Ординатура / Офтальмология / Английские материалы / Ophthalmic Drugs Diagnostic and Therapeutic Uses 5th edition_Hopkins, Pearson_2007

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CYCLOPLEGIC EXAMINATION

The essential components of the cycloplegic examination are retinoscopy, subjective refraction (where possible) and ophthalmoscopy.

RETINOSCOPY

To confirm that adequate cycloplegia has been achieved to undertake retinoscopy, the level of residual accommodation can be determined either objectively, using dynamic retinoscopy, or subjectively. In the latter method, a pair of plus spheres, usually +2.50D are placed in the trial frame or phoropter and the push-up method used to measure the assisted accommodation. Residual accommodation is calculated by deducting 2.50D from this measured value.

In all cases, retinoscopy will be undertaken. The use of a streak instrument can facilitate accurate location of cylinder axis, but the use of a spot retinoscope has been advocated by Edgar & Barnard (1996) for paediatric use on the grounds that it is preferable for the techniques of dynamic and near retinoscopy that they describe.

Two essential practical precautions should be observed when undertaking retinoscopy under cycloplegia. First, due to the pronounced mydriasis present it is essential to observe the movement of the light reflex in the central 3–4 mm pupillary area only, ignoring the light movement in the periphery, which can be either in the same or the opposite direction.

Movement of the light reflex in the peripheral parts of the dilated pupil can show positive or negative aberrations due to the differing refractive conditions in this area as compared to the central ‘axial’ region. In positive aberration (four or five times more common than negative aberration) the peripheral area is more myopic or less hypermetropic than the central area, and thus an ‘against’ movement still persists in the periphery when the central axial zone is neutralized.

A negative aberration causes the opposite effect. Regardless of any peripheral movement, the central 3–4 mm diameter zone alone must be neutralized and the outer area reflex ignored. Due to peripheral aberrations a scissor movement of the light reflex may be observed on occasion, usually near the neutralization point. Movement of the refractionist’s head forwards and backwards over a range of about 25 cm, giving first a ‘with’ then an ‘against’ movement, is a useful check that neutralization has been attained. It can readily be seen that, with positive aberration, if the peripheral zone is erroneously corrected, too little plus or too much minus power will be recorded and vice versa when negative aberration is present. These retinoscopy rules concerning peripheral aberrations apply in all cases of dilated pupils, whether drugs have caused the mydriasis or not.

If too bright a light source is used when performing a retinoscopy with dilated pupils it is difficult to differentiate clearly the central from

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SUBJECTIVE REFRACTION

With children from about 5 years of age it might be possible to undertake some subjective results. Edgar & Barnard (1996) advocate a ‘bracketing’ technique in which the child’s reaction to modification of the retinoscopy findings with ±1.00D spheres is assessed. If sensible responses can be elicited, the practitioner can attempt to refine the refraction using ±0.50D, then ±0.25D spheres. A similar approach can be followed to check cylinder axis and power by the use of crossed cylinders of decreasing power.

As the subjective refraction is carried out immediately following the cycloplegic retinoscopy, it must be expected that the sphere power will vary somewhat from that determined objectively because of the presence of spherical aberration caused by mydriasis. Usually, as positive spherical aberration is more common, less plus sphere will be accepted in hypermetropia and more minus sphere in myopia. A further discrepancy under these conditions (also attributable to this aberration) will frequently be the loss of a line or so in the visual acuity.

It is helpful to record all refractive findings obtained under cycloplegia in red ink to differentiate them readily from results obtained without its use.

OPHTHALMOSCOPY

Pre-cycloplegic findings can be verified, taking advantage of the dilated pupil.

EFFECT OF CYCLOPLEGIA ON OCULAR COMPONENTS

Cycloplegia in children has been reported to increase anterior chamber depth and to decrease crystalline lens thickness and vitreous chamber length irrespective of the nature of the refractive error. Axial length increased in hyperopic eyes and decreased in myopes. Using a computerized video keratoscope, corneal power increased in hyperopes and decreased in myopes (Gao et al 2002).

CHOICE OF CYCLOPLEGIC

Many antimuscarinic agents were used in the past but today only three are used regularly (Table 6.1). Arranged alphabetically, and in descending order of efficacy, they are: atropine, cyclopentolate and tropicamide. Of the others, homatropine is still used infrequently, whereas hyoscine is hardly ever employed at all.

