We recommend having an eye examination at least every two years, or more frequently, if advised by the optometrist. The importance of regular eye tests is not solely, to determine the need for glasses, but also, to assess the general health of the eyes. Signs of many underlying health conditions, such as, Diabetes, High Blood Pressure, High Cholesterol can be detected during the eye examination.
The Eye Examination is a procedure carried out by an optometrist to determine if a patient requires spectacles and if so to determine the correct prescription for him/her. The examination also involves checking to see if the eyes are healthy. An optometrist cannot diagnose disease but he/she is trained to recognize pathology and will refer the patient on to the relevant specialist.
An optometrist will follow a certain routine during an examination. This routine may vary from individual to individual but the basic procedure would be as follows:
Initial discussion with the patient.
This would include problems the patient is experiencing, e.g. headaches, difficulty seeing up close or in the distance.
It would also include discussion on the patient’s general health and medical history. It is important that the optometrist is aware of this as they can have a direct effect on the eye and vision, e.g. arthritis may cause a dry eye or other problems (i.e. difficulty in reading). Someone on long-term steroid treatment may have raised Intraocular pressure (i.e. glaucoma). Family history is also important. Certain conditions may be hereditary such as glaucoma and diabetes.
The optometrist will then check the patient’s distance vision by seeing how far down the letter chart he/she can read. This is done without any spectacles (i.e. unaided vision) and with the patient’s present spectacles (i.e. aided vision) if they have a pair.
This is a method the optometrist uses to determine the prescription the patient may need without actually getting him/her to read the letter chart, i.e. it is an objective test.
The optometrist uses a retinoscope to shine a light on the back of the eye (the retina) and he/she sees a red reflection called the retinal reflex. When the optometrist moves the retinoscope this reflex moves also. He/she will put test or trial lenses in the trial frame in front of the eye until the reflex stops moving. This is called neutralisation and gives the objective result of the power of the spectacles needed for the patient to see clearly.
This is a very helpful procedure where the patient may be unable to communicate to the optometrist, e.g. a mute patient or a very young child.
Some optometrists may use an auto-refractor to determine this objective result. This is an automated instrument, which will give an estimation of the prescription required, it is useful in giving the optometrist a starting point for his/her subjective test.
Subjective Test (Refraction)
The optometrist will now determine the final strength of the spectacles required. In this case the patient is fully involved (i.e. it is subjective) and will read the letters on the letter chart for the optometrist. The optometrist will show stronger and weaker lenses to the patient until the best vision is obtained. The optometrist will also determine if there is any astigmatism present by using a lens called a cross/cylinder. The final result is fine tuned by using a duochrome or red/green test. Each eye is tested separately first and the final prescription is determined binocularly, i.e. with both eyes together.
The optometrist uses the ophthalmoscope to view inside the eye. This instrument has a series of tiny lenses on a revolving rack and by changing these the optometrist can focus on the front of the eye (the cornea) right through to the back (the retina). This part of the examination is a vital health check for the eye. It can reveal conditions such as cataract, glaucoma and retinal detachment.
The eye is the only part of the body where the blood vessels can be seen without surgery. The optometrist can see the arteries and veins in the retina and may therefore be able to detect signs of vascular problems such as high blood pressure and diabetes. However, the optometrist is not qualified to diagnose these problems so if he/she detects anything abnormal he/she will refer the patient on to their G.P. or directly to a specialist if necessary.
Slit Lamp Examination
This is a large table mounted instrument and is used to view the cornea and eyelids and anterior part of the eye in more detail than the ophthalmoscope. It is a binocular microscope with variable magnification and light source. It is used most frequently in contact lens fitting but it is also very useful as part of the general eye examination. The optometrist may also use this in conjunction with a small but powerful lens called a Volk lens to look at the retina.
Tonometry: Pressure check
This is done using an instrument called a tonometer, which measures the pressure of the fluid inside the eye (called the aqueous fluid). If the pressure is high it may indicate that the patient is suffering from glaucoma or at risk of developing the condition.
It is very important to measure the pressure for patients who have a family history of glaucoma, or for patients over the age of 40 or for patients that the optometrist feels might have a problem when examined through ophthalmoscopy.
Generally speaking glaucoma can be easily controlled by medication but any damage caused before detection is irreparable. This is why early detection is so important.
The contact tonometer such as the Perkins or Goldmann touches the cornea so the optometrist must instil a drop of anesthetic before carrying out the procedure.
The non-contact tonometer, such as the Pulsair, does not touch the cornea so an anesthetic is not required. This instrument emits a puff of air, which bounces off the cornea and back into the instrument. The Pulsair calculates the speed of the returned air and converts it into a pressure reading, i.e. the faster the air is returned, the higher the pressure reading.
Visual Field Test
This test is used to test the central and peripheral fields of vision and to determine if there are any “gaps” in this field of view. If the field of vision is not full or complete it could indicate problems such as detached retina or a hemorrhage caused by high blood pressure. Glaucoma will also affect the visual fields, as can many neurological conditions.
The above auxiliary tests will not necessarily be carried out on every patient but only on those the optometrist feels it is necessary.