In this section we explore the structure and function of the eye, retinitis pigmentosa (RP) and how vision is tested in children.
The eye consists of several parts that are somewhat similar to a camera
The pupil constricts in response to bright light and dilates in low levels of light.
The eye condition those with Usher syndrome suffer with is Retinitis Pigmentosa (RP). RP is caused by a degeneration of the cells at the back of the eye reducing the persons ability to see in dark conditions, along with a narrowing of the field of vision. You can read more about RP in the next section 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 Australia 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. It is also a good way to keep
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.
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.
The below educational video titled Assessment of Retinitis Pigmentosa, Eye Appointments and Eye Care & the Importance of Regular Eye Monitoring will help viewers gain an understanding of the various tests to assess visual function, retinal imaging and specialised vision electrophysiology assessments for patients with Usher syndrome and how clinicians can work with families to provide the best outcomes from eye appointments.
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.
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 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.
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.
Using special cameras, photographs are taken of the retina to document RP related eye changes.