The refracting power of the eye results from the static power of the eye (the combined ability of the cornea and the lens to bend incoming rays of light) and the accommodative power of the eye (the variable force of accommodation that alters the path of light rays by causing the ciliary body to change the curvature of the lens). The total increase in plus power that accommodation invokes is known as the amplitude of accommodation. Cycloplegia inhibits the accommodative power of the eye by blocking the action of the ciliary muscle, allowing the static or objective refractive error of the eye to be measured. The best way to achieve paralysis of accommodation is to instill cycloplegic drugs into the eye. Cycloplegic drugs are classed as anticholinergic because they block the muscarinic action of acetylcholine. The three most commonly used cycloplegic drugs, tropicamide, cyclopentolate, and atropine, inhibit cholinergic stimulation of the iris sphincter and ciliary muscle, which results in cycloplegia and mydriasis.

The younger the patient, the greater the patient’s amplitude of accommodation and the more difficult it is to inhibit it. Because of the powerful accommodation and the inability of young pediatric patients to respond with accurate subjective responses, cycloplegic refraction, rather than manifest or subjective refraction, is usually necessary. In older children and adults, noncycloplegic methods of inhibiting accommodation, such as fogging, become more practical clinically, but cycloplegics can be used selectively to determine accurately the refractive state of the eye.

Cycloplegia is necessary for controlling accommodation and obtaining an accurate refraction in infants and young children. No other refractive technique, such as fogging, can match the preciseness of cycloplegia in determining refractive errors in early childhood because the results are independent of patient cooperation and fixation distance. Handheld autorefractors (Nikon Retinomax Plus and Welch Allyn Sure Sight Vision Screener) have shown value as screening tools for assessing refractive error in pre- and school-aged children with and without cycloplegia, but do not have the accuracy of cycloplegic retinoscopy in the hands of experienced clinicians.^1,2,3,4

Cycloplegic examinations not only allow the refractive error to be determined accurately, the concomitant mydriasis prepares the patient for an ophthalmoscopic examination. A thorough ophthalmoscopic examination is critical in infants and children to rule out opacities in the ocular media and abnormalities in the inner eye.

In older children and young adults, cycloplegic refraction can confirm the diagnosis of accommodative spasm, a constant or intermittent involuntary increase in ciliary contraction. Ciliary spasm can be caused by high ciliary tonus, or by secondary factors, such as hyperopia or convergence insufficiency.⁵ Patients with low hyperopia may present as myopic during examination. This so-called pseudomyopia can be identified by cycloplegic evaluation.

Although indicated for all patients with ocular motility disorders, cycloplegic refraction is indispensable to the clinician in diagnosing and managing accommodative vergence anomalies. The role of accommodation in esotropia is well recognized. A fully accommodative deviation exists when no tropia is present after accommodation is fully relaxed. Most esotropic vergence disorders are partly accommodative and are characterized by an abnormal accommodative convergence to accommodation (AC/A) ratio. Treatment includes blocking accommodation by the use of plus lenses and miotics.

The entire hypermetropic refractive error is usually prescribed in the spectacle correction of children with esotropia who are less than 5 years of age. Complete blockade of the ciliary hypertonus is necessary to eliminate all accommodation-induced convergence. If adjustment to the full hypermetropic correction is not prompt, a short course of a cycloplegic drug, such as atropine, is useful. The key to determining the full hypermetropic correction is a proper cycloplegic refraction, repeated one or two times a few months apart. In most cases of esotropia with hypermetropia in children less than 5 years of age, either atropine or scopolamine is preferred to ensure that no residual accommodation goes unrecognized. Several investigators have shown atropine to be more effective than cyclopentolate in blocking accommodation and the drug of choice for children with esotropia 5 years of age and younger and children with dark irides.^6,7,8 Above age 5 years, cyclopentolate 1% and atropine 1% produce equal cycloplegic effects.^9,10

Cycloplegia can be used in pharmacologic occlusion therapy when the nonamblyopic eye is sufficiently hypermetropic that effective blurring of vision can be obtained by instilling a cycloplegic drug into that eye alone. The success of this penalization therapy can be predicted in the office by blurring the sound eye with a short-acting cycloplegic agent, removing the spectacle correction, and evaluating the fixation preference. If under test conditions the patient switches from using the good eye to fixating with the amblyopic eye, chances are excellent that he or she will also do so during treatment and that penalization will force the amblyopic eye to be used. The degree of penalization is related to the amount of the patient’s uncorrected hyperopic refractive error.¹¹ Comparisons of atropine and patching treatments for moderate amblyopia have shown equal effectiveness as initial treatments.^12,13 Atropine drops administered twice a week are equally effective as a daily dose of atropine for treating moderate amblyopia in children.¹³

Cycloplegics have a role in the treatment of myopia as well. Several controlled clinical trials have proved that atropine can retard myopia progression in children.^14,15,16,17 An M1 muscarinic receptor antagonist, pirenzepine (2% gel, not commercially available), is less likely to cause mydriasis and cycloplegia, and shows promise as a safe agent for slowing the progression of myopia in children.^18,19,20,21

