Factors determining the potency of mydriatic drugs in man

Mydriasis in eastern box turtles (Terrapene carolina carolina) following topical administration of proparacaine, 10% phenylephrine, and rocuronium bromide

OBJECTIVE

To determine the mydriatic effect of topical 10% phenylephrine with 10 mg/mL rocuronium bromide and compare this protocol with and without pretreatment with proparacaine.

ANIMALS STUDIED

Ten client-owned pet adult eastern box turtles (Terrapene carolina carolina).

PROCEDURES

All turtles were sedated with 8 mg/kg alfaxalone intramuscularly. One group of four turtles received four 20 μL drops of 10% phenylephrine and four 20 μL drops of rocuronium bromide in the right eye. Another group of four turtles received one standard drop of proparacaine followed by four 20 μL drops of 10% phenylephrine and four 20 μL drops of rocuronium bromide in the right eye. Two control group turtles received four 20 μL drops of saline in the right eye. The left eye was untreated in all turtles. Drops of the same type were separated by 2 min while drops of different types were separated by 5 min. Pupil size was recorded at 0, 15, 30, 60, 90, 120, 180, 240, and 360 min after administration of the final drop.

RESULTS

Treatment with 10% phenylephrine and rocuronium bromide resulted in pupil diameter changes from baseline that were statistically significant from zero at 60, 90, and 120 min in the non-proparacaine group and 90 min in the proparacaine group. The time to peak effect was 90 min in the proparacaine group and 75 min in the non-proparacaine group. Saline-treated pupils in the control group decreased in diameter over the study period. Overall, the treated eyes of the proparacaine group and non-proparacaine group were not different from each other, but both dilated more than the control group.

CONCLUSIONS

Rocuronium bromide and 10% phenylephrine can produce effective and safe mydriasis in eastern box turtles, but there was wide interindividual variation in effectiveness. Proparacaine did not improve the mydriatic effect.

Ocular Cyclopentolate: A Mini Review Concerning Its Benefits and Risks

Cycloplegic and mydriatic agents are essential in ophthalmological clinical practice since they provide the means for diagnosing and treating certain eye conditions. In addition, cyclopentolate has proven to possess certain benefits compared to other available cycloplegics and mydriatics. Still, the incidence of some adverse drug reactions related to this drug, especially in susceptible patients, has created interest in reviewing the literature about the benefits and risks of using cyclopentolate. A literature search was conducted in Medline/PubMed and Google Scholar, focusing on identifying cyclopentolate's benefits and risks; the most important benefit was its usefulness for evaluating refractive errors, especially for hyperopic children, pseudomyopia, anterior uveitis, treatment of childhood myopia, idiopathic vision loss, and during examinations before refractive surgery, with particular advantages compared to other cycloplegics. While the risks were divided into local adverse drug reactions such as burning sensation, photophobia, hyperemia, punctate keratitis, synechiae, and blurred vision, which are relatively frequent but mild and temporary; and systemic adverse drug reactions such as language problems, visual or tactile hallucinations and ataxia, but unlike ocular, systemic adverse drug reactions are rare and occur mainly in patients with risk factors. In addition, six cases of abuse were found. The treatment with cyclopentolate is effective and safe in most cases; nevertheless, special care must be taken due to the potential severe ADRs that may occur, especially in susceptible patients like children, geriatrics, patients with neurological disorders or Down's syndrome, patients with a low blood level of pseudocholinesterase, users of substances with CNS effects, and patients with a history of drug addiction. The recommendations are avoiding the use of 2% cyclopentolate and instead employing solutions with lower concentrations, preferably with another mydriatic such as phenylephrine. Likewise, the occlusion of the nasolacrimal duct after instillation limits the drug's absorption, reducing the risk of systemic adverse events.

The penetration and distribution of topical atropine in animal ocular tissues

PURPOSE

To conduct a multi-tissue investigation on the penetration and distribution of topical atropine in myopia treatment, and determine if atropine is detectable in the untreated contralateral eye after uniocular instillation.

