What is it?
A small aperture or an opening or hole in an opaque disc that is commonly used to detect and differentiate between a pathological error from a refractive based error as cause of reduced vision. The pin hole could also help predict the potential post operative visual acuity in patients with cataract.
How does it work?
In any refractive based error, like myopia or hyperopia, the rays of light do not fall on a single point on the macula. This causes a diffused smeared quality of image, or a blur circle on the retina. The pinhole cuts down the peripheral rays thus reducing the blur circle and thereby increasing the depth of focus, hence increasing vision. Depth of focus is a range of area over which an object remains to be seen fairly sharp. Think of a butterfly in the air. With greater depth of focus, you will be able to see over a distance as it moves away in different directions from your eye. Thus the image is equally sharp within the range.
The pin hole also helps by cutting down the scattering of light if corneal opacities ( and lenticular opacities in case of phakic eye) is present. A cornea (or lens) with high aberration may benefit with a pin hole and vision often improves.
The depth of focus increases as the pin hole size decreases. In effect there are different sizes of pin hole, but the most common ones used in clinics are of around 1mm to 1.75 mm of size, with most useful size the provides a balance between diffraction, retinal illumination and depth of focus is around 1.2 mm. Less than 1 mm size of pin hole may not be beneficial due to the effect of diffraction and scattering of light.
Careful placement of pinhole over the centre of the patient’s miotic pupil helps to get the best benefit. If the small aperture of the pin hole is not aligned to the patient’s miotic pupil, it may not give the best of results.
Pinhole as a diagnostic tool?
After checking the patient with a pinhole, the following could happen:
1. Vision improves and the patient may see more than his/her uncorrected vision.
Interpretation: Refractive error present. In case of post IOL implantation, it would largely mean a biometry or ELP error
2. Vision does not improve even with pin hole
Interpretation: Ocular or even neurological pathology could be present. Check the retina or ocular condition in the anterior segment.
3. Vision declines with a pin hole
Interpretation: Central opacities either in the retina (macula) or corneal opacities. In case of phakic patients, this may indicate lenticular opacities.
4. Vision improves to better than BCVA
Interpretation: Aberropia, lens tilt, irregular astigmatism