Objective Refraction Technique: Retinoscopy

Excerpt

Refractivity is a principal property of light and explains the changes in the path of light rays following transmission or propagation via various materials or optical media. The cornea and crystalline lens are the principal refractive tissues of the human eye. Geometrical optics is a branch of physics that studies the observed changes in the path of light rays refracted by various media and is directly applicable to vision.

Although D'Armate designed the first pair of spectacle lenses in the 13th century, refractive errors remained a significant irredeemable impairment. In the 19th century, the French ophthalmologist Cuignet was credited with developing what later came to be known as retinoscopy based on the work of Foucault. The invention of the retinoscope, sometimes referred to as a skiascope, pupilloscope, shadowscope, scotoscope, or umbrascope, was a milestone that made it possible to determine the refractive status of the eyes.

Various procedures or techniques are employed when evaluating the ocular refractive state. Objective techniques include those without subjective input from test subjects or patients; retinoscopy is an objective refraction technique. Subjective refraction includes input from patients. (See Image. Optometrist Assessing Subjective Refraction During Ophthalmoscopic Examination)

Retinoscopy is an exam technique that utilizes an external light source supplied by a retinoscope to project light rays through the transparent ocular media to observe their reflection off the retina. (See Image. Angle of incidence) Early retinoscopes comprised single or separate perforated mirrors through which illumination from an external source could be directed toward the eyes of the patient. Wolff invented the first internally illuminated electric retinoscope in 1901. These early electric retinoscopes projected an evenly round spot of light, which did not permit the assessment of orthogonal planes. Copeland invented the streak retinoscope in 1927 to more accurately detect astigmatism; the streak technique is still in use in modern practice. While continued advancements have led to marked improvements in automated optical devices, retinoscopy remains a useful clinical technique.

Retinoscopic procedures can be broadly categorized as static or dynamic. Static retinoscopy requires the patient to be in a relaxed accommodative state, which may be natural or pharmaceutically induced. Dynamic retinoscopy requires active accommodation.

Static Retinoscopy

During static retinoscopy, the examiner observes for reflex neutralization during consistent patient fixation at an immovable visual stimulus, target, or optotype situated at optical infinity, defined as 6 m or 20 ft. Static retinoscopy can be employed during routine noncycloplegic or cycloplegic objective refraction assessments. Noncycloplegic objective refraction using the retinoscope is carried out while the accommodative facility of the patient remains naturally intact; this is colloquially termed dry retinoscopy. Cycloplegic retinoscopy is performed after the instillation of cycloplegic agents to restrict the natural accommodative response temporarily and may be referred to as wet retinoscopy.

Cycloplegic retinoscopy is particularly useful when assessing patients who cannot or will not fixate on a target or follow instructions, such as pediatric patients or patients with special needs. Wet retinoscopy may also be required in patients with amblyopia, tropias, accommodative phorias, accommodative spasm, latent hyperopia, moderate-to-high anisometropia, high lenticular astigmatism, high myopia, other pediatric ametropias, or oscillatory nystagmus. Commonly utilized topical cycloplegic agents include tropicamide, cyclopentolate, scopolamine, homatropine, and atropine; selecting the most appropriate agent requires consideration of the age of the patient, allergy profiles, and other comorbidities.

In cases where cycloplegic agents are not indicated or inappropriate, the Mohindra retinoscopy technique may be employed. This technique presents the retinoscope light as a near stimulus to elicit direct fixation from the patient throughout the assessment and derives final objective values using an algebraic sum of objective findings and a constant factor of -1.25 D from the habitual working distance and resting state of accommodation.

Dynamic Retinoscopy

Dynamic retinoscopy requires fixation at a near target at a distance less than optical infinity and is employed to determine innate accommodative response to presented stimuli at given distances. Many dynamic retinoscopic techniques have been described. While the most commonly utilized retinoscopic techniques are the monocular estimation method and the Nott method, other techniques include the Bell, Book, stress point, and binocular cross-cylinder retinoscopic methods.

The monocular estimation method (MEM) of dynamic retinoscopy proposed by Haynes in 1960 requires maintaining the near-fixation target and retinoscope at equivalent distances while utilizing lenses to attempt reflex neutralization. The impression of an accommodative lag or lead can be derived from the resultant dioptric powers.

Nott dynamic retinoscopy employs a static near-fixation target and moves the retinoscope until reflex neutralization can be observed. The distance between the instrument and the patient is used to derive the accommodative response.