Glossary

Pachymetry - Measurement of the corneal thickness using an instrument called a Pachymeter and is essential information for the planning of refractive eye surgery. It is usually performed using ultrasound.

Pentacam - Diagnostic instrument that calculates a 3-D model of the anterior segment of the eye by capturing images from a rotating camera. These images give information from the anterior corneal surface to the posterior crystalline lens and include precise measurement of the central cornea, 3-D chamber analysing, lens density and intra-ocular lens calculation. The information is used in refractive surgery, cataract surgery and glaucoma screening.

Perimetry - Determines the extent and shape of the field of vision by using an instrument showing test objects of different sizes and light intensities. A perimeter is used primarily in the screening and management of glaucoma and for detecting damage to the sensory visual pathway.

Peripheral Vision - Side vision formed by stimuli falling on retinal areas in the outer regions of the retina.

Phacoemulsification - Modern cataract surgery using a high frequency ultrasound probe to break up a cataract so that it can be removed through a small incision in the eyeball.

Phenylephrine - A drug that affects the sympathetic nerve pathway and is used primarily as a decongestant by causing constriction of blood vessels. In the eye, it causes dilation of the pupil and constriction of blood vessels and is used in conjunction with another dilating drug to keep the pupil open during surgery or when examining the internal part of the eye.

Photophobia - An abnormal intolerance to light that produces intense discomfort of the eyes causing excessive tearing and blinking and other reactions aimed at avoiding the light. It can be due to active inflammation in the eye, trauma to the outer surface of the eye, migraine or as a result of certain eye drops.

Photoreceptors - Specialised photosensitive cells found in the eyes retina that transmit information from the visual field to the brain to form a complete representation of the visual world. There are 2 types of photoreceptor: rods and cones. There are more rods than cones in the human retina. The rods function at low levels of light (night vision) and are used in motion and form detection and not colour perception. They are more numerous in the peripheral retina. The cones function in high levels of light (day vision) and are used to determine colour, depth and intensity. They are predominantly in the central retina. The 3 types of cone are sensitive to different wavelengths and intensities of light.

Phropter - An instrument commonly used by Optometrists and Ophthalmologists during an eye examination to determine an individuals refractive error. This will help determine the refractive power of the eye for prescriptive spectacles or contact lenses.

Posterior Chamber - The rear section of the eye that lies behind the crystalline lens to the retina and which is filled with vitreous humour.

Presbyopia - Inability of the eye to focus objects close up clearly due to the gradual loss of elasticity of the crystalline lens, as occurs with age. Usually becomes significant in individuals over 40 years of age.

Ptosis - Drooping of the upper eyelid. It may be congenital or caused by a disorder to the 3rd cranial nerve (oculomotor) or sympathetic nerves that cause muscle weakness or paralysis. A drooping eyelid that mimics a Ptosis (Pseudo-Ptosis) can be caused by excessive weight of the upper eyelids.

Punctal Occlusion - Treatment option for dry eyes that prevents drainage of tears from the tear ducts. This can either be permanent or temporary and is achieved by inserting either silicone or collagen plugs into the tear ducts and closing the openings off to the drainage system.

Punctum - The two openings of the tear ducts in the inner corners of the upper and lower eyelids.

Pupil - Circular opening in the middle of the iris that regulates the amount of light entering the eye.

PVD - Posterior Vitreous Detachment - A condition in the eye where the vitreous humour separates from the retina either due to age-related changes or trauma. The vitreous humour is attached in several places to the retina but is more firmly attached anteriorly at the vitreous base. The weakest point is around the optic nerve head. As a posterior vitreous detachment proceeds, there are symptoms of flashes, floaters or a large ring shaped floater in the vision. Occasionally there is an associated vitreous haemorrhage due to tractional forces on the retinal blood vessels.