Retinoscopy

An infant cannot read a letter chart, and a patient with aphasia cannot say which of two lenses looks better. Their refraction can still be measured, from the light that comes back out of their eyes. Retinoscopy does this with nothing more than a light, a peephole and a box of lenses, and it remains the standard objective starting point for a refraction.

#Light that comes back out

A retinoscope shines a band of light, the streak, into the patient’s pupil. The retina scatters some of it back, and a patch of illuminated retina becomes a light source inside the eye. The light from it passes back out through the eye’s optics, and we already know where it goes: light leaving a point on the retina is focused at the eye’s far point (Chapter 6). A myopic eye sends it converging toward a far point in front of the eye; an emmetropic eye sends it out parallel; a hyperopic eye sends it diverging, as if from a far point behind the eye.

The examiner looks through a peephole in the retinoscope, at a working distance of about an arm’s length, and sees the pupil filled, or partly filled, with a red-orange glow: the reflex. As the streak is swept across the eye, the reflex moves, and the way it moves depends on one thing only (in the usual setting; see The sleeve): where the far point lies relative to the peephole.

#With, against and neutral

On the left, the light leaving the patient’s eye heads for its far point; the examiner’s peephole is at the working distance. On the right is what the examiner sees: the streak sweeping across the face and the reflex in the pupil. Change the patient’s refractive error and watch both:

There are three possibilities:

• Far point beyond the peephole, or behind the patient’s eye (low myopia, emmetropia, hyperopia): the reflex moves the same way as the streak. This is with motion.
• Far point between patient and examiner (myopia stronger than the working distance): the light crosses over at the far point before it reaches the peephole, and the reflex moves the opposite way. This is against motion.
• Far point at the peephole: all the light that leaves the pupil from the illuminated patch converges on the peephole, so the examiner sees the whole pupil light up at once, with no motion at all. This is neutral, or the reversal point.

Retinoscopy is therefore a search for the lens that moves the patient’s far point to the peephole. With motion means the far point is too far away, so add plus to pull it in; against motion means it is too close, so add minus. The rule is easy to remember: with, add plus.

#Reading the reflex

Far from neutral the reflex is dim, narrow and slow. As the lens brings the far point toward the peephole, the reflex becomes brighter, wider and faster, until at neutral it fills the pupil in a flash. Watch for this in the figure above as you move the refractive error toward −1.50 D, the neutral point at 67 cm. The changes are a useful guide to how much lens is still needed: a dull, sluggish reflex calls for big steps, a bright, fast one for quarter dioptres.

Near neutral, the reflex can be hard to judge. Clinicians bracket it: add lens until the motion just reverses, then go back a step. Moving a little closer to the patient should then show with motion, and moving back, against.

#The working distance

At neutral the far point is at the peephole, not at infinity. The lens in front of the eye has made the patient myopic by exactly the vergence of the working distance, so that has to be taken away to find the prescription for distance:

At 67 cm the working distance is worth $1/0.67=1.50\text{D}$; at 50 cm, 2.00 D. The lens at neutral is the gross retinoscopy, and the result after this subtraction the net. A patient neutralised with +4.00 D at 67 cm needs +2.50 D. Some clinicians put a working-distance lens (+1.50 D) in the phoropter instead and remove it at the end; the arithmetic is the same.

Here is a patient whose prescription is hidden. Neutralise the reflex, then work out the prescription:

#Astigmatism: finding the axis

In an astigmatic eye, each principal meridian has its own far point. The streak tests one meridian at a time: swept across the pupil, it tests the meridian at right angles to its length. A vertical streak, swept from side to side, measures the horizontal (180) meridian; a horizontal streak, swept up and down, measures the vertical (090) meridian. So the streak is aligned with the axis of the cylinder that would correct the meridian it tests.

