Retinoscopy is an objective measure of refractive error. It needs no response from the patient, so it is essential in children and in patients who cannot communicate
Principle: light from the retinoscope illuminates the fundus. The examiner watches the red reflex in the pupil as the beam is moved, and adds lenses until the patient's far point is brought to the retinoscope

At that point, all the light from the illuminated retina converges on the examiner's peephole, the whole pupil fills with light, and no movement is seen: neutral

## The retinoscope

Streak retinoscope (the usual type): projects a line of light whose orientation can be rotated, which makes it easy to find and neutralise the axes of astigmatism
The sleeve changes the vergence of the beam

Plane mirror effect (divergent beam): the standard setting. All the movements described here assume it
Concave mirror effect (convergent beam): the source is imaged in front of the patient, which reverses the movements

Observation and illumination share one path (through a semi-silvered mirror or an aperture), so the examiner sees along the beam

## With, against and neutral

With movement: the reflex moves the same way as the beam. The far point is beyond the examiner (low myopia less than the working-distance dioptres, emmetropia or hypermetropia). Add plus
Against movement: the reflex moves the opposite way. The far point lies between patient and examiner (myopia greater than the working-distance dioptres). Add minus
Neutral: the pupil fills with light, or the reflex moves infinitely fast
The reflex tells you how far from neutral you are

Approaching from the with side is easier: confirm neutral by adding +0.25D, which should give a just-visible against movement

## The working distance

The examiner sits at a working distance, and neutral corrects the eye for that distance, not for infinity
Refraction = gross retinoscopy result − working-distance dioptres

Working distance dioptres = 1 ÷ distance (m). At 67 cm, 1.50D; at 50 cm, 2.00D
Equivalently, use a working-distance lens (+1.50D at 67 cm) in the trial frame; then the lenses that neutralise are the distance prescription

Worked example: at 67 cm, neutral is found with +4.00DS. Refraction = 4.00 − 1.50 = +2.50DS
Worked example: at 67 cm, neutral is found with no lens. The far point is at 67 cm: the patient is a −1.50D myope
Error from a wrong working distance: if the examiner actually works at 50 cm but deducts 1.50D, the result is 0.50D too plus (2.00 − 1.50)

## Astigmatism

Break: the reflex is not aligned with the streak unless the streak lies along an axis
Width: the reflex is narrowest when the streak lies along an axis
Intensity: the reflex is brightest along an axis
Skew: off axis, the reflex moves obliquely rather than with the streak

Method: rotate the streak until there is no break, then neutralise each principal meridian in turn with spheres (or a sphere, then a cylinder)

Sweeping a vertical streak side to side neutralises the horizontal (180°) meridian. The power found acts in the meridian perpendicular to the streak

Vertical streak, horizontal meridian: neutral with +2.00D. Horizontal streak, vertical meridian: neutral with +3.00D
Subtract 1.50D: +0.50D in the 180° meridian and +1.50D in the 90° meridian
Prescription: +1.50 / −1.00 × 90, or in plus-cylinder form +0.50 / +1.00 × 180
The vertical meridian needs more plus, so it is the weaker (flatter) one: against-the-rule astigmatism, compound hypermetropic, since both meridians are plus (see Astigmatism)

## Cycloplegic retinoscopy

Accommodation during retinoscopy makes the result too minus. Ask the patient to fixate a distant target, and fog the other eye with a plus lens
Cycloplegia abolishes accommodation and dilates the pupil. It is needed for children, suspected latent hypermetropia, accommodative esotropia, and pseudomyopia

Cyclopentolate 1% (0.5% in infants), one or two drops, measured after 30–45 minutes
Atropine 1% for the fullest cycloplegia, e.g. in dark irides or accommodative esotropia; it lasts up to two weeks

The cycloplegic result includes the latent hypermetropia normally held by ciliary tone. Whether to deduct an allowance before prescribing depends on age, the drug and whether there is a squint. In accommodative esotropia, the full cycloplegic hypermetropia is prescribed

## Pitfalls and reflexes

Finding	Cause
Scissors reflex: two bands moving towards and away from each other	Irregular astigmatism, especially keratoconus; also after refractive surgery
Oil-drop (Charleux) reflex: a dark central disc in the red reflex	Keratoconus, lenticonus, early nuclear cataract
Different movements in the centre and periphery of the pupil	Spherical aberration in large (dilated) pupils. Neutralise the central reflex
A fluctuating, myopic result that changes with cycloplegia	Active accommodation or accommodative spasm
Unexpected oblique cylinder	Working off the visual axis
Poor or absent reflex	Media opacity, small pupil, very high ametropia (very dim, slow reflex: try a large lens first)

Dynamic retinoscopy (e.g. the monocular estimation method): the patient fixates a near target at the retinoscope. It measures the lag of accommodation and helps assess near adds and accommodative insufficiency
Retinoscopy is the starting point for subjective refraction, which then refines the sphere, cylinder axis and cylinder power (see Subjective Refraction)
Autorefractors find the far point electronically, using the optometer or Scheiner principle (see Autorefraction and Wavefront)