Structure and function of the eye

In this section we will give information about the structure and function of the eyes, and how vision is affected by Usher syndrome.

The eye consists of several parts that are somewhat similar to a camera

  • Sclera:  the white material which we normally see as the white of the eye. It is the eye‘s outer protective coat.
  • Cornea:  the transparent, curved structure at the front of the eye
  • Iris:  through the cornea the iris can be seen, which is that part of the eye which gives it colour – blue, brown, green, grey etc. It is a circular muscle which responds to bright light by contracting, making the pupil smaller.
  • Pupil:  in the middle of the iris is the pupil, a transparent aperture which appears black. ​

The pupil constricts in response to bright light and dilates in low levels of light.

  • Lens:  situated immediately behind the iris and pupil is a transparent disc with both sides being convex. The lens focuses light onto the retina. It divides the eye into two parts – anterior and posterior chambers.
  • Anterior Chamber:  a fluid with the consistency of water circulates around this chamber. This fluid is called the aqueous humour.
  • Posterior Chamber:  this is filled by the vitreous humour which has the appearance of transparent jelly.
  • Retina:  situated at the back of the eye, the retina consists of millions of nerve cells which are divided into two main groups – rods and cones. They are so described because of their appearance under a microscope.
  • Cones: concentrated around an area of the retina called the macula.
  • Rods: although some are placed near the macula, the majority of rods are spread out to cover the rest of the retina.
  • Retinal Pigment Epithelium:  a dark coloured layer of cells underlying the retina responsible for providing oxygen and other nutrients to the rods and cones.
  • Choroid:  located behind the retina, it consists of a large network of blood vessels that transport oxygen and other nutrients to the retinal pigment cells.
  • Optic Disc: the nerve cell connections from all the rods and cones travel to the optic disc, a small, yellow, oval structure which can be seen through an ophthalmoscope.
  • Optic Nerve and beyond:   the optic disc is the front part of the optic nerve which passes from the eye to make connections throughout the brain.​

How vision is affected by Usher syndrome

Vision loss in Usher syndrome is caused by retinitis pigmentosa (RP), which affects the area at the back of the eye which senses light (the retina). The first symptom is usually difficulty seeing in the dark, followed by progressive loss of peripheral vision (reduced visual field).

Retinitis Pigmentosa

Retinitis Pigmentosa also known as RP is an inherited eye condition affecting the retina light-sensitive tissue at the back of the eye (retina). When optometrists or ophthalmologists use an ophthalmoscope to look into the eye of a person with RP, they usually see scattered areas of pigment across the retina. By contrast, in a normally sighted person, the pigmented cells are covered by the living nerve cells – the rods and cones. The pigmented appearance gives rise to the name – retinitis pigmentosa.

People affected with RP have difficulties with seeing at night, loss of peripheral (side) vision and may have a tendency to trip over curbs and stairs. Symptoms and rate of visual loss can vary between individuals. At present there is no treatment or cure, but many research groups both in UK and overseas are looking at potential future therapies. Although there is no current treatment for Usher syndrome, it is important to maintain eye checks as cataracts and macula odema (swelling) can be associated with RP and are able to be managed.

When do symptoms of RP begin?

Individuals diagnosed with Usher syndrome type 1 will usually start to develop symptoms of RP within the first decade of their life. Those with Usher syndrome type 2, the symptoms usually appear in the second decade of life.
The rate of vision loss is usually slow, this is why the diagnosis without genetic testing is often not made until the child is in their early teens.

A child having an eye test

Optical coherence tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section pictures of your retina, the light-sensitive tissue lining the back of the eye.

How is vision tested in children?

There are many procedures used to test different visual functions. Some tests can be carried out by professionals such as nurses, special education teachers or general practitioners, while others may require more specialist knowledge or equipment such that an optometrist or ophthalmologist may possess.

Visual Acuity

Visual Acuity measures the ability to read the letters on an eye chart. It is the most common clinical measurement of visual function. The standard definition of normal vision is 6/6 (metric) or 20/20 (imperial). In young children vision is measured using symbols and pictures. Children who can’t understand these tests are shown different sized stripes and their behaviour is observed.

Visual Fields

A Visual Field test measures the field of vision (side vision). This is where you look straight ahead at a particular point in a bowl-shaped machine. Each time you spot dots of light you click a button. This is referred to as computerized visual field testing. Alternatively, it can be performed manually whereby a light is brought in from the side of the machine and slowly moved to the center of vision. You press a button as soon as you see the light. The results are then marked on a chart and compared with the results a person with normal vision would be expected to see. A normal visual field is described as being about 140 degrees in every direction. Assessment of visual fields becomes relevant around 8 years of age.

Colour Vision

Colour vision can be assessed by different methods, but the most commonly used test is the Ishihara Colour Vision Test. The Ishihara is a booklet that shows numbers composed of different coloured dots. The numbers are printed within different coloured dotty backgrounds.

Electroretinogram (ERG)

The ERG is used to measure the function of the retina and identify layers of the retina that are not working properly. In some cases it can detect very early signs of RP. It involves placing special electrodes either in the eye or close to the eye. Flashes of light are used to stimulate the retina and the electrodes measure the electrical response of the retinal cells to the flashing lights.

Optical Coherence Tomography (OCT)

The OCT takes cross- sectional images of the retina. It can measure the thickness of the retina and detect any retinal abnormalities such as macula odema which can occur in patients with RP.

Fundus photographs

Using special cameras, photographs are taken of the retina to document RP related eye changes.

A child having an eye test

The slit lamp microscope magnifies the retina during a routine Ophthalmology eye test.

With thanks to medical experts who have provided and verified the information contained in this section.

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