Initiating The Process

Initiating the process during clinical refraction refers to establishing the goal and direction that allows the practitioner to achieve the desired end results. The examiner applies certain procedures that allow him to get into the refractive status of the patient, and then he makes the objective evaluation of the

refractive error. Based on the information gathered, the examiner makes the predictions about the refractive status and decides a course that will lead him to write the final prescription. The flow of the process should follow the sequence:

1. History taking

2. Lensometry

3. Visual acuity test

4. Objective evaluation of refractive error

5. Think and go ahead.

History Taking

The process of refraction starts with extensive case history. History taking establishes a relation between the patient and the practitioner and helps the practitioner in differential diagnosis and patient education. It should start with an aim to establish the reason for the patient’s visit. The questions may be asked in the following order:

1. What is the reason for your visit today?

2. Is it a routine check up or you have specific reason?

3. Ask the chief complains. Each complains should be enquired with its frequency, onset, duration,          associated factors and severity.

4. The practitioner needs to understand the patient’s visual demands. Ask questions like What do you      do? What are your hobbies? Do you drive?

5. When was your last eye examination done? By whom and what was its outcome?

6. How long have you been using the glasses? Is it for distance vision or near vision or both? Can            you see clearly and comfortably with them now?

7. General health including pressure and diabetes.

8. Family history of cataract, glaucoma or blindness. A brief record of all information should be              made. Care must be taken to record the information derived in the manner it is being presented by     the patient.

Lensometry

If the patient is already an user of spectacles, the lens power of the old spectacle should be found out using electronic equipment known as Lensometer. The procedure as how to use the equipment has been dealt with separately. The lens power of the old spectacle lens provides an important platform on which the refraction procedure can be initiated. Sometimes it is also used as initial trial lenses. It also helps during the intellectual process to decide the final lens prescription and helps to compare

the refractive status of the patient’s eyes during the process of counseling. Hand neutralization is an alternate method which utilizes loose trial lenses of known power to find the power of unknown spectacle lenses.

Visual Acuity Test

Visual acuity is the spatial resolving capacity of the visual system. It is the ability to see and discriminate a certain size target at a specified distance. Monocular visual acuity must be measured

on all patients following the history taking at a distance of 20 feet or 6 meters. This is most commonly measured using Snellen’s Acuity Charts. Normal acuity on Snellen’s Test Chart for a healthy eye is recorded as 6/6 or 20/20. A line is said to be read when more than its half of the letters are identified correctly. The optical system should project sharp image of the object onto the retina.

The retina must be able to translate the image into the neural impulse and finally the neural impulse must travel to the brain, where they are interpreted. Thus the visual acuity is the good indicator that all the levels of the visual system is functioning properly. The measurement of visual acuity is done when the room is sufficiently illuminated so that the extra illumination will not improve the visual acuity readings. If the patient cannot read the largest letter in the test chart at the standard distance,reduce the test distance between the patient and the chart. In case the patient cannot see letters at any distance, you may follow the following sequential procedure to record the acuity:

1. Counting fingers

2. Hand motion

3. Light projection

4. Light perception.

Visual acuity needs to be recorded in three states of condition:

1. Uncorrected visual acuity (UCVA)

2. Visual acuity with corrected correction (VAcc)

3. Pinhole acuity.

Uncorrected visual acuity (UCVA)

UCVA refers to the measurement of acuity when no glasses or contact lenses are used and is most commonly measured immediately after history taking. This is an important measurement to know the current refractive status of the eye and it becomes a benchmark against which the benefits of using refractive correction may be referred. Care must be taken to ensure that the patient does not squint or reduce the palpebral aperture to reduce the blur created by defocus or optical irregularities.

Visual acuity with corrected correction(VAcc)

A well taken visual acuity measurement is critical to ensure an accurate spectacle correction—just right …..not too strong….. and not too weak. This implies that examiner is looking for the

finest detail that the visual system can resolve. Measuring the visual acuity with existing correction reveals information as to the possibilities of improvements. Many a times the patient carries

a perception that his acuity is better with his old correction than the new one. Recording the acuity with old correction provides opportunity to compare with the new correction and is also a

strong indicator on which the subjective refraction may be initiated.

Pinhole acuity

Pinhole acuity can be measured if the visual acuity is worse than

20/30. It is a critical measurement to determine whether the

decreased visual acuity is correctable by lenses. When pinhole

acuity is used, the size of the blur circle on the retina created by

uncorrected refractive error is reduced which establishes that

better visual acuity may be improved with refraction .

Care must be taken in patients with keratoconus and cortical or

posterior subcapsular cataracts because it can channel the light

through a better region of the eye’s optics. But it will not have

significant impact on visual acuity that is reduced because of

amblyopia or retinal disorder.

