A few central nervous system diseases damage
nerve transmission of visual signals from the retinas to
the Edinger-Westphal nucleus, thus sometimes blocking
the pupillary reflexes. Such blocks frequently occur as a
result of central nervous system syphilis, alcoholism,
encephalitis, and so forth.The block usually occurs in the
pretectal region of the brain stem, although it can result
from destruction of some small fibers in the optic
nerves.
The final nerve fibers in the pathway through the pretectal
area to the Edinger-Westphal nucleus are mostly
of the inhibitory type. When their inhibitory effect is
lost, the nucleus becomes chronically active, causing the
pupils to remain mostly constricted, in addition to their
failure to respond to light.
Yet the pupils can constrict a little more if the
Edinger-Westphal nucleus is stimulated through some
other pathway. For instance, when the eyes fixate on a
near object, the signals that cause accommodation of
the lens and those that cause convergence of the two
eyes cause a mild degree of pupillary constriction at
the same time. This is called the pupillary reaction to
accommodation. A pupil that fails to respond to light
but does respond to accommodation and is also very
small (an Argyll Robertson pupil) is an important diagnostic
sign of central nervous system disease—often
syphilis.
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