Indirect opthalmoscopy with direct ophthalmoscope as light source

Ai-Hou Wang, M.D. Ph.D.

Pediatric ophthalmological examinations often use direct ophthalmoscopes for Hirschberg corneal light reflex strabismus test, interocular distance measurement during spectacle fitting, and other procedures. Clinical experience has shown that indirect ophthalmoscopy for fundus examination, including fundus torsion test, will be more convenient if using a direct ophthalmoscope as the light source (see figure).

How many of you ophthalmologists have actually done this?

If you look through the observing hole of a direct ophthalmoscope, corneal light reflex would interfere with the fundus image. Therefore, we usually view close to the upper edge of the ophthalmoscope. This is similar to Neitz handheld indirect ophthalmoscope, where the visual line is typically from its upper edge (see figure).

The upper part of the light source of Neitz handheld indirect ophthalmoscope is borderless, allowing for extremely close between the visual line and the light path. This is crucial for indirect ophthalmoscopic fundus examination.

In direct ophthalmoscopes, the light path is almost at the same level as the viewing hole. There is a space between the instrument border and the viewing hole. Among the direct ophthalmoscopes from four well-known brands – Neitz (Japan), Welch Allyn (USA), Heine (Germany), and Keeler (UK) (see figure) – Heine has a shortest of this space (d) (see figure). This configuration makes it best suitable for indirect ophthalmoscopy, when the visual line is from the upper edge of the instrument.

The optics of indirect ophthalmoscopy – specifically, the distance between the visual line and the light path – are similar to those of photorefraction. If the distance is too short (as observing through the viewing hole of direct ophthalmoscope), corneal light reflex will interfere; if it’s too long (as viewing along the upper border of direct ophthalmoscope), the dark zone of photorefraction will make the fundus invisible.

The formula for photorefraction is shown in the figure, where d is the distance between the visual line and the light path, and is the product of (1) the ratio of the dark area to the pupil (Dark Fraction), (2) the pupillary diameter, (3) the test distance, and (4) the relative refraction (see figure).

Relative refraction refers to the refraction with the reciprocal of the test distance as the zero point. For example, at a test distance of 50cm, -2D (1/0.5m) is the zero point. The relative refraction of -2D is 0D [(-2D)-(-2D)=0D], the relative refraction of -4D is -2D [(-4D)-(-2D)=-2D], and the relative refraction of +1D is +3D [(+1D)-(-2D)=+3D].

With a fixed d-value, fixed pupil size, and fixed testing distance, the relationship between refraction and dark fraction exhibits a hyperbolic relationship (see figure).

Many photorefractive instruments measure the light fraction of the pupil, also known as the crescent reflex. The light fraction is 1-dark fraction (DF). The relationship between refraction and light fraction is thus shown on the figure – the larger the refraction, the larger the light fraction (crescent). However, in high refractive errors, the size of the light fraction reaches a plateau, resulting in decreased resolution in reading the refractive value.

A large d-value results in a relatively large dark zone in areas of low refractive error (see figure), meaning that the red reflex of the pupil is not visible over a relatively large range of low refractive errors (most people have relatively low refractive errors). A small d-value results in a smaller dark zone, where the red reflex of the pupil is not visible only in a relatively small range of low refractive errors.

Returning to the topic of ‘using direct ophthalmoscope as the light source of indirect ophthalmoscopy, a larger d value will result in a larger dark zone. In most people with lower refractive error, the red reflex of the pupil will be invisible and the fundus examination will thus be impossible.

If direct ophthalmoscope manufacturers would make the edge above the viewing hole as shorter as possible, or even make it as the Neitz handheld indirect ophthalmoscope which has no upper edge, then direct and indirect ophthalmoscopes could be combined into one, which would be really terrific!