METHODS

Nine mature New Zealand white rabbits were evenly divided into three groups. Each group was killed at 5, 24 and 72 hr, respectively, following uniocular instillation of 0.05 ml of 1% atropine. Tissues were sampled after enucleation: conjunctiva, sclera, cornea, iris, ciliary body, lens, retina, aqueous, and vitreous humors. The assay for atropine was performed using liquid chromatography-mass spectrometry (LC-MS), and molecular tissue distribution was illustrated using matrix-assisted laser desorption ionization-imaging mass spectrometry (MALDI-IMS) via an independent experiment on murine eyes.

RESULTS

At 5 hr, the highest (mean ± SEM) concentration of atropine was detected in the conjunctiva (19.05 ± 5.57 ng/mg, p < 0.05) with a concentration gradient established anteriorly to posteriorly, as supported by MALDI-IMS. At 24 hr, preferential binding of atropine to posterior ocular tissues occurred, demonstrating a reversal of the initial concentration gradient. Atropine has good ocular bioavailability with concentrations of two magnitudes higher than its binding affinity in most tissues at 3 days. Crossing-over of atropine to the untreated eye occurred within 5 hr post-administration.

CONCLUSION

Both transcorneal and transconjunctival-scleral routes are key in atropine absorption. Posterior ocular tissues could be important sites of action by atropine in myopic reduction. In uniocular atropine trials, cross-over effects on the placebo eye should be adjusted to enhance results reliability. Combining the use of LC-MS and MALDI-IMS can be a viable approach in the study of the ocular pharmacokinetics of atropine.

The effect of topical atropine on the choroidal thickness of healthy children

The purpose of the current study was to investigate the effect of topical atropine on choroidal thickness using spectral-domain optical coherence tomography. A total of 30 healthy eyes from 30 children were analyzed in this study. A single drop of 1% atropine gel was administered twice daily for a week. Choroidal thickness (CT) was measured using SD-OCT, and changes in CT before and after administration of the eye drops were analyzed at the subfovea and at 1.0-mm intervals (up to 3.0 mm) from the fovea at superior, inferior, nasal, and temporal locations. Pre- and post-cycloplegic axial length (AL) was also measured using the IOLMaster. We observed that administration of 1% atropine gel led to a significant increase in the choroidal thickness under the fovea and at all intervals from the fovea. The greatest change in CT was observed in the inferior meridian, while the nasal meridian exhibited the least change. AL did not significantly differ before and after cycloplegia, and there was no significant correlation between the changes in AL and subfoveal CT. It was concluded that administration of 1% atropine gel can significantly increase CT in the eyes of young Chinese children, albeit with different magnitude at different locations.

Cycloplegic refraction is the gold standard for epidemiological studies

Many studies on children have shown that lack of cycloplegia is associated with slight overestimation of myopia and marked errors in estimates of the prevalence of emmetropia and hyperopia. Non-cycloplegic refraction is particularly problematic for studies of associations with risk factors. The consensus around the importance of cycloplegia in children left undefined at what age, if any, cycloplegia became unnecessary. It was often implicitly assumed that cycloplegia is not necessary beyond childhood or early adulthood, and thus, the protocol for the classical studies of refraction in older adults did not include cycloplegia. Now that population studies of refractive error are beginning to fill the gap between schoolchildren and older adults, whether cycloplegia is required for measuring refractive error in this age range, needs to be defined. Data from the Tehran Eye Study show that, without cycloplegia, there are errors in the estimation of myopia, emmetropia and hyperopia in the age range 20-50, just as in children. Similar results have been reported in an analysis of data from the Beaver Dam Offspring Eye Study. If the only important outcome measure of a particular study is the prevalence of myopia, then cycloplegia may not be crucial in some cases. But, without cycloplegia, measurements of other refractive categories as well as spherical equivalent are unreliable. In summary, the current evidence suggests that cycloplegic refraction should be considered as the gold standard for epidemiological studies of refraction, not only in children, but in adults up to the age of 50.

