Webster's New World Medical Dictionary

asthma, thermally induced
See 
asthma, exer-
cise-induced.
astigmatism
A common form of visual impair-
ment in which part of an image is blurred due to an
irregularity in the dome-shaped curvature of the
front surface of the eye, the cornea. With astigma-
tism, light rays entering the eye are not uniformly
focused on the retina. The result is blurred vision at
all distances. Significant astigmatism can cause
headaches, eye strain, and seriously blurred vision.
Astigmatism is often not detected during routine eye
screening in schools. It may coexist with other
refractive errors such as nearsightedness and far-
sightedness. Astigmatism is corrected with slightly
cylindrical lenses that have greater light-bending
power in one direction than the other. Use of these
lenses elongates objects in one direction and short-
ens them in the other, much like looking into a dis-
torting wavy mirror.
astrocytoma
A tumor that begins in the brain or
spinal cord in small, star-shaped cells called astro-
cytes. The location of the tumor depends on the age
of the person. In adults, astrocytomas most often
arise in the cerebrum, whereas in children, they may
arise in the brain stem, cerebrum, and cerebellum.
asymptomatic
Without symptoms. For example,
an asymptomatic infection is an infection with no
symptoms.
asystole
A dire form of cardiac arrest in which
the heart stops beating and there is no electrical
activity in the heart. As a result, the heart is at a total
standstill.
ataxia
Poor coordination and unsteadiness due
to the brain’s failure to regulate the body’s posture
and regulate the strength and direction of limb
movements. Ataxia is usually due to disease in the
cerebellum of the brain, which lies beneath the
back part of the cerebrum.