Pupil assessment and abnormalities
Pupil Evaluation
The pupil’s function is to control the amount of light entering the eyes, providing the best visual function under varying degrees of light intensity. The normal diameter of a pupil is 3 to 4 mm under average lighting conditions. In dim lighting the pupil size is larger and in bright light it is smaller.
Pupils are usually larger in children and smaller in the elderly.
Twenty percent of fibers in the optic tract are for pupillary function.
Changes of the pupil in accomodation:
The pupil changes when focusing on near objects. With accommodative effort, a “near synkinesis” is evoked, including increased accommodation of the lens, convergence of the visual axes of the eyes and pupillary constriction or miosis.
How to accurately assess the pupils
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The testing conditions for evaluating pupils include: Evaluate pupils in dim illumination Have the patient fixate at a distance (eliminates miosis from accomodation) Use bright light source Note size of pupils before assessing the responses look for anisocoria (unequal pupil size) and corectopia (irregular shaped pupil) Direct light response In watching the eye receiving the direct light, you can then determine the direct response by: Shining the penlight directly in the right, look at the response including speed of response note (brisk, sluggish) Repeating for the opposite eye If the pupils are normal, they will react equally to the direct light. Indirect light response Placing the penlight in front of the right eye (OD) The examiner watches the left eye for a consensual response. In a normal eye the pupils should constrict
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Lesions of the parasympathetic system cause a dilated pupil Acute third nerve palsy May signal compression of the third nerve due to a posterior communicating artery aneurysm.
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A third nerve palsy will be usually associated with other signs of dysfunction including ocular motility disturbances and ptosis.
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An isolated dilated pupil does not usually signify a III nerve palsy.
Tonic pupil or Adie's pupil
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Characterized by a dilated pupil with very poor or no light reaction with tonic constriction to near and tonic redilation.
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Under the slit lamp sectoral palsies of pupillary sphincter may be seen with light stimulation. Because the abnormality is post-ganglionic (that is, damage to the ciliary ganglion), one can demonstrate supersensitivity of the sphincter muscle to pilocarpine 1/10%. Adie's syndrome consists of a tonic pupil and loss of deep tendon reflexes. The lesion causing pupillary dysfunction is generally thought to be in the ciliary ganglion and the cause is basically unknown
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Caused by damage to peripheral pathways to the pupil (parasympathetic neurons in the ciliary ganglion Argyll Robertson pupils
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Bilateral small pupils that constrict when the patient focuses on a near object, but do not constrict when exposed to bright light (they do not “react” to light). They were formerly known as "prostitute's pupils" because of their association with syphilis and because, like a prostitute, they “accommodate but do not react.
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The primary lesion is thought to be caused by damage to central pathways for pupillary constriction. Specifically caused by selective damage to pathways from the retina to the Edinger-Westphal nucleus. These light-sensitive pathways allow the pupil to constrict to bright light. The accommodation pathways (pathways to the Edinger-Westphal nucleus that cause the pupils to constrict with near vision) are thought to be spared because of their more ventral course in the brainstem.
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They are a highly specific sign of neurosyphilis.
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Pupils that “accommodate to near objects but do not react to light are said to show light-near dissociation.
Lesions of the sympathetic system cause a small pupil Horner's Syndrome Horner's Syndrome, consists of pupillary miosis which is more accentuated in darkness. Light and near reactions are intact. There is ipsilateral ptosis due to paresis of Muller's muscle (not the levator muscle of the eyelid) There is an pseudo enophthalmos because of the associated ptosis. Occasionally anhydrosis of the face will also be seen.
Pharmacologic testing, to confirm the diagnosis of a Horner's syndrome:
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Cocaine test (Diagnose horners as a cause of miosis, however cannot distinguish preganglionic from post ganglionic) 10% cocaine is instilled into the eye The cocaine prevents the re-uptake of norepinephrine into the normal nerve and hence the norepinephrine will cause dilation of the pupil If there is a lesion anywhere along the sympathetic pathway, there will be less norepinephrine coming down the nerve to be released COCAINE will NOT DILATE the eye with a sympathetic defect (Horners syndrome)!!!
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Hydroxyamphetamine test (Differentiate between preganglionic and postganglionic horners) o To localize whether the Horner's syndrome is preganglionic (that is before the superior cervical ganglion) or postganglionic (after the cervical ganglion). Hydroxyamphetamine (Paredrine) is instilled into both eyes Hydroxyamphetamine actively releases norepinephrine from the nerve endings. If the lesion is preganglionic, there will be normal norepinephrine store present at the iris and therefore the pupil dilates. A dilation to Hydroxyamphetamine in a Horner's eye means the lesion is preganglionic. If the pupil does not dilate, the lesion is postganglionic because there is no norepinephrine to be released.
Parinaud syndrome:
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Another cause of light-near dissociation is Parinaud syndrome, also called dorsal midbrain syndrome This uncommon syndrome involves vertical gaze palsy associated with pupils that “accommodate but do not react.
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The causes of Parinaud syndrome include: Brain tumors (pinealomas) Multiple sclerosis Brainstem infarction.
