Pupillary Reflexes or Reactions in Central Nervous System Disease

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.