Pupil
|
Pupil |
|
| The resting size of each pupil and their
respective responses to light shone through them are the best indicators
of the status of a patient's brain. These are also the best
predictors of the likelihood of good or bad outcome in cases of brain
tumors, head trauma, hydrocephalus, etc.
The pupil is the opening into the eyeball through which light passes to reach the retina and stimulate the neural pathways of visual perception. The size of the pupil determines the amount of light that passes into the eyeball (a globe lined inside by layers of cells collectively called "the retina"). The size of the pupil is regulated by the amount of light in the around the eye. The brighter the exterior environment, the less light is required by the retina and the smaller the pupil should be. The darker the exterior, the larger the pupillary opening required for adequate retinal stimulation. The size of the pupil is controlled by two different nerves: one that dilates (widens, enlarges), the other that constricts, it. The nerve that enlarges the pupil comes from the sympathetic nervous system that controls the "fight or flight" response to stressful and/or dangerous situations (when people are afraid thier pupil actually becomes larger). The nerve that constricts the pupil comes from the parasympathetic nervous system that controls the vegetative (eating, sleeping, eliminating, ...) functions. Causes of pupillary dilatation would include overactivity of the sympathetic pupillary nerve or underactivity of the parasympathetic. The parasympathetic and sympathetic nerves to the pupils are themselves controlled by the brain. The brain regulates pupillary size in response to retinal activity which depends on light input. Any condition or disease that prevents the retina from receiving or transmitting light information will affect the ability of the brain to regulate pupillary size in response to light. Clinical conditions associated with dilated pupil: Overactive sympathetic Disease: stress response Underactive parasympathetic Disease: aneurysm posterior communicating artery, Mechanism: pushing against parasympathetic nerve to pupil Disease: diabetes, Mechanism: ischemia due to narrowing of small vessels that supply and sustain the nerve No light input: injury to eye, optic nerve, etc. Anisocoria: asymmetry (unequal size) of the pupils.
|
|||||
|
Pupillary responses - Operations for head trauma: Evaluation Pupillary exam abnormalities - table Anisocoria -- differential diagnosis
Pupillary Reaction Abnormalities on Neurologic Examination
|
|||||
| Pupil | Anatomy | Physiology | Examination | Significance | |
| Adie ("tonic" pupil) | ciliary ganglion, post-ganglionic parasympathetic fibers | fibers constrict pupil and muscles that control
lens (focus)
|
absent or significantly reduced reaction to
light. unable to accomodate (focus on an object brought close to the eyes)
very small amount of pilocarpine causes denervated hypersensitive pupil to constrict |
most common in women 20-40 years old. associated with loss of normal ankle jerk reflex. a mild neuropathy simultaneously and similarly affecting many nerves. | |
| Argyll Robertson | |||||
| diabetes mellitus | |||||
| Horner's | |||||
| Marcus Gunn | pathology that interferes with light reception (retina) and/or transmission (optic nerve) can cause abnormal clinical findings. | pupil constricts in response to stimulation by
light.
Essentially a Marcus Gunn pupil dilates when it should constrict in response to a light shining into the eye.
|
light:
slow response with inability to sustain constriction "swinging flashlight": light shone into right eye makes left pupil constrict (consensual response). if light then shone into left eye the pupillary response is too weak and with the consensual stimulus (light in the opposite eye) now gone, the pupil dilates. |
present to some degree in many normal persons but most apparent in patients with damage to the retina or optic nerve. | |
Return to uscneurosurgery.com Homepage