The principle of ‘as little as possible but as much as necessary’ should apply to the use of drugs by doctors, optometrists and patients alike. Having decided that a particular patient will benefit from a cycloplegic

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refraction, it is incumbent on the optometrist to use the weakest agent consistent with achieving the required depth of cycloplegia. Antimuscarinic drugs can affect the whole body and the stronger the cycloplegic the stronger will be the side-effects. Additionally, stronger agents will produce longer effects and the inconvenience to the patient of dilated pupils and the inability to read or do close work will be prolonged.

The popularity of atropine has declined significantly, principally due to the disadvantages of slow action and long duration compared with alternative drugs. Indeed, it now tends to be used only when cyclopentolate has failed to achieve satisfactory cycloplegia. Ingram & Barr (1979) consider that atropine is absolutely contraindicated in the first 3 months of life because of the danger that its prolonged action could result in stimulus deprivation amblyopia. The use of atropine cycloplegic refraction in cases of intermittent squints and high heterophorias was rejected long ago on the grounds that there is a possible danger of converting these conditions into constant squints. The shorter-acting but less complete effects of cyclopentolate are preferable in these conditions, regardless of age. Zetterton (1985) compared atropinization with the use of a phenylephrine–cyclopentolate combination and found that the cycloplegia produced by the latter was adequate for all clinical purposes.

Amos (1978) considers 5 years to be the maximum age for the use of atropine; this view should be respected because the long duration of action of atropine would either cause serious disruption of school work in the case of older children or its use would have to be restricted to school holidays. The preceding considerations demonstrate that, despite the efficacy of its action, atropine should no longer be regarded as the cycloplegic of first choice.

It is difficult to be rigid about which drug should be used on which patient. One cannot rely on age as the only determining factor. For example, heavily pigmented might may require a stronger cycloplegic than light-coloured eyes of the same age. The following are recommendations, not hard-and-fast rules:

Cyclopentolate will suffice for all cases irrespective of refractive and binocular states. Up to the age of 12 years, 1% is the recommended strength with 0.5% for older children and adults.

Tropicamide can be used as a less effective alternative cycloplegic, especially when a quick reversal of the cycloplegia is desirable. However, it is unsuitable for use with young children. In the main, 1% is used as a cycloplegic with older children and adults; the results with 0.5% are too variable.

In a study that compared tropicamide 1% with cyclopentolate 1% as cycloplegics in myopic children, the former was found to act more quickly, at around 30 min, its effect remaining stable for 75 min. The maximal cycloplegic effect of cyclopentolate was around 45 min, and it remained stable until 90 min after the last instillation. As the difference in refractive findings between the two agents was less than 0.1D, the authors concluded that tropicamide 1% is a suitable cycloplegic for such patients (Lin et al 1998).

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Cycloplegia commences in half an hour, the action is slow and full recovery may take 7–10 days, although adequate accommodation for near work has usually returned within 4–5 days (after the usual six applications). Even with one application, full ciliary muscle recovery might take 3–7 days; the resulting cycloplegia reached in 1–3 h, although marked, is not complete.

Because of the different time courses of mydriasis and cycloplegia, the size of the pupil is a poor indicator of cycloplegic effect (Amos 1978). Wide dilation of the pupil causes photophobia and sometimes the patient complains of micropsia. Normal pupillary reflex constrictions to light and to accommodation-convergence are completely abolished.

Very powerful miotics (such as ecothiopate 0.3%) will overcome the mydriatic effects of atropine 1% but this drug is not readily available in the UK. It has been withdrawn and is now available on a limited basis.

Tonus allowance The ciliary muscle, like all other smooth muscles in the body, has both a dependent and an independent tone, the former being conditional on an intact nerve supply whereas the latter is not. The independent tone in the ciliary muscle is very small and does not give rise to symptoms, neither is it affected by cycloplegics. On the other hand, the dependent tone of the ciliary muscle is totally abolished by complete atropine cycloplegia, but not by the full effects of other cycloplegics.

An allowance therefore has to be considered only in the case of complete atropine cycloplegia for the return of the dependent tone of the ciliary muscle on its recovery from the effects of atropine paralysis. This tonus allowance is an adjustment of the spherical element only of the retinoscopic findings in such cases, to take into account the fact that the eyes, when fully returned to normal, will once more usually have their overall refractive power increased slightly in a positive direction by the constant effect of the (dependent) tone of the recovered ciliary muscle. The quantitative effect of this tone will vary slightly, depending on the nature of the refractive condition of the eye being considered. Traditionally, it has been suggested that −l.00D should be added as an allowance in all refractive errors up to one dioptre whereas a smaller modification is made for higher amounts of myopia. At −3.00D or more, zero allowance might be made. In practice, any prescription issued should not be based on an arbitrary modification to the cycloplegic retinoscopy findings but must take into account factors such as any previous spectacle correction and the binocular status of the patient.