When the streak lies along a principal meridian, the reflex band is parallel to it and moves straight across. Turned off axis, the band breaks away from the streak. Turn the streak in this eye and watch the reflex line up with the streak only at the principal meridians:

Clinicians use four clues to find the axis; the figure shows the break, the width and the skew:

• Break: off axis, the band is not parallel to the streak.
• Width: the band’s width changes as the streak turns; it is extreme (thinnest for the meridian farther from neutral) on axis.
• Intensity: the band is brightest on axis.
• Skew: off axis, the band moves obliquely rather than straight across the pupil as the streak is swept.

#Neutralising both meridians

With the axis found, neutralise each principal meridian in turn: one with a sphere, the other with a minus cylinder whose axis lies along the streak (or with a second sphere, recording each result on a power cross). When the reflex is neutral whichever way the streak points, take away the working distance from every meridian. If neutral needs +3.00 D with the streak vertical and +2.00 D with it horizontal, at 67 cm, the eye needs +1.50 D in the 180 meridian and +0.50 D in the 090 meridian: +1.50 / −1.00 × 180, or +0.50 / +1.00 × 090.

Try a complete retinoscopy on a mystery patient. Find the principal meridians with the streak, and neutralise the more plus meridian with the sphere: it is the last to lose its with motion as plus is added, or the first to lose its against motion as minus is added. Then turn the streak to the other meridian, which now shows against motion, and add minus cylinder with its axis along the streak:

#The sleeve

Everything so far assumes the usual setting, in which the light leaving the retinoscope diverges slightly, as from a plane mirror. On a Copeland retinoscope this is with the sleeve up; on a Welch Allyn, with the sleeve down. In the other position the instrument behaves like a concave mirror: its light converges to a focus before reaching the patient, the illuminated patch on the retina moves the other way, and every motion reverses. That setting is sometimes used to sharpen the streak and read its axis, but the patient is always neutralised in the plane mirror position.

#In practice

• Relax accommodation. Without cycloplegia the patient fixates a distant target past the examiner’s head with the other eye, which is fogged by the plus lenses in front of it (the working-distance lens does this). In children and in suspected latent hyperopia, cycloplegic retinoscopy is the standard.
• Stay on axis. Working off the visual axis adds oblique astigmatism and errors. The examiner uses the right eye for the patient’s right eye and the left for the left, as close to the line of sight as possible without blocking the fixation target.
• A dull reflex may mean a large refractive error (try a big lens first) or media opacities.
• A scissors reflex, two bands moving toward and away from each other, signals irregular astigmatism, as in keratoconus or after corneal surgery. An “oil droplet” reflex is another sign of keratoconus.
• Dynamic retinoscopy, with the patient looking at a near target attached to the retinoscope, measures how much the eye actually accommodates for that target (the lag of accommodation).

Autorefractors automate objective refraction, some on the retinoscopic principle and others on the Scheiner principle, with infrared light and a Badal optometer (Chapters 10 and 22). They can be fooled by accommodation (instrument myopia), small pupils and irregular corneas. Retinoscopy remains the check.

#Recap

• Light from the illuminated retina leaves the eye focused toward its far point. Retinoscopy finds the lens that puts the far point at the peephole.
• Far point beyond the peephole or behind the eye: with motion, add plus. Between patient and examiner: against motion, add minus. At the peephole: neutral.
• Toward neutral the reflex brightens, widens and speeds up.
• Prescription = lens at neutral − $1/w$: −1.50 D at 67 cm, −2.00 D at 50 cm.
• The streak tests the meridian at right angles to it. Break, width, intensity and skew locate the axis; neutralise each principal meridian.
• A scissors reflex suggests irregular astigmatism such as keratoconus.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, Quick-Start Guide and chapter on clinical refraction.
• D. G. Hunter and C. E. West, Last-Minute Optics: A Concise Review of Optics, Refraction, and Contact Lenses. SLACK.
• W. J. Benjamin (ed.), Borish’s Clinical Refraction. Butterworth-Heinemann.