Refractive errors 

The brain is not able to form a clear image if the light waves are not focused on the retina. A short-sighted (or near-sighted, myopic) eye for example would have its image formed anterior to the retina due to the eyeball being long lengthwise. In other words, an object at a distance would appear blurry, a condition also known as myopia. A long-sighted (or far-sighted, hypermetropic) eye would have it formed posterior to the retina due to the eyeball being short. Close objects would appear blurry in this condition also known as presbyopia. Such problems are known as refractive errors.

Depending on age, ethnicity and health conditions, biannually, annually or biennially eye tests are recommended. In addition to those, some countries have diabetic eye screening programmes in place to detect diabetic retinopathy at an early stage. In the UK for example, “from the age of 12, all people with diabetes are offered an annual diabetic eye test to check for early signs of diabetic retinopathy.” (NHS, 2017) This is motivated by the following facts:

• Diabetic retinopathy is the leading cause of preventable blindness in the 20-74 years old population in western countries.

• Diabetes diagnoses are growing exponentially around the world

• Almost all patients with diabetes type 1 and > 60% of diabetes type 2 who have had the disease for over 20 years develop retinopathy.
• Retinopathy can generally be treated if detected early. If the retina is not severely damaged, even patients presenting with an advanced retinopathy have a 95% chance of keeping their vision when they get treatment.

The International Council of Ophthalmology recommends that a retinal examination be performed either by direct or indirect ophthalmoscopy or slit-lamp biomicroscopy of the retina, or by “retinal (fundus) photography with any of the following: 30-degree to wide field, mono or stereo photography, and dilated or undilated photography” (Vujosevic et al., 2020) . For the latter, telemedicine is a suitable approach.

Direct ophthalmoscope: handheld instrument that consists of an eye piece, a light source, a concave mirror, a set of lenses and a handle containing batteries. Most commonly used during routine examinations. The image of the fundus can be magnified up to 15x and is rendered upright but it is difficult to visualize the peripheral retina using a direct ophthalmoscope. No 3D rendition of the interior of the eye is available. Click here to learn more about direct ophthalmoscopes and their working principles.

Indirect ophthalmoscope: comes in monocular or binocular form and produces an inverted image. In both types, the field of view is wider than with the direct ophthalmoscope but the magnification is lower. With the binocular version, a 3D rendition of the interior of the eye is obtained which allows for a more thorough examination and therefore makes it a more popular choice. The peripheral retina can be viewed. Most commonly used for patients presenting with an eye condition or certain complexities. Typically consists of a headband, binocular lens with mirrors and a light source.

Retinoscopy is a procedure used to measure how well one can see objects at varying distance and determine the refractive error so that vision correction can be adapted. Its purpose is to inspect the light moving across the fundus. It is an indispensable test for paediatric patients and can prove very helpful with non-communicative and non-cooperative patients. Using a retinoscope, the examiner can determine the correct prescription for eye glasses or lenses without needing the patient’s feedback. 

Streak retinoscope

Reflects a beam of light from a linear light source. 

• Operates with plane mirror effect (plano mode) or concave mirror effect

• Ideal to detect astigmatism
• More commonly used as offers dual modes

Spot retinoscope

Reflects a beam of light from a circular light source.

• Operates with plane mirror effect

• More visual information available (optical phenomena that take place away from the center, subtle changes in brightness, colour or cylinder…) 

This video created by Dr. Tim Root from OphthoBook brilliantly explains, in a simplified manner, the working principle of the retinoscope.

Ophthalmoscopes and retinoscopes come with either a filament bulb relying on a gas (halogen or xenon) or an LED bulb relying on diodes and electrons. 

Halogen bulbs are the least efficient in terms of lighting, lifetime and heat release, but they are usually the cheapest. Xenon bulbs last longer, produce a brighter and whiter light (the halogen is more yellowish; the xenon, blueish) and release less heat. LED bulbs offer similar improvements as the xenon bulbs but take them one step further – while LEDs typically shine less bright than xenon light, they produce a more accurate pigmentation, stay cool to touch and last exponentially longer, making them cost effective despite the higher cost upfront.