The effect of topical adrenergic and anticholinergic agents on the choroidal thickness of young healthy adults

The human choroid is capable of rapidly changing its thickness in response to a variety of stimuli. However little is known about the role of the autonomic nervous system in the regulation of the thickness of the choroid. Therefore, we investigated the effect of topical parasympatholytic and sympathomimetic agents upon the choroidal thickness and ocular biometrics of young healthy adult subjects. Fourteen subjects (mean age 27.9 ± 4 years) participated in this randomized, single-masked, placebo-controlled study. Each subject had measurements of choroidal thickness (ChT) and ocular biometrics of their right eye taken before, and then 30 and 60 min following the administration of topical pharmacological agents. Three different drugs: 2% homatropine hydrobromide, 2.5% phenylephrine hydrochloride and a placebo (0.3% hydroxypropyl methylcellulose) were tested in all subjects; each on different days (at the same time of the day) in randomized order. Participants were masked to the pharmacological agent being used at each testing session. The instillation of 2% homatropine resulted in a small but significant increase in subfoveal ChT at 30 and 60 min after drug instillation (mean change 7 ± 3 μm and 14 ± 2 μm respectively; both p < 0.0001). The parafoveal choroid also exhibited a similar magnitude, significant increase in thickness with time after 2% homatropine (p < 0.001), with a mean change of 7 ± 0.3 μm and 13 ± 1 μm (in the region located 0.5 mm from the fovea center), 6 ± 1 μm and 12.5 ± 1 μm (1 mm from the fovea center) and 6 ± 2 μm and 12 ± 2 μm (1.5 mm from the fovea center) after 30 and 60 min respectively. Axial length decreased significantly 60 min after homatropine (p < 0.01). There were also significant changes in lens thickness (LT) and anterior chamber depth (ACD) (p < 0.05) associated with homatropine instillation. No significant changes in choroidal thickness, or ocular biometrics were found after 2.5% phenylephrine or placebo at any examination points (p > 0.05). In human subjects, significant increases in subfoveal and parafoveal choroidal thickness occurred after administration of 2% homatropine and this implies an involvement of the parasympathetic system in the control of choroidal thickness in humans.

Parasympathetic control of the pupillary light response in the red-eared slider turtle (Pseudemys scripta elegans)

OBJECTIVE

We investigated effects of both vecuronium bromide, a nicotinic cholinergic antagonist, and atropine, a muscarinic cholinergic antagonist, on the pupil of the turtle to determine whether responses to light are controlled by parasympathetic innervations acting on the iris.

ANIMAL STUDIED

Three red-eared slider turtles, Pseudemys scripta elegans.

PROCEDURE

Turtles were secured to immobilize their head movements and then inserted into a light-integrating sphere. Both pupils were monitored through small apertures by digital video cameras. Pupil diameters were measured manually with a digital caliper. During each trial, drugs (0.4%) were topically applied, four times at 15 min intervals, to the corneas of each eye. One eye was randomly selected for treatment of the drug while the other, treated with saline (0.9% NaCl), was used as control. Pupil sizes under adaptation to light were tracked after drug or saline applications.

RESULTS

Mean pupillary diameters of eyes treated with vercuronium bromide increased by 28%, reaching peak size in 90 min. Onset of response occurred 20 min after drug application and then increased at a rate having a time constant of 26 min. Recovery began at 120 min after initial application. Atropine had no effect on pupil size. No systemic side effects by drugs were observed in turtles.

CONCLUSIONS

Although atropine does not cause mydriasis, vecuronium bromide does. These results suggest that the parasympathetic system in turtles acts through acetylcholine onto nicotinic receptors to stimulate pupillary light constriction.

Comparative study on the safety and efficacy of different cycloplegic agents in children with darkly pigmented irides

BACKGROUND

The ideal cycloplegic drug that is safe, effective and convenient in children is not yet available. This study aimed to evaluate the safety and efficacy of three cycloplegic regimens in hyperopic children with pigmented irides. The responses to cycloplegia in different age groups and presence of strabismus were also compared.

METHODS

Tropicamide 0.5% and phenylephrine 0.5% (regimen I), tropicamide 1.0% and cyclopentolate 1.0% (regimen II), and atropine 1.0% (regimen III) were evaluated in 25 children using a crossover study design. Cycloplegic refractions were assessed.

RESULTS

The mean age of the children was 5.7 +/- 2.0 years (range 2.5-10.8 years). Six (24.0%) of them had strabismus. The spherical equivalent (SE) refraction for regimens I, II and III were +5.11 +/- 2.04 D, +5.29 +/- 1.89 D and +5.71 +/- 1.90 D, respectively, and were significant different from the manifest SE (+3.95 +/- 2.17 D) (P < 0.001). There was no statistical difference between regimen I and II in children without strabismus (P = 0.258) or aged older than 5 years (P > 0.050).

