Subjective refraction

The following equipment is used to complete a Subjective Refraction: • Trial frames • Trial lens box & confirmation set: Including the pinhole and occluder • Jackson Cross Cylinder: a combination of two cylinders whose powers are numerically equal and of opposite sign (+/-) and whose axis are perpendicular to one another. This is used to search for astigmatism. • Snellen chart • Duochrome test: used to check the spherical component of the refraction == Performing the test ==

Test Requirements Before commencing a Subjective Refraction, ensure that: • The patient is seated at a 6-metre distance from the Snellen Chart. • The illumination in the testing room is at a comfortable level of brightness for an indoor setting Preliminaries • The examination begins by testing the patient's BCVA in both eyes separately, without correction. • 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 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 • A patient without astigmatism should be fully corrected after this step alone. However, if astigmatism is present, the aim of this step is to position the 2 focal lines so as to straddle the retina. This is known as the Circle of Confusion. • First we need to establish whether the patient is myopic or hypermetropic, therefore determining the need for a minus or plus lens to correct their refractive error. • Whilst holding the spherical lenses on the confirmation set centrally in front of the patient's eye, the clinician asks the patient: "Is it clearer with, or without the lens?" • Adjust the spherical component by adding, increasing and refining the power as required. Initially, adjust by 0.50DS, then refine with 0.25DS. • When prescribing minus lens correction, it is crucial that the clinician asks the patient if the letters appear clearer or if they appear smaller and darker (If smaller and darker, do not add the extra minus power). • This process continues until the patient states that their visual acuity is the same or the letter is just smaller and darker with the extra lens power. 2) SEARCH FOR Astigmatism • The Jackson cross cylinder (JCC) is used in this step. 4) REFINE CYLINDER POWER • The cylinder power is refined by superimposing the 0.50JCC over the correcting cylinder axis in the trial frames. • The patient is asked to indicate whether it is clearer with or without the lens. • Dioptric changes are then made in 0.25 increments in order refine cylindrical correction. 5) ADJUST FOR BEST SPHERE CORRECTION • Recheck VA and direct patient's fixation to the smallest line visible. • This step follows the principles in step 1. • Using a 0.50DS lens on the confirmation set, adjust spherical lens power and if needed refine with 0.25DS == Duochrome Test ==

of the eye. 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. == Recording ==

1) It is important to record the initial level of visual acuity, including the pinhole: • VA. sc R. 6/12 c ph: 6/9 L. 6/9 c ph: NI KEY: • sc = without correction • c ph = with pinhole • NI = no improvement 2) The final prescription is recorded as follows: • -2.50/-1.00 x 90° -> 6/5 KEY: • Spherical Power/Cylindrical Power x Axis -> BCVA achieved with the prescription 3) The Duochrome result is recorded as follows: • Duochrome: R=G KEY: • R = Red • G = Green == Evaluation ==

Overall, the reliability of subjective refraction is high. However, it comes with advantages and disadvantages. Advantages • Based on the patient's preference • Not invasive • Time effective Disadvantages • Relies on patient's cooperation and ability to provide accurate responses • Can be difficult when there are language barriers and communication issues • Age (more difficult with children) == See also ==