An optometrist does a retinoscopy procedure to obtain an objective measurement of the refractive error in the patient’s eyes. Refractive error includes nearsightedness, farsightedness and astigmatism. Eyeglasses can alleviate these conditions and the doctor prescribes them based on the results of this procedure. The instrument for retinoscopy is called a retinoscope. The process is quick and painless and does not require a high degree of compliance on the patient’s part. Below are some of the steps that need careful consideration when carrying out retinoscopy.
- Understand Prescription for Retinoscopy Success
It is essential to first understand the various components in a glasses’ prescription. An eye has two powers: spherical and cylindrical.
The first number denotes the spherical power in diopters. If the power is positive, it denotes hyperopia or farsightedness. Minus power denotes myopia or nearsightedness. If extra spherical power needs to be denoted in the case of bifocals, it will be under additional.
The second and third number denote cylindrical power. If this exists, it will show the presence of astigmatism. The second number, in either minus or plus form, represents the power of the cylinder in diopters. The third number specifies the neutralizing axis of the cylinder.’
- Working Distance for a Retinoscopy
To begin the procedure, you must ask the patient to fixate on a distant spot. This is your first step and your next requirement should be to tell the patient to relax. You can dilate their eyes by a cycloplegic agent. Sit at an arm’s distance from the phoropter and begin determining the appropriate distance for retinoscopy.
This distance is an important consideration when figuring out the final spherical error for a patient. To calculate the number of diopters to offset, we require the inverse of the distance in meters. For example, a distance of 25 cm (or 0.25 meters) will correspond to 4 diopters and one needs to subtract this from the final spherical correction.
- Procedure for Retinoscopy
Start on the right. Shine the retinoscope streak in the eye and move from one side to another. Note the reflex of the patient and whether it moves with or against the streak. Look at the reflex in different meridians when you rotate the streak’s axis. If a patient has astigmatism, the reflex will become dimmer and brighter on alternating axis.
- Script Adjustment
If cylindrical power is not present, we need to account for spherical error only. The acronym SPAM is a handy reminder. It stands for: “Same requires Plus. Against motion requires Minus”. Therefore, if the light reflex moves with the pupil, it is plus power and if light reflex moves against the motion of the pupil, it denotes negative power. Achieve neutrality by adjusting the spherical power dial of the phoropter. Once a bright reflex can be seen without any motion from the pupil, you have achieved neutrality.
- Adjustment for Astigmatism
For astigmatic patients, the reflex moves obliquely on side-to-side movement of the streak. Rotate the beam and make it parallel to the reflex motion. Neutralize on both eyes using the second meridian. Subtract the working distance before making the prescription.
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