T

tonometry A test that measures INTRAOCULAR
PRESSURE(the pressure within the EYE). The pri-
mary 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
CELLULITISand GRAVES’S OPHTHALMOPATHY. Tonome-
try 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
  CORNEAto 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

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 alsoOPHTHALMOSCOPY;SLIT LAMP EXAMINATION.

toxic optic neuropathy Damage to the OPTIC
NERVE, RETINA, and other structures of the EYEas a
SIDE EFFECTof medications or exposure to environ-
mental or systemic toxins. Many substances can
harm vision, and any substance that is a neuro-
toxin (damaging to the NERVOUS SYSTEM) has the
potential to damage the optic nerve. Long-term
cigarette smoking or ALCOHOLabuse, and especially

a combination of these practices, causes various

disturbances of vision and ocular function. Severe

or chronic MALNUTRITION, notably vitamin B 12 defi-

ciency, 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 ANTI-

pyridoxine INFLAMMATORY DRUGS(NSAIDS)

radiation exposure quinine

tamoxifen sulfonamide

ultraviolet light tobacco use

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 contin-

ued exposure to the toxic substance. Symptoms

may begin in one eye though nearly always

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