the pre-determined concentration of allergen. Subjects who again responded with at least a grade 2 bilateral ocular itching and conjunctival redness score at Visit 2 were eligible for drug evaluation. At Visit 3, subjects received olopatadine in one eye and levocabastine in the contralateral eye according to a computer-generated randomization scheme generated prior to commencement of the study. Ocular discomfort was then graded in both eyes. Subjects were bilaterally challenged with the predetermined concentration of allergen 27 min after topical drug administration, such that the first post-challenge assessment was made 30 min post-drug instillation. Allergic signs and symptoms were evaluated at 3 min, 10 min, and 20 min postchallenge and safety and efficacy analyses were performed.

RESULTS

Sixty-eight subjects received study drug and were included in the safety and efficacy analyses. Ocular itching scores for olopatadine were significantly lower than levocabastine at 3 min and 10 min post-challenge (p < 0.001). Ocular redness scores for olopatadine were significantly lower than levocabastine at all time points post-challenge (p < 0.0001). Of all subjects, 4.41% reported ocular discomfort in the olopatadine eye and 26.5% in the levocabastine treated eye.

CONCLUSION

Olopatadine treated eyes had significantly less itching and redness than levocabastine treated eyes after conjunctival allergen challenge. Olopatadine was also associated with less discomfort upon instillation than levocabastine.

Emedastine ophthalmic solution 0.05% versus levocabastine ophthalmic suspension 0.05% in the treatment of allergic conjunctivitis using the conjunctival allergen challenge model

PURPOSE

To compare a new ocular antihistamine, emedastine difumarate (Emadine Ophthalmic Solution 0.05%; Alcon Laboratories, Fort Worth, Texas), with the marketed ocular antihistamine, levocabastine hydrochloride (Livostin Ophthalmic Suspension 0.05%; CIBA Vision, Atlanta, Georgia), in the treatment of allergic conjunctivitis after conjunctival allergen challenge.

METHODS

We performed a prospective, double-masked, randomized, contralateral eye study comparing emedastine 0.05% in one eye with levocabastine 0. 05% or emedastine vehicle (placebo) in the contralateral eye. Efficacy was determined 10 minutes and 2 hours after administration of study medications. Ocular itching and redness scores were recorded 3, 5, and 10 minutes after conjunctival allergen challenge.

RESULTS

A total of 97 subjects with a history of allergic conjunctivitis and a positive response to a diagnostic test were evaluable for safety analysis, and 91 subjects were evaluable for the efficacy analysis. Emadastine 0.05% was statistically significantly more effective than levocabastine 0.05% in reducing ocular itching after conjunctival allergen challenge in both the 10-minute and the 2-hour challenge (P <.05). Emedastine 0.05% and levocabastine 0.05% were statistically equivalent in reducing conjunctival redness after conjunctival allergen challenge, although emedastine tended to be more efficacious than levocabastine at every observation time point.

CONCLUSIONS

After conjunctival allergen challenge, emadastine 0.05% is significantly more effective than levocabastine 0.05% in reducing ocular itching associated with allergic conjunctivitis. The two compounds are equivalent in controlling the conjunctival redness associated with allergic conjunctivitis.

Age, stability of the precorneal tear film and the refractive index of tears

PURPOSE

To investigate the relationship between age, stability of the precorneal tear film and refractive index of tear samples drawn from a normal population.

METHODS

The stability of the precorneal tear film was estimated by measuring the Tear Thinning Time MT) using the Bausch and Lomb keratometer. Microlitre tear samples were obtained from the lower tear meniscus using glass capillary tubes. The refractive index (RI) was measured using a digital electronic Abbe refractometer with an accuracy of +/- 0.00005 units. Measurements were obtained from the right eyes of 110 subjects (55 male and 55 female) ranging in age from 18 to 89 years. They were all healthy, non-contact lens wearing individuals with no history of ocular disease or dry eye symptoms.