CONCLUSION

In older children, regimen I was as effective as regimen II and can be used to avoid cyclopentolate toxicity.

Systemic adverse effects of topical ophthalmic agents. Implications for older patients

Topical ophthalmic medications are widely prescribed by growing numbers of eye-care professionals. Increasingly, these agents are being prescribed by optometrists and ophthalmic-trained nurses in addition to ophthalmologists and general practitioners. As the number and variety of topical agents on the market rises, and as the number of clinicians involved in prescribing those agents increases; the risk of systemic adverse effects will also increase. Thus, professionals involved in the care of these patients must be aware of the risks associated with these drugs in order to minimise the likelihood of complications. Moreover, inadequate training may result in the clinician failing to associate a topical medication with a systemic condition, allowing an adverse effect to pass unrecognised. It is therefore in the interest of the ophthalmic and pharmaceutical communities to improve awareness of the potential dangers intrinsic in the use of topical eye medications. It is the elderly population who are at greatest risk of experiencing systemic adverse effects of topical agents. Chronic ophthalmic diseases, and hence long term ophthalmic drop treatments, are more prevalent among older people. Such individuals are also likely to have other medical conditions (e.g. cardiac, respiratory or neurological disease) that may be induced or exacerbated by topical ophthalmic agents. Moreover, polypharmacy is common in elderly people, and this is associated with an increased risk of drug interactions.

Plasma concentrations and ocular effects of cyclopentolate after ocular application of three formulations

1. Eight volunteers received in randomized order two 30 microliters drops of either 1% w/v cyclopentolate hydrochloride or a corresponding amount of cyclopentolate polygalacturonate in saline or in acetate buffer in one eye. Cyclopentolate concentrations in plasma were measured by a radioreceptor assay. 2. Peak plasma drug concentrations of about 3 ng ml-1 occurred within 30 min after all formulations. Occasionally, a second concentration peak in plasma, probably reflecting drug absorption from the gastrointestinal tract, was seen after 2 h. The mean elimination half-life of cyclopentolate was 111 min when all subjects and formulations were considered together. There were no statistically significant differences between the formulations with respect to the time-course of plasma drug concentration. 3. The maximal mydriatic effect was reached within about 15 min and was maintained for several hours, often being 1/3 of its peak value after 30 h. Similarly, an intense cycloplegic response was achieved within a few minutes, the peak changes in the near-point of vision being 9 to 10 dioptres. The cycloplegic response was more intense after one of the polygalacturonate complexes, especially at later time points.

Receptor-responses in fresh human ciliary muscle

The physiological and pharmacological properties of ciliary muscle isolated from fresh human eyes were investigated. The muscle exhibited no spontaneous activity. Concentration-dependent contractions in response to carbachol were competitively antagonized by atropine (pA2 = 8.95). The muscle, precontracted by carbachol (2.7 X 10(-4)M), responded to the application of isoprenaline by concentration-dependent relaxation blocked by propranolol (3.5 X 10(-9)M to 3.5 X 10(-8)M; pA2 = 9.15). Angiotensin-evoked contractions were antagonized by 8-Ala-angiotensin II (4.5 X 10(-8)M) in a competitive manner, but were not inhibited by phentolamine or propranolol. Contractions generated by electrical stimulation of the muscle (30 ms, 20 Hz, 60 pulses) were antagonized by atropine (10(-7) M) and tetrodotoxin (6.3 X 10(-7) M). Phentolamine and propranolol did not influence these responses. An increase of the external potassium concentration ([K+]o) from 5.4 to 158.8 mM produced a mechanical response, antagonized by atropine, but not influenced by tetrodotoxin, phentolamine or propranolol. The human ciliary muscle appears to carry muscarinic and angiotensin receptors and beta 2-adrenoceptors. The estimate of Katropine for muscarinic receptors mediating carbachol-induced contractions agrees with estimates of Katropine reported for human and rabbit iris.

