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Acanthamoeba
A free living protozoa in tap water,
in the air and in the tear film. Can cause devastating corneal infection
and blindness. Strong association with contact lens wearers who make
their own saline, but due to changing technology and cleaning methods
this is no longer a risk.
Accommodation
The ability to focus from one object
to another at different distances.
Aphakia
Absence of the eye's internal lens due to surgical
removal of a cataract, and where extremely high prescription spectacles
were worn to compensate. Seldom seen now due to lens implants.
Astigmatism
An irregularity of the shape of the cornea.
Unlike myopia and hyperopia where poor vision is due to lens function,
Astigmatism causes poor vision for both near and distance. Best demonstrated
by squeezing a half tennis ball.
Base curve
The flatter of the two curves on a lens.
Cataract
The opacification of the crystalline lens.
The lens turns milky, most often with age. Result is like looking through
a dirty window. Easily remedied with lens removal and implant of synthetic
replacement. Most successful of all operations today, often performed
under local anaesthetic.
Cone cells, or cones
These
are cells in the retina of the eye which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea. They gradually become more sparse towards the outside of the retina. Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of colour. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods.
Concave lens
A lens thinner at its center than at the
edges, for correcting myopia or short-sightedness.
Conjunctavitus
A condition which usually affects both
eyes, usually one at a time. Extremely contagious, and drops are usually
prescribed to give comfort and prevent further bacterial infection. It
is not dangerous but must be correctly diagnosed.
Convex lens
A lens which is thicker in the center
than at the edge, used to correct hyperopia or farsightedness. Prescriptions
are in + dioptres.
Convergence
The ability to move both eyes in at the
same degree as the object moves closer.
Cornea
The transparent layer that forms the front
wall of the eye, covering the iris.
Dark Adaptation
The time taken for the rods of the
retina to become active.
Diabetes
Changes often occur in the retina, at the
back of the eye where the blood vessels become fragile and leak. This
is often detected by the optom before the GP, as due to sugar level variances
the vision fluctuates. Untreated diabetes can result in blindness.
Dioptre
Unit used to measure the power of a lens in
measures of .25.
Divergence
The ability to move both eyes out as object
moves away.
Fundus
See retinal fundus.
GPC
Giant Papillary Conjunctivitis is pimple like bumps
found under the upper lid, and often associated with dirty contact lenses.
Disposable contact lens wearers who take their lenses out daily don't
get this condition which is extremely difficult to treat.
Glaucoma
Fluid comes into the eye at one point and
leaves at another. If the fluid is not moving out at the same rate that
it is flowing in, pressure begins to build up. This will kill off the
fibres of the optic nerve and cause irreparable loss of sight. Chronic
glaucoma has no symptoms to indicate warning, therefore this is an essential
part of the visual examination. There are four tests, the internal pressure
test using the non-contact Tonometer, the visual fields (these two usually
performed by one of our pre-testers) and the optometrist uses the opthalmoscope.
The disease can be hereditary and it is important that family members
are screened.
Hyperopia
- "Farsightedness"
The ability to see far, however, requires intense focusing in order to see. The closer the object the greater the difficulty in focussing. These customers complain of headaches, teary eyes and poor concentration.
Intra-ocular pressure
The pressure of the viscous fluid
inside the eye. The measurement is recorded in mm of mercury. Anything
under 20mmHg is usually acceptable but one cannot rely only on the pressure
for a diagnosis.
Kerataconus
The cornea bulges in the shape of a cone.
Lazy
Eye
Correct term Amblyopia. The amblyopic eye is one which, even with
the best optical correction, has reduced vision but no pathology.
Low
vision
Abnormal vision despite the best visual correction.
Monovision
A form of contact lens fitting whereby one
lens used is for distance vision and the other for near vision. It usually
works extremely well. Indicated for over 40's who are current contact
lens wearers who dislike the thought of spectacles. Multifocal contact lenses are now easily available and are more popular.
Myopia - "Short-sighted"
Can
see up close but poor distance vision. A farsighted eye is too “short,” causing
light to converge behind the retina and blurring near vision.
Macular Degeneration
Breakdown of the macular, the
back wall of the eye, often through age, resulting in central blindness
which spreads outward.
Opthalmologist
A specialist of eye surgery and pathology.
Opthalmoscope
Instrument with a light source used to
detect abnormalities within the eye.
PDPupillary Distance
Measured
from the inner of one pupil to the outer of the other.
Presbyopia
The inability to focus on close objects
due to the ageing process. The crystalline lens loses its flexibility,
and one has to hold reading material further away in order to focus.
This results in the need for reading spectacles or contact lenses suitable
prescribed.
Pinguecula
Soft yellow patches over the sclera (the
white of the eye) at 3 and 9 o'clock.
Prism
A pyramid shaped glass through which light is
bent and is always refracted toward the base.
Pterigium
Membranes extending over the visible section
of the cornea, at the inner corner of the eye. Causes light and dust
sensitivity. Easily removable when pterigium starts extending over the
iris.
Ptosis
Droopy eyelid.
Radial Keratometry
An operation initially performed
with a scalpel and now utilises the Excima Laser. Fine cuts are made
in the cornea to collapse it to reduce myopia. Only certain prescriptions
are acceptable for this procedure.
Refract
To bend light. As glass is a
denser medium than air, the path of light passing through it is altered.
The optometrist changes the focus point with spectacles, in order to
cause the light to fall correctly on the retina in a point of focus.
Refraction
The method used to determine what prescription
is required for the best sight of the eye.
Retinal Detachment
This is a real optometric emergency,
where visual disturbances like flashing lights and a "curtain" coming
down over the eye is noticed. Callers with these symptoms should be seen
immediately. This will then be referred to an opthalmologist for treatment.
Retinal fundus
The interior lining of the eyeball, including the retina (the light sensitive screen), optic disc (the head of the nerve to the eye), and the macula (the small spot in the retina where vision is keenest). The fundus is the portion of the inner eye that can be seen during an eye examination by looking through the pupil.
"Fundus" is the Latin word for the bottom. In medicine, fundus refers to the bottom or base of an organ.
Retinitus
Pigmentosa
Untreatable inherited disease of the eye, which begins with
night blindness and progresses through tunnel vision to blindness.
Retinoscope
Instrument with a light source (like a
torch) used to objectively determine the prescription of the eye.
Rod cells, or rods
These
are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Since they are more light-sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 100 million rod cells in the human retina.
Slit
lamp
Magnifies the eye, enables the optom to view the layers of the
cornea, lens, a contract lens fit and general observation of the eye.
Also known as a biomicroscope.
Strabismus
The eyes do not align in the same direction
- a squint.
Stye
A common bacterial infection of a little canal
within the lid. Does not respond well to antibiotics. The best treatment
is to hold hot compresses against it. The bump is a collection of fluid
in a little sac inside the lid.
Ultra Violet
Light travels in wavelengths measured
in nanometers. The short wavelengths below 400 nm are called Ultra Violet.
UVA causes ageing through a breakdown of the tissue and UVB causes burning.
Sunglasses must provide 100% protection from UV rays.
Vertex distance
The distance between the back surface
of the spectacle lens and the front of the cornea. This becomes relevant
with prescriptions of 4 dioptres and over. Contract lens and spectacle
lens prescriptions start to differ, and all measurement pertaining to
the processing of spectacles must be exact for Rx's of 4 dioptres and
over.
