Eye Examinations
There is no standard eye examination but some of the techniques behind the eye examination in general are described below. The patient may well speak of long-standing or recent eye problems, and about occupational and recreational activities relevant to the eye (like using a computer). From the history and Symptoms the optometrist will tailor the examination appropriately. There is no upper or lower age limit for an eye examination. A pre-school examination is advisable though by age three we are able to establish the presence of normal binocular vision, refraction and acuity (see below). Earlier examinations are recommended if either parent has strabismus ('squint') or amblyopia ('lazy' eye).
Acuity: The Snellen chart with letters allows the examiner to check for reduced acuity (poor vision). We then establish whether this is a focussing (refractive) error, whether the eye is poorly developed (amblyopic or 'lazy') or whether there is damage or disease to the eye. Different tests are used to establish pre-school children's acuity.
The motility test: This is a check on the function and co-ordination of the extra-ocular muscles. The position of each eye is controlled by six separate muscles which wrap around the eye from the back of the orbit. Pursuit of a moving target should be smooth and the eyes should appear yoked. Eyes are capable of rapid refixation movements called saccades during which the eyes can travel in excess of 500 deg/sec. Damage to the brain cortex, the midbrain, the oculomotor nerves or the muscles themselves can produce distinctive anomalies of eye movement control. Worfolk, R. (1992), 'The control of eye movements', Parts 1, 2, 3, Optometry Today, 5/10/92, 11/1/93, 8/3/93; Worfolk, R. (1994) 'Internuclear Ophthalmoplegia', British Journal Optometry and Dispensing, 2, 480-483. Reprints available on request.
The cover test: This is a check for manifest and latent strabismus ('squint'). Particularly important in the developing system of children as early detection of stabismus can lead to beneficial treatment not possible in adults. Adults with unsightly squints can usually have cosmetic surgery
Pupil responses: Pupils should be equal in size (though small variations can occur in the normal population) and they respond to direct, consensual (so that a light in one eye induces contriction of the fellow pupil) and near stimuli. Abnormal pupil responses require investigation to establish whether the limitation is in constriction (parasympathetic nervous system) or dilatation (sympathetic nervous system) and whether the inflowing or outflowing signals are involved.
Biomicroscopy: The slit-lamp microscope gets a magnified view of the outer layers of the eye, which are carefully checked for signs of injury or disease. Particularly with contact lens wear where the cornea is critically assessed.
Ophthalmoscopy: An ophthalmoscope, the handheld device with a bright light, looks through the eye to the retina, checking the blood vessels for damage due to high blood pressure or diabetes for instance. We inspect the optic nerve head in detail as part of a glaucoma assessment. In our practices we are also able to gain a three dimensional view of the retina (useful for diagnosing retinal detachments) with a technique known as binocular indirect ophthalmoscopy.
Inter-ocular pressure: A tonometer tests the tension of the eye (see glaucoma section). Tonometers use a fast puff of air and do not require drops. Alternatively eye drops are used anesthetize the eye and a probe touches the eyeball to determine intraocular pressure.
Refraction: Using a trial, frame lenses are inserted before the eyes to establish the precise additional focusing power required. Many patients are concerned that they may give misleading or contradictory answers. This rarely happens but an experienced practitioner is quite aware of the degree of change being assessed and the expected and previous responses. The combination of this information does ensure that you will be given an appropriate and accurate precription. Sometimes there is literally no appreciable difference of vision between one lens and another a situation which is fully understandable and explicable by the physics involved. Children often cannot respond reliably to questions in which case we use retinoscopy alone to check the focus
Retinoscopy: By focusing on the reflections from the eye (rather like the image of a cat's eye at night) we are able to determine the state of focus quite accurately. This is known as an objective technique as the subject is passive and does not need to answer questions clarity. We begin refraction with retinoscopy and refine these results for adults by offering small changes to the precription to see whether the image appears 'better or worse'.
Visual fields: Patients are shown a series of small dim lights spread across a white background. This test checks for insensitive areas of vision. These arise as a damage anywhere along the visual pathway from the eye to the brain. The pattern of the defects in the two eyes gives a clue as the site of the problem. Field tests are a sensitive means of detecting glaucoma. We use very advanced field screeners in all practices. You need not worry if you feel you are missing some of the target presentations. You are not supposed to see them all!
The following people qualify for an eye examination funded by the NHS...
- People suffering from diabetes or glaucoma.
- All children under 16 and those under 19 in full-time education.
- Those on low income who have been assessed by the Department of Social Security as having no contribution to pay.
- People receiving Family Credit, Income Support or Disability Working Allowance (benefit capital must be £8,000 or less when it was claimed).
- Registered blind or partially sighted.
- People who need complex lenses.
- Those aged 40 and over who are parents, brothers, sisters or children of diagnosed glaucoma sufferers.
- People aged 60 or over.