Cholinergic systems and multiple cholinergic receptors in ocular tissues

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscarinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

Pupillary dilatation with single eyedrop mydriatic combinations

We studied the mydiatic effect of three solutions containing a combination of two mydriatic drugs in 80 adult patients. The solutions tested were cyclopentolate HCl 0.5% with phenylephrine 2.5%, tropicamide 0.5% with phenylephrine 2.5%, and tropicamide 1.0% with phenylephrine 2.5%. We evaluated the effect of prior instillation of proparacaine 0.5% eyedrops. All three mydriatic combination solutions evaluated produced pupillary dilatation of about 7 mm within 60 minutes. Additional pupillary dilatation of 1 mm occurred when proparacaine was instilled before the mydriatic combination eyedrop. Mydriasis was resistant to bright light during indirect ophthalmoscopy in all patients. Pupils of younger patients dilated better than those of older patients, but sex and iris color were factors in the amount of pupillary dilatation obtained. Wide and sustained pupillary dilatation can be obtained for satisfactory indirect ophthalmoscopy by the instillation of one drop of proparacaine solution followed by a single drop of any of the three mydriatic combination solutions evaluated. By eliminating the need for multiple instillations of drugs, the use of a single eyedrop mydriatic combination is convenient in terms of time saved and also lessens the change of systemic drug toxicity.

Mydriatic effect of anticholinergic drugs used during reversal of nondepolarizing muscle relaxants

Large doses of anticholinergic drugs (atropine, glycopyrrolate) produced mydriasis in a group of adults with no eye abnormalities except strabismus, though the usual intramuscular and intravenous doses of these drugs do not have this tendency. Such large doses are often given intravenously during general anesthesia to prevent the side effects of neostigmine methylsulfate, which is used to reverse the effect of nondepolarizing muscle relaxants. Neostigmine methylsulfate (Prostigmin) reduced the mydriatic effect when given intravenously in conjunction with atropine or glycopyrrolate. Mydriasis was more likely to occur in lightly pigmented eyes than in eyes with dark irides. Pilocarpine eyedrops instilled at the beginning of anesthesia caused miosis that persisted after the large intravenous doses of atropine or glycopyrrolate were given. To prevent an attack of acute angle-closure glaucoma in any patient who is to receive large doses of anticholinergic drugs during general anesthesia, miotic drug therapy should be continued before, during, and after anesthesia at the same frequency as when awake.

The affinity of atropine for muscarine receptors in human sphincter pupillae

Concentration-dependent contractions in response to carbachol were determined on isolated pieces of human iris. Atropine competivtively antagonized the effect of carbachol. An apparent dissociation equilibrium constant of 0.4--0.7 nM was estimated for the muscarine receptor-atropine complex.

Factors determining the potency of cholinomimetic miotic drugs and their effect upon the light reflex in man

1. Television pupillometry was used to measure the effect of six topically applied cholinomimetic drugs on the resting diameter and light reflex amplitude of the human pupil. Drug potency was obtained from dose response curves. 2. The tertiary amines arecoline, aceclidine and pilocarpine were considerably more effective miotics than the choline esters carbachol, methacholine and acetylcholine. 3. All the drugs which caused miosis also reduced light reflex amplitude proportionally. 4. The in vitro potency of these drugs was also measured on preparations of rabbit iris sphincter and guinea pig ileum. 5. Dose response relationships for pilocarpine in man and in vitro showed evidence of partial agonist activity on the rabbit iris only. 6. A comparison of the in vivo and in vitro results showed that three factors influenced the potency of topically applied miotics: accessibility to the iris; sensitivity to cholinesterase; and cholinoceptor agonist potency.

Gas chromatographic and gas chromatographic-mass spectrometric analysis of ampicillin

A method was developed for the quantitative gas chromatographic (GC) determination of ampicillin. The procedure requires silylation iwth hexamethyldisilazane, trimethylchlorosilane, trimethylsilylimidazole and N,O-bis(trimethylsilyl)acetamide in pyridine and subsequent GC on an OV-17 column, using 5 alpha-cholestane as an internal standard. The method was applied to the determination of ampicillin in some pharmaceutical products. The characteristics of the mass spectra and the derivatization GC of ampicillin are also discussed.