Pinhole Test

Objective Evaluation of Refractive Error

Objective refraction is the method to determine the refractive error

of an eye without taking any responses from the patient,i.e.

the patient is not asked to read anything and provide any input

to the practitioner. This gives a good starting point for the subjective

refraction for the regular patients and is the only technique

for the patient whose subjective responses are absent, limited or

unreliable. There are several ways to obtain an estimate of refractive

error of a patient objectively. The most common of them are:

1. Retinoscopy

2. Keratometry

3. Autorefractometry.

Retinoscopy

Retinoscopy has gained the recognition of gold standard and is

an excellent method for not only estimating the refractive error,

but also to serve as first opportunity to view the internal structure

of the eye. However, accurate measurement requires a lot

of training and practice. It is best done on undilated eyes with

an exceptions of patients with pupil less than 2mm and young

patients with active accommodation. In case the retinoscopy is

performed on a dilated eye, ignore the confusing reflexes seen

at the pupil edge and pay attention to central reflex only. 

video of retinoscopy is given below:

Keratometry

Keratometry is an instrument used to measure the front surface

curvature of the cornea. The readings provide information as to

the corneal astigmatism which may be used as baseline cylinder

correction needed for the patient. They are also of great importance

for fitting contact lenses and for calculating the power of

intraocular lens to be implanted.

Axis of the corneal astigmatism can be measured by

rotating the keratometer tube until the left mire and the focus-

ing mire plus signs are not staggered and perfectly in line. For

moderate to high corneal astigmatism, this is simple. To verify

alignment for low astigmatism, the focusing mire can be thrown

slightly out of focus, and the left mire plus sign should line up

exactly between the doubled plusses.

With two position instruments, rotate the body of the keratometer

through 90° to locate and align the second meridian.

An estimation of corneal astigmatism can be done by measuring

the curvatures of the cornea at two principal meridians

and recording the values as under:

Flat K Values: 44.00D @ 180° or 7.80mm @ 180°

Steep K Values: 45.00D @ 90° or 7.70mm @ 90°

Estimated Corneal Astigmatism : -1.00D @ 180°

However, the amount of astigmatism derived by keratometer

may differ from that of spectacle refraction. It may be more than

the spectacle refraction, or less than the spectacle refraction or

may be same.

Autorefractometry

Autorefractometer is an automated, fastest and very reliable

method of objective refraction. The results obtained from

autorefractometer are highly repeatable. It not only provides

several estimates of the refractive error but also suggests an

average reading. Some autorefractometers also provide the

reliability coefficient values of the readings taken which helps

the examiner judge the accuracy of the readings.

Autorefractometer basically comprises of an infrared source

(800–900nm), a fixation target and a Badal Optometer. The light

is reflected back from the deeper layer of the eyes and the ocular

refraction is determined by measuring how the light is changed

as it enters a person’s eye. The fixation targets are used to relax

the accommodation from less interesting stars to pictures with

peripheral blur to further relax the accommodation.

The procedure takes hardly a few seconds. The patient is

seated on a chair behind the autorefractometer and is asked to

rest his chin on the chinrest with brow against the forehead rod.

He is asked to fixate at the picture inside the autorefractometer.

The operator must align the instrument on the center of the

entrance pupil and focus the instrument on the plane of the

pupil (iris). This is usually performed with the use of joystick

and the reading is displayed on the display panel. Before each

reading target is blurred which comes back to focus when the

reading is displayed. Several readings are taken that the machine

averages to form a prescription. No feedback is required from

the patient during this process. Within seconds an approximate

measurement of a person’s prescription can be made by the

machine and printed out.

Although many studies have evaluated the accuracy

and repeatability of autorefractometer relative to subjective

refraction, the ability of patients to adapt and tolerate these

prescriptions has not been addressed. Experience has shown that

autorefractometer usually provides minus spherical correction

at a higher side. This could be probably because of the chromatic

aberration coupled with ocular spherical aberration.

Autorefractometer readings are unobtainable in the following

cases:

1. Small pupils— In such cases the patient may be asked to sit

    in the dark room for some time before putting him onto the

    autorefractometer.

2. Inadequate fixation.

3. Opacities or cloudiness of ocular medias.

4. Certain geriatric and pediatric patients are difficult to

    measure because of their inability to keep the head in

    position and eyes fixated, and patients with Parkinson’s

    disease or nystagmus may prove impossible to clinically

    perform autorefractometry.

5. Some posterior segment abnormalities reduce the intensity

    and definition of fundus reflex.

6. Corneal irregularities arising naturally or resulting from

    refractive surgical procedures can cause autorefractometer

    results to differ from subjective results.

7. Ametropia beyond the range of the instruments.

8. Accommodative abnormalities as a result of such latent

    hypermetropia or pseudomyopia. Young patients with active

    accommodative spasm may produce more in minus than

    revealed in retinoscopy or subjective refraction and the

    amount of over minus reduces with age.

Think and Go Ahead

Now with the completion of first step of the clinical refraction

the examiner gathers the information about patient’s history, his chief complains, current status of the visual system of the eye and also the results of the objective assessment of the refractive

errors. Its time to relate all collected information, make a judgment

as to the further course of action and decide a goal for

the treatment. The routine visit implies that the patient has no

complains with his old correction, whereas visit with specific

reason asks the examiner to address the same. An asthenopic

symptoms may be relieved by looking at cylinder and its axis

correction. Reduced vision with habitual correction implies that

a change in refraction may yield an improvement in acuity. A

quick and in depth brain storming exercise is needed to relate

the symptoms with the objective assessment results. The general

guideline for probable refractive error which is given as under

has to be kept in the mind during the process:

The idea behind this exercise is to think ahead the nearest

final prescription and set a goal for the rest of the process. The

process also helps to decide the selection of the initial lenses for

subjective refraction to be followed.