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cases have been reported by von Noorden (1981). Experiments in monkeys raised with unilateral cycloplegia have demonstrated shrinkage of cells in the lateral geniculate nucleus and loss of cortical binocularity and of neurons responding to stimulation of the atropinized eye. The results of such animal experiments and the occurrence of atropine-induced amblyopia in children point to the need to exercise the greatest caution in the application of this technique.

A retrospective study found no statistical or clinical differences in the visual outcome of treatment of strabismic amblyopia when atropine penalization and occlusion were compared (Simons et al 1997).

A combination of atropine and optical penalization for the treatment of strabismic and anisometropic amblyopia has been assessed in patients with anisometropic and strabismic amblyopia. It was well tolerated and was considered to be an effective treatment when conventional occlusion therapy had failed (Kaye et al 2002).

CYCLOPENTOLATE

Cyclopentolate is the most widely used cycloplegic today, and is now the one of choice. The paralysis of accommodation is not complete but it gives a depth of cycloplegia that is sufficient for the majority of cases. Havener (1978) aptly sums up the great value of cyclopentolate when he describes its effects in the field of cycloplegia as superior to homatropine (even the 5% concentration of the latter) in its rapidity of onset, shortened duration of action and greater intensity of effect.

Cyclopentolate is available in single-dose form (‘Minims’) and in multidose containers in two strengths for cycloplegia, 0.5% and 1.0%. An interesting alternative application method for cyclopentolate has been evaluated by Ismail et al (1994), who used a spray application to the closed eye and compared it with conventional eyedrop administration. They found that the resulting refractions using the two applications were not significantly different but that the spray application was easier to administer and was more acceptable to the patient.

Aged up to 12 years Only one drop of the 1% solution is usually necessary but a further drop should be instilled if little effect is measurable after 15 min. A further factor in determining the appropriate concentration is the degree of iris pigmentation and, when this is very light, the 0.5% solution might be suitable. Retinoscopic refraction can then be performed in 40–60 min (or sooner if desired, when the maximal cycloplegic effect is obtained earlier than this).

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necessary to administer two drops of cyclopentolate, it has in the past been accepted procedure to allow a 5-min interval between drops so that the conjunctival sac can drain and allow space for the second drop. This has been challenged by Stolovitch et al (1994), who found that an interval of 1 min was sufficient.

For dark-skinned adults, one drop of the 1% solution should be instilled and the dose repeated only if the amplitude of accommodation is not falling at a satisfactory rate. Again, retinoscopy is generally carried out in 40–60 min, that is, the average time taken for the maximum effect of the drug to reduce the accommodation to less than 2.00D.

Time course One or two drops of the cyclopentolate solution instilled into the conjunctival sac produces a cycloplegia commencing in a few minutes and becoming maximal in 30–60 min, but sometimes as rapidly as in 15 or (on rare occasions) even in 10 min, especially in patients with light irides (Manny et al 1993). Because of the variation in the time taken to produce maximum cycloplegia, and also in view of the fact that the duration of this condition varies from 10 to 60 min (averaging about 40 min), the amplitude of accommodation should be measured every 10 min after a time lag of 20 min following instillation, until no further fall in the accommodation is recorded (Mitchell et al 1958). In very young children and others unable to respond to this test, the accommodative state can be assessed using dynamic retinoscopy. Like most drugs that affect accommodation and the pupil, the time course of the mydriasis is different to that of the cycloplegia and pupil diameter should not be relied on to give a measure of the remaining accommodation.

Depth of cycloplegia As in nearly all cases the residual accommodation is 1.50D or less around 40–60 min after instillation (although not infrequently a second drop of the solution might be necessary to reduce the accommodation level to this), a period during this interval is the most usual time for retinoscopic refraction. Priestley & Medine (1951) considered that cyclopentolate (then known as Compound 75 G.T.) more closely approximated their ideal criteria for a cycloplegic or mydriatic than any other drug discovered up to that time.

Priestley & Medine compared the depth of cycloplegia reached 1 hour after instillation of two drops of a 0.5% solution of cyclopentolate with the same dosage as a 5% solution of homatropine in a group of over 50 patients, which included children and young adults. The cyclopentolate was instilled in the right eye and the homatropine in the left eye. Their results showed that the residual accommodation for cyclopentolate ranged between 0.50 and 1.75D, with an average of 1.25D, whereas with homatropine this range was between 1.00 and 3.00D, with an average of 2.00D. In an endeavour to ensure that any anisocycloplegia present did not vitiate their findings, the series was later repeated using cyclo-