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Stenopaic Slit Refraction
PURPOSE
To subjectively determine the power required to correct the refractive error of each major meridian individually．
INDICATIONS
The stenopaic slit refraction is useful for confirming the results of other refraction techniques for patients with irregular astigmatism or reduced visual acuity．It is helpful for patients who have difficulty understanding the complex instructions associated with other subjective techniques．
It is important to note that, like the pinhole，the stenopaic slit may be used diagnostically to determine a patient’s potential visual acuity．
EQUIPMENT
·Trial frame．
·1 mm stenopaic slit (2 to 3 slits of various widths are usually included in trial lens kits)．
·Trial lens kit or lens rack．
·Distance visual acuity chart．
SET-UP
·Place the trial frame on the patient’s face so the patient is comfortable and his eyes are centered relative to the lens wells (see the section on trial frame refraction)．
·Project the visual acuity chart．Expose a range of letters in which the patient’s best known acuity corresponds to the lowest line．For example, if the patient’s VA is 20/40, expose lines ranging from 20/70 down to 20/40．
STEP-BY-STEP PROCEDURE
1．Occlude the patient’s left eye．
2．Using a lens rack, perform an MPMVA on the right eye．
3．Add +1.00 to +1.50 diopters to the result found in step 2 and place a lens of this power in the rear cell of the trial frame．
4．Place a 1 mm wide stenopaic slit in the trial frame in front of the fogging lens．If the patient’s acuity is significantly reduced it may be necessary to use a wider slit．
5．Rotate the slit until the position of best acuity is found．At this time, the slit will run parallel to the axis of the correcting minus cylinder．
6．Decrease the fog, using a lens rack．until MPMVA is achieved．
7．The combined power of the lens in the lens rack and the fogging lens is the correction for the meridian which runs parallel to the stenopaic slit．Record the power and the meridian on an optical cross．
8．Remove the lens rack from in front of the eye and rotate the stenopaic slit until the position of worst acuity is found．If this is not 90° away from the meridian found in step 5, an irregular astigmatism is present．
9．Again, reduce the fog using an MPMVA．
10．Record the resulting power and meridian as in step 7．
11．Calculate the resulting spherocylindrical correction and place the correction in the trial frame．Remember to remove the stenopaic slit and the fogging lens．If an irregular astigmatism is present，the resulting spherocylindrical correction may be determined by placing two cylinders in a trial frame and measuring the total power through the lensometer or by calculating the result using a formula for obliquely crossed cylinders．
12．Measure the patient’s VA for the right eye．
13．Occlude the patient’s right eye and repeat steps 2 through l2 for the left eye．
RECORDING
·Record the refraction technique used．
·If the patient has a regular astigmatism, record the spherocylindrical correction in minus cylinder form for each eye．
·If the patient has an irregular astigmatism record the correction for the major meridians separately．
·Record the visual acuity for each eye．
EXAMPLE
·Refraction (Stenopaic Slit)
0D +1.00=-1.00×90 20/20+2
OS +0.75=-1.25×90 20/20+1
·Refraction (Stenopaic Slit)
OD -3.50 at 170, -5.75 at 105 20/70
0S -4.00 at 160, -5.00 at 80 20/40
Note：“at” indicates the meridian of power，not the axis． |
