The visual impairment associated with HPS is very similar to that of OculoCutaneous Albinism (OCA). There are three key differences to those of normal eyes.
The wearing of prescription glasses cannot correct the above physical differences.
The fovea is a small but important area of the retina inside the eye. The function of the retina is to sense patterns of light (images) that enter the eye and to send nerve signals to the brain about what the eye sees. The fovea is the area of the retina capable of the sharpest vision, such as reading. In HPS and Albinism the retina and the foveae are underdeveloped. This is called ‘retinal hypoplasia’. This occurs because the developing eye lacks a pigment called melanin. Melanin enables the cells of the retinas and optic nerves to develop correctly as a foetus grows.
The effect is to reduce the eyes ability to see detail. The term ‘Visual Acuity’ refers to how much detail can be seen. The better ‘acuity’ is the finer the detail that can be seen. In HPS and albinism acuity is typically low. Click here for more.
Studies have shown that people with albinism have an unusual pattern of nerve fibres between the retinas and those areas of the brain involved with vision. Each eye has an optic nerve that contains nerve fibres. The diagram to the right shows that in normal eyes some of the nerve fibres from each eye cross over and connect to structures on the opposite of the brain. This is called ‘decussation’. In HPS and albinism too many nerve fibres cross over, there is too much decussation. This unusual pattern prevents the eyes from working well together and causes strabismus, poor depth perception, poor binocular (3D) vision. Click here for more.
Melanin in the eyes
Structures in the eye lack melanin pigment. In normal eyes there is a layer of melanin containing cells behind the iris (called the melanin epithelium). Normally the melanin reduces the amount of light that enters the eyes. The melanin is either completely absent or reduced in the irises (Irids) of people who have HPS or albinism.
There is also a layer of melanin epithelium behind the retinas in normal eyes. The melanin stops light from being reflected back from the inside surface of the eye (the fundus or fundi). It is important for producing a crisp image on the retina. Again this is absent or very reduced in people with HPS or albinism. This causes the image on the retinas to be blurred.
This red reflex is similar to that which occurs when a flash photograph is taken of a person looking directly at the camera, and the eyes appear red. Ophthalmologists sometimes shine a light into patients’ eye when they suspect albinism to see if the light reflects back through both the pupil and the iris. This is called ‘Iris trans-