Fundus Examination in Pediatric Patients: Direct Ophthalmoscope and PanOptic Ophthalmoscope

Background

The direct ophthalmoscope head connects to a handle (the power source). The head contains plus and minus power lenses used to compensate for refractive errors. There is an illumination system and a viewing system (Figure 1). The illumination system is composed of an incandescent light source, an aperture, two lenses and a small 45^ᵒ mirror (Timberlake & Kennedy, 2005). It also contains a wheel that has multiple different apertures. Most direct ophthalmoscopes have two or three different size white light circles, blue and green filters, a slit and a visuoscopy target. A half circle is another type of aperture available. The viewing system is composed of condensing lenses and a viewing aperture. Condensing lenses can range in power from high plus to high minus lenses with the range varying depending on the ophthalmoscope manufacturer. The role of the condensing lenses is to allow for the image of the retina to be brought into focus if the eye is not emmetropic.

Figure 1.

Image of the different systems that compose a direct ophthalmoscope. Source: Tak-Man Kimberly Fung OD, 2021

(Adapted from [Cordero, 2016])

In the viewing system, the light rays are reflected off the patient’s retina and exit through their pupil. The light rays then pass through the lens in the direct ophthalmoscope and enter the examiner’s eye and create an image on the examiner’s retina. This image the examiner sees is a virtual, upright and magnified image. The angular magnification produced by this system is approximately 15X. The standard field of view with a direct ophthalmoscope is 5^ᵒ (Cordero, 2016). With an emmetropic patient, there is no need for a compensating lens to focus the image on the examiner’s retina, because the rays that leave the patient’s eye are parallel and ultimately the light rays comes into focus on the examiner’s retina (Figure 2). This is not the case when the patient is ametropic.

With a myopic patient, the light rays converge as they exit the patient’s eye, therefore the light rays will come into focus in front of the examiner’s retina. To correct this, a concave lens is needed to diverge the rays to make them parallel to ultimately allow a clear image to be produced on the examiner’s retina (Figure 3). The power of the concave lens needed depends on what the patient’s refractive error is and the distance from the patient that this technique is being performed. With a hyperopic patient, the light rays come into focus behind the examiner’s retina, therefore a convex lens is needed to converge the light rays so that they come into focus on the examiner’s retina (Figure 4). The power of the lens needed for the hyperope is determined the same way as previously stated for a myope.

Figure 2.

A simplified ray tracing diagram of an emmetropic observer (O) examining the retina of an emmetropic subject (S) using a direct ophthalmoscope. Fp: focal point. Source: Tak-Man Kimberly Fung OD, 2021

(Adapted from [Timberlake & Kennedy, 2005]).