RESULTS

A significant relationship was found between age (x) and tear stability (TTT) in both genders. Females, TTT = 32.89-0.33x, (Spearman correlation coefficient, r = -0.537, P = 0.0001). Males, TTT = 29.25-0.29x (Spearman correlation coefficient, r = -0.353, P = 0.008). A significant correlation was found between age (x) and refractive index (RI) in females only RI = 1.33815-0.000023x, (Spearman correlation coefficient, r = -0.299, P = 0.027).

CONCLUSION

Tear stability is lower in the aged eye regardless of gender. Tear refractive index is lower in the older female eye.

Non-steroidal treatment of pollen-induced conjunctivitis: comparison of different pharmacological protocols

Pharmacological treatment of allergic conjunctivitis may be based on the therapeutic use of topical steroids; however, because of the side effects, steroids are not clinically suitable in either prolonged treatment, or pollinosis. Because non-steroidal topical drugs are suitable in prolonged treatment, this single-blind study evaluated the following topical drugs: cromoglycate plus chlorphenamine, cromoglycate plus tetryzolin and imidazolin plus nafazolin, compared with placebo. The results confirm significant clinical efficacy of these drugs on signs and symptoms compared with placebo. Thus, "non-steroidal" topical drugs should be the preferred choice in prolonged forms of pollen-induced (e.g. Parietaria and/or grasses) conjunctivitis.

Effects of topical pre-treatment with dexamethasone on the immediate and late phases of topically induced ocular anaphylaxis in the rat

We tested the effects of pre-treatment with dexamethasone on topically induced ocular anaphylaxis in the rat. Rats were immunized with dinitrophenylated Ascaris suum extract and challenged with di-DNP-lysine. Dexamethasone was administered topically once (24, 6, or 1 h before challenge) or three times (6, 4, and 2 h before challenge). A single pre-treatment given at 24 or 6 h had no significant effect. A single pre-treatment 1 h before challenge reduced the extent of edema assessed histologically but not clinically, and had no significant effect on the eosinophil count in conjunctival tissue examined 6 h after challenge. Eyes pre-treated with dexamethasone 6, 4, and 2 h before challenge showed a significant reduction in conjunctival edema assessed histologically and clinically 1 h after challenge. In addition, 6 h after challenge the number of eosinophils was significantly reduced. We conclude that repeated pre-treatment with dexamethasone can suppress both the immediate phase and the cellular late phase of topically induced ocular anaphylaxis.

Adverse external ocular effects of topical ophthalmic medications

Medical and pharmacologic research of recent years has led to the development of many potent and efficacious topical ophthalmic medications. Unfortunately, many of these drugs are potentially toxic or allergenic, and their adverse effects have themselves become important external ocular diseases. This paper presents a classification of these adverse effects and provides a review of their etiology, pathogenesis, histopathology, diagnosis, and management. It is hoped that this information will be helpful to ophthalmologists in their efforts to anticipate, prevent, recognize, and treat drug-induced ocular problems and that it will serve to emphasize the importance of avoiding the ill-considered use of medications.

Microscopically and chemically detected haemolacria

Two methods have been developed for the detection of occult haemolacria (occult blood in tears): 1) Microscopy of conjunctival fluid aspirated by means of the quantitative pipette method. 2) Chemical stix method employed on a sample of conjunctival fluid transferred to cotton-wool plug. Double samples from 306 eyes were subjected to microscopy, and chemical tests were performed on samples from303 other eyes. Microscopy has been found to disclose occult blood in 13 per cent of normals (greater than or equal to 100 erythrocytes/3.1 mm2 in 3 per cent of normals), while blood is detected chemically in 3 percent. Occult blood is most frequently found in cases of acute infectious conjunctivitis (65 per cent microscopically and 21 per cent chemically), less frequently in subacute and chronic infectious conjunctivitis (26 and 20 per cent respectively). Haemolacria is most feequently seen in relation to neutrophilia (45 per cent), less frequently with lymphocytosis (35 per cent), and even more rarely in the absence of both neutrophilia and lymphocytosis (11 per cent). Ciliary congestion is an unlikely cause of haemolacria. Postoperatively blood is present for two or three days, wheras blood instilled into the conjunctival sac of a normal eye disappears in less than one hour. The protein concentration in tears is raised in association with haemolacria.