Webster's New World Medical Dictionary

symphysis pubis
The area in the front of the
pelvis where the pubic bones meet.
symptom
Any subjective evidence of disease. In
contrast, a sign is objective. Blood coming out a
nostril is a sign; it is apparent to the patient, physi-
cian, and others. Anxiety, low back pain, and fatigue
are all symptoms; only the patient can perceive
them.
synapse
A specialized junction at which a neural
cell (neuron) communicates with a target cell. At a
synapse, a neuron releases a chemical transmitter
that diffuses across a small gap and activates special
sites called receptors on the target cell. The target
cell may be another neuron or a specialized region
of a muscle or secretory cell. Neurons can also
communicate through direct electrical connections
(electrical synapses).
synchronic study
A study that is done at a single
point in time rather than over the course of a period
of time (longitudinally).
syncope
Partial or complete loss of conscious-
ness, with interruption of awareness of self and sur-
roundings and spontaneous recovery that is due to
a temporary reduction in blood flow and therefore
a shortage of oxygen to the brain. This leads to light-
headedness or a “blackout” episode (loss of con-
sciousness). Heart conditions that can cause
syncope include abnormal heart rhythms, abnor-
malities of the heart valves, high blood pressure in
the arteries supplying the lungs (pulmonary artery
Swan-Ganz catheter
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hypertension), tears in the aorta (aortic dissection),
and widespread disease of the heart muscle (car-
diomyopathy). However, syncope is most commonly
caused by conditions that do not directly involve the
heart, including postural (orthostatic) hypotension,
a drop in blood pressure due to changing body
position to a more vertical position after lying or sit-
ting; dehydration, which can cause a decrease in
blood volume; blood pressure medications that lead
to overly low blood pressure; diseases of the nerves
to the legs which can dilate blood vessels in the legs
and reduce blood flow to the heart; high altitude;
stroke or transient ischemic attack; and migraine
attack. Another common form of noncardiac syn-
cope is known as situational syncope because the
fainting occurs after certain situations. Triggers for
situational syncope include having blood drawn,
urinating (micturition syncope), defecating (defe-
cation syncope), swallowing (swallowing syncope),
and coughing (cough syncope). In some individu-
als, one or more of these situations can trigger a
reflex of the involuntary nervous system called the
vasovagal reaction, which slows the heart, dilates
blood vessels in the legs, and causes the person to
feel nausea, sweating, or weakness just before faint-
ing. No treatment is needed for many noncardiac
causes of syncope, as the person regains conscious-
ness by simply sitting or lying down. Also known as
fainting. See also 
syncope, situational; vasovagal
reaction.