Retinoscopy: Principle, Procedure And Inferences

Retinoscopy also called skiascopy or shadow test is an objective method of finding out the error of refraction by the method of neutralization.

Principle

Retinoscopy is based on the fact that when the light is reflected from a mirror into the eye, the direction in which the light will travel across the pupil will depend upon the refractive state of the eye.

Requirement for Retinoscopy

Mirror retinoscope;
A: Plain mirror
B: Pristley Smith

Trial Frame

Streak Retinoscope

1. A darkroom,  preferably 6-m long, or which can be converted into 6 m by use of a plane mirror.
2. A trial box containing spherical and cylindrical lenses of variable plus and minus powers, a pinhole, an occluder, and prisms.
3. A trial frame preferably of the adjustable type which can be used in children as well as adults.
4. A Snellen’s self-illuminated vision box
5. Retinoscope is a simple device to perform the retinoscopy. Broadly, retinoscopes available are of two types:

(a) Mirror retinoscopes are cheap and the most commonly employed. A source of light is required when using mirror retinoscope, which is kept above and behind the head of the patient. A mirror retinoscope may consist of a single plane mirror or a combination of plane and concave mirrors (Pristley-Smith mirror).

(b) Self-illuminated retinoscopes are costly but handy. Two types of self-illuminated retinoscope available are -
a spot retinoscope and a streak retinoscope.

The streak retinoscope is more popular. In it, the usual circular beam of light is modified to produce a linear streak of light by using a planocylindrical retinoscopy mirror. The streak retinoscopy is more sensitive than spot retinoscopy in detecting astigmatism.

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Procedure

The patient is made to sit at a distance of 1 m from the examiner.  With the help of a retinoscope, light is thrown onto the patient’s eye, who is instructed to look at a far point (to relax the accommodation). However, when a cycloplegic has been used, the patient can look directly into the light and have the refraction assessed along the actual visual axis.  Through a hole in the retinoscope’s mirror, the examiner observes a red reflex in the pupillary area of the patient. Then the retinoscope is moved in horizontal and vertical meridia keeping a watch on the red reflex (which also moves when the retinoscope is moved).

In low degrees of refractive errors the shadow (red-reflex) seen in the pupillary area is faint and moves rapidly with the movement of the mirror; while in high degrees of ametropia it is very dark and moves slowly.In the presence of astigmatism, when the axis does not correspond with the movement of the mirror, the shadow appears to swirl around.

Use of Cycloplegics in Retinoscopy

Cycloplegics are the drugs which cause paralysis of accommodation and dilate the pupil. These are used for retinoscopy when the examiner suspects that accommodation is abnormally active and will hinder the exact retinoscopy. Such a situation is encountered in young children and hypermetropes. When retinoscopy is performed after instilling cycloplegic drugs it is termed as wet retinoscopy in converse to dry retinoscopy (without cycloplegics). The commonly employed cycloplegics are as follows:

1. Atropine is indicated in children below the age of 5 years. It is used as 1 percent ointment thrice daily for 3 consecutive days before performing retinoscopy. Its effect lasts for 10 to 20 days.
2. Homatropine is used as 2 percent drops. One drop is often instilled every 10 minutes for 6 times and the retinoscopy is performed after 1 to 2 hours. Its effect lasts for 48 to 72 hours. It is used for most of the hypermetropic individuals between 5 and 25 years of age.
3. Cyclopentolate is a short acting cycloplegic. Its effect lasts for 6 to 18 hours. It is used as 1percent eye drops in patients between 8 and 20years of age. One drop of cyclopentolate is instilled after every 10-15 minutes for 3 times(Havener’s recommended dose) and the retinoscopy is performed 1 to 1/½ hours or 60 to 90 min. later, after estimating the residual accommodation which should not exceed one dioptre.
4. Only mydriatic (10% phenylephrine) may be needed in elderly patients when the pupil is narrow or media is slightly hazy.

Observation and Inferences

Depending upon the movement of the red reflex when a plane mirror retinoscope is used at a distance of 1 meter) the results are interpreted as: 

1. No movement of red reflex indicates myopia of 1D. 
2.  With the movement of red reflex along the movement of the retinoscope, indicates either emmetropia or hypermetropia or myopia of less than 1 D.
3.  Against movement of red reflex to the movement of the retinoscope implies myopia of more than 1 D.

Neutralization

When the red reflex moves with or against the movement of retinoscopy we do not exactly know the amount of refractive error. However, when the red glow in the pupil does not move then we know for certain that patient has myopia of 1D. Therefore, to estimate the degree of refractive error, the movement of red reflex is neutralized by addition of increasingly convex (+) spherical lenses (when the red reflex was moving with the movement of plane mirror) or concave (–) spherical lenses (when the red reflex was moving against the movement of plane

Two end point of retinoscopy

• With simple plane mirror retinoscope, the end point of retinoscopy is neutralization of red reflex in all the meridia, i.e., either no movement or just reversal of the movement.
• With a streak retinoscope at the end point streak disappears and the pupil appears completely illuminated or completely dark.

Problem in retinoscopy

Certain difficulties encountered during the procedure of retinoscopy-

1.  The red reflex may not be visible or may be poor. This may happen with small pupil, hazy media and the high degree of refractive error. In most cases, this difficulty is overcome by causing mydriasis and/or use of converging light with concave mirror retinoscope.
2. Changing retinoscopy findings are observed due to abnormally active accommodation and is corrected by use of cycloplegia.
3. Scissors shadows may sometimes be seen in patients with regular astigmatism with dilated pupils. Mostly this difficulty is diminished with the undilated pupil.
4. Conflicting shadows moving in various directions in different parts of the pupillary area are seen in patients with irregular astigmatism.
5.  Triangular shadow may be observed in patients with conical cornea (keratoconus), with its apex at the apex of the cone. On moving the mirror the triangular reflex appears to swirl around its apex (yawning reflex).