Subjective refraction

Subjective Refraction is an attempt to determine, by trial and error using the patient’s cooperation, the combination of lenses that will provide the best corrected visual acuity (BCVA).[1] It is a clinical examination used by orthoptists, optometrists and ophthalmologists to determine a patient's need for refractive correction, in the form of glasses or contact lenses. The aim is to improve current unaided vision or vision with current glasses.

(Top) 0.50 Confirmation Set(Middle) Trial lens box, including pinhole and occluder(Bottom) Snellen Chart

Equipment & Requirements

The following equipment is used to complete a Subjective Refraction:

Performing the test

Test Requirements

Before commencing a Subjective Refraction, ensure that:

Trial frames fitted comfortably on the patient and occluder over left eye.

Setting up the patient

  1. Comfortably fit the trial frames onto the patient, by adjusting the nose piece, Inter-Pupillary Distance (IPD) and vertex distance to ensure that they are properly centered.[4]

Preliminaries

  1. The examination begins by testing the patient's BCVA in both eyes separately, without correction.[4] Conventionally, the right eye is tested first. An occluder is placed over the eye that is not being tested (e.g.: over the left eye, to test the right eye's vision).
  2. A pinhole occluder is then placed before the patient's eye, and their vision is then tested again (each eye separately) to determine if the patient's poor visual acuity is a result of optical irregularities, or pathological issues. If the patient is able to read more lines on the Snellen chart with the use of the pinhole, this indicates the presence of refractive error. This is based on the principle that the pinhole blocks out any peripheral rays of light, so that only the principal ray falls on the fovea, decreasing the size of blur circles.[5]
  3. In the presence of refractive error in most patients, visual acuity will improve with the use of the pinhole. The examiner aims to achieve this level of visual acuity, or better, by the end of the Subjective Refraction.

Steps

Step 1: Using the +/-0.50DS on the confirmation set to determine the initial best sphere correction.
Step 2: Presenting the 0.50JCC initially @ 90deg to determine any presence of astigmatism on that axis.
Step 3: The axis of the JCC must straddle the axis of the correcting cylinder in the trial frames, in both flip positions.
Step 4: The axis of the JCC must superimpose the axis of the correcting cylinder lens in the trial frames.
Step 5: Spherical lens power is altered and refined, if required.

The entire process of Subjective Refraction involves the patient fixating at the Snellen Chart, whilst the clinician presents a variety of lenses and alters the power of the lenses in the trial frames according to the patient's subjective responses regarding improvements to their vision.

1) INITIAL BEST SPHERE CORRECTION

2) SEARCH FOR ASTIGMATISM

3) REFINE CYLINDER AXIS

4) REFINE CYLINDER POWER

5) ADJUST FOR BEST SPHERE CORRECTION

WATCH THIS TUTORIAL AND PATIENT DEMONSTRATION OF SUBJECTIVE REFRACTION

Duochrome Test

"Do the letters stand out more on the red or green background?"
Duochrome is based on chromatic aberration of the eye.
The patient's subjective response to the duochrome test is a perfect indicator as to whether you have under-corrected, over-corrected, or adequately corrected their refractive error during the subjective refraction process.

The red-green duochrome test is performed monocularly, and is used to refine the spherical component of the refraction. It is based on the principles of chromatic aberration; red (longer wavelength) is refracted less than green (the shorter wavelength). Therefore, a myope (generally with a longer axial length) sees red clearer as red focuses closer to retina than green. The examiner asks the patient: "Do the black letters stand out more on the red or green background? Or do they appear equal?"
Neutrality is achieved when the patient subjectively reports that the letters on both backgrounds appear equally as prominent.[7]

Recording

1) It is important to record the initial level of visual acuity, including the pinhole:

KEY:

2) The final prescription is recorded as follows:

KEY:

3) The Duochrome result is recorded as follows:

KEY:

Evaluation

Overall, the reliability of subjective refraction is high. However, it comes with advantages and disadvatages.

Advantages

Disadvantages

References

  1. Khurana. (2008). Theory and Practice of Optics and Refraction (2nd ed.). Elsevier: Okhla, New Delhi. pp147
  2. Rabbetts, R.B. (1998). Bennett and Rabbett‟s Clinical Visual Optics. (3rd ed.). Oxford: Butterworth-Heinemann. pp99
  3. 1 2 Rabbetts, R.B. (1998). Bennett and Rabbett‟s Clinical Visual Optics. (3rd ed.). Oxford: Butterworth-Heinemann. pp94
  4. 1 2 3 4 5 Khurana. (2008). Theory and Practice of Optics and Refraction (2nd ed.). Elsevier: Okhla, New Delhi. pp148
  5. Benjamin, W.J. (2006). Borish's Clinical Refraction (2nd ed.) Elsevier: Butterworth-Heinemann.
  6. 1 2 3 4 5 6 7 8 Michaels, D. D. (1980). Visual Optics and Refraction: A Clinical Approach. (2nd ed.). Missouri: C.V. Mosby Company. pp379-383
  7. Colligon-Bradley, P (1992). "Red-green Duochrome Test". Journal of Ophthalmic nursing and technology. 11 (5): 246. PMID 1469739.

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