Encyclopedia of Health and Medicine

T
tonometry A test that measures INTRAOCULAR
PRESSURE (the pressure within the EYE). The primary
reason for tonometry is to screen for GLAU-
COMA, a condition for which elevated intraocular
pressure is a characteristic symptom. Increased
intraocular pressure is also common with ORBITAL
CELLULITIS and GRAVES’S OPHTHALMOPATHY. Tonometry
is a standard component of the OPHTHALMIC
EXAMINATION.
There are several methods of tonometry. The
most commonly used are
• applanation, in which the ophthalmologist puts
anesthetic drops in the eyes and then touches a
device called a tonometer to the surface of the
CORNEA to measure how much pressure it takes
to depress the cornea
• noncontact, or air puff, in which the person
stares at a focused light while a device blows a
quick puff of air at the cornea, then measures
the change in reflected light from the cornea
a combination of these practices, causes various
disturbances of vision and ocular function. Severe
or chronic MALNUTRITION, notably vitamin B 12 deficiency,
also results in toxic optic neuropathy. As
well, numerous medications cause temporary
visual disturbances, among them sildenafil (color
shifts) and antidepressants (distorted perception
and VISUAL ACUITY).
POSSIBLE SOURCES OF TOXIC OPTIC NEUROPATHY
amiodarone
carbon monoxide
chloroquine
digoxin
ethambutol
ethanol (drinking ALCOHOL)
ethylene glycol (antifreeze) industrial chemicals
isoniazid
lead
mercury
methanol (wood ALCOHOL)
methotrexate
NONSTEROIDAL ANTIpyridoxine
INFLAMMATORY DRUGS (NSAIDS)
radiation exposure
quinine
tamoxifen
sulfonamide
ultraviolet light
tobacco use
Elevated intraocular pressure, or intraocular
HYPERTENSION, requires further evaluation to detect
and correct the cause. Intraocular pressure that
remains elevated damages or destroys the OPTIC
NERVE, resulting in total vision loss in the affected
eye.
See also OPHTHALMOSCOPY; SLIT LAMP EXAMINATION.
toxic optic neuropathy Damage to the OPTIC
NERVE, RETINA, and other structures of the EYE as a
SIDE EFFECT of medications or exposure to environmental
or systemic toxins. Many substances can
harm vision, and any substance that is a neurotoxin
(damaging to the NERVOUS SYSTEM) has the
potential to damage the optic nerve. Long-term
cigarette smoking or ALCOHOL abuse, and especially
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Symptoms and Diagnostic Path
Most often toxic optic neuropathy develops slowly
as the consequence of cumulative exposure.
Symptoms may include
• dimness and diminished clarity
• altered color perceptions (dyschromatopsia)
• blind spots in the visual field (SCOTOMA)
• PHOTOPHOBIA (extreme sensitivity to light)
• DIPLOPIA (double vision)
Symptoms progressively worsen with continued
exposure to the toxic substance. Symptoms
may begin in one eye though nearly always