A number of indications exist for cycloplegic refraction in adults. As amplitude of accommodation gradually decreases with age, a closer agreement between cycloplegic and manifest refraction findings occurs. The 17 diopters (D) of accommodation of a 3-year-old child decreases to 14 D at 10 years of age, 10 D at 25 years of age, 6 D at 40 years of age, and 2 D or less at age 50. Cycloplegia may be necessary in handicapped or mentally disabled individuals because of their unresponsiveness to subjective refraction. Cycloplegic refraction is useful in objectively evaluating the suspected malingerer or hysteric. Young adults with asthenopic symptoms who use their accommodation to compensate for their hypermetropic refractive error may require a cycloplegic examination when the manifest refraction reveals too little hypermetropia to account for their symptoms of blurred vision or headaches after sustained reading. The full optical cycloplegic correction may be necessary to alleviate headaches until the age of 45.²² Cycloplegic refraction is a required occupational test used by the United States Navy in screening student aviator candidates for latent hyperopia.²³ With the advent of modern laser refractive surgery, cycloplegic refraction has become a valuable preoperative test for accurately determining refractive error, particularly in hypermetropic patients and young patients with myopia. Excessive accommodation during wavefront capture can lead to inaccurate treatment plans.^24,25 Table 41.1 summarizes the indications for cycloplegic refraction.

Because it is so difficult to suppress ciliary body muscle function completely, cycloplegic findings are not always consistent or complete. Retinoscopy can be uncertain because of the ocular aberrations caused by the wide mydriasis that accompanies cycloplegia. Repeated refractions often are necessary to achieve accurate results.

Cycloplegic refraction after childhood may require a postcycloplegic manifest refraction in those patients found to have significant hypermetropia. Eyeglasses cannot be prescribed based on cycloplegic findings alone because visual acuity may be reduced when the eye returns to its normal state of accommodative tonus. In those patients without a motility problem, the amount of reduction in plus lens correction is determined by the difference in manifest and cycloplegic refractions and the age and visual demands of the patient.

A patient having cycloplegic refraction is inconvenienced by the loss of accommodation and mydriasis that may last several hours to several days after the examination. Some patients are hypersensitive to cycloplegic agents, and experience extended effects beyond the period of expected duration of action of the drug. Pilocarpine does not reverse, and may even increase, the patient’s discomfort owing to ciliary spasm.²⁶ The drug dapiprazole hydrochloride (Rev-Eyes, 0.5%, Bausch & Lomb), an α-adrenergic blocking agent, can be used to reverse the effects of phenylephrine and, to a lesser degree, those of tropicamide.^27,28

All cycloplegic drugs have potentially serious systemic side effects, especially in infants and children. Both allergic and toxic reactions can occur with their use. Specific adverse effects are included in the comments section for each individual cycloplegic drug detailed later in this chapter. The authors of this chapter have reviewed the toxic effects of topical eye medications, including cycloplegics, in infants and children in great detail.²⁹

Proper techniques of drug instillation can greatly reduce the hazards of cycloplegics. One drop of each drug is sufficient after introduction directly into the eye because the eye normally holds only about one-fifth of one drop. Overflow onto the skin should be blotted away quickly and gentle eyelid closure should be attempted for several minutes after drug instillation. Digital pressure should be applied at the periphery of the nasolacrimal system at the medial canthus for several minutes, if possible. A second set of the same or different drops may be indicated 5 minutes or so later in darkly pigmented irides for complete cycloplegia. Increased iris pigmentation delays the onset and magnitude of cycloplegia.³⁰ The use of one drop of tropicamide 1% followed by one drop of cyclopentolate 1% can be effective in providing cycloplegia in black children.³¹ The lowest dose necessary to produce adequate cycloplegia should always be used.

The pediatric age group is easily agitated after the instillation of irritating cycloplegic eyedrops. Cycloplegics, administered as a spray to closed eyelids, have been shown to be as efficacious as instilling the same drugs as eyedrops directly into the opened eye. Sprays cause less discomfort, less dilution of the drops from tearing, and are effective even in pediatric populations with dark irides.^{32,33,34,35,36,37} No cycloplegic spray preparation are commercially available, however.

Cycloplegic drugs can cause a significant elevation of intraocular pressure in patients with narrow-angle³⁸ and open-angle glaucoma.^39,40 No significant change in intraocular pressure has been reported in open-angle, nonglaucomatous patients with tropicamide 1%⁴¹ and cyclopentolate 1%⁴² Cycloplegics can precipitate an attack of narrow-angle glaucoma in predisposed patients. Gonioscopy is therefore advised in patients suspected of having narrowed angles, such as older patients with hypermetropia. A large decrease in intraocular pressure has been reported in a 6-year-old aphakic child after the administration of cyclopentolate 1%, tropicamide 1%, and phenylephrine 2.5%.⁴³ Cycloplegics, such as tropicamide, can increase corneal thickness after instillation, affecting the accuracy of applanation tonometry.⁴⁴ It is recommended that intraocular pressure be checked before dilation in all ophthalmic examinations.