List the causes of the various pupillary abnormalities which may assist in the differential diagnosis of the comatose patient

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College Answer

ABNORMALlTY

CAUSE

NEUROANATOMICAL 
BASIS

Miosis{<2mm isize)

Unilateral

Homer's syndrome
Localpathology

Sympathetic paralysis
Trauma to sympathetics

Bilateral

Pontine lesions
Thalamic haemorrhage
· Metabolic encephalopathy

Sympathetic paralysis

Drug ingestion

Organophosphate

Cholinesterase inlnoition

Barbiturate

Narcotics

Cetrtral Effect

ABNORMALITY

CAUSE

NEUROANATOMICAL 
BASIS

Mydriasis (>5mm in size)

Unilateral fixed pupil

Midbrain lesion
Uncal herniation

3 'nerve damage 
Stretch of3 'nerve against the petroclioid ligament

. Bilateral fixed pupils

Massive midbrain
Haemonhage 
Hypoxic cerebral injury

Bilateral 3rd nerve damage

Mesencephalic damage

Drugs

Atropine

Paralysis of                    athetics

Tricyclics

Prevent local reuptake of catecholamines by nerve
endings

Sympathomimetics

Stimulation of   sympathetics

Discussion

The college produces a nice table of explanations. Unfortunately, my copy-and-paste process has done some serious harm to its layout. In reponse to this failure, I have produced my own table, which neither better nor worse than the college table. This table can be found in the chapter on Examination of pupil reactivity and diameter (CN II, III)

A Summary of Pupil Examination Findings
Unaided observations of the pupillary diameter
Miosis Bilateral miosis Bilateral pontine lesion, with damage to the descending sympathetic fibers
Bilateral thalamic lesion, also with damage to decsending sympathetic fibers
Opiate intoxication
Organophosphate poisoning (thus, a pharmacological excess of parasympathetic stimulation)
Barbiturate poisoning
Unilateral miosis Horner's syndrome
Sympathetic damage at any level:

Ipsilateral thalamic lesion

Ipsilateral pontine lesion

Ipsilateral sympathetic chain lesion

Mydriasis Bilateral mydriasis That is what the end of brainstem herniation looks like
Bilateral midbrain lesion- eg. basilar artery infarct
Bilateral 3rd nerve damage, eg. due to severe base of skull fracture
Severe global brain injury (eg. due to hypoxia)
Anticholinergic drugs
Sympathomimetic drugs
Serotonin syndrome
 
Unilateral mydriasis Midbrain lesion- ipsilateral damage to the Edinger-Westphal nucleus of the 3rd nerve (thus resulting in loss of parasympathetic input to the ipsilateral eye)
Uncal herneation - stretch of the 3rd nerve across the petroclinoid ligament
Direct trauma to the eyeball

The reaction to light
Normal consensual reaction of both pupils The optic nerve on the tested side, the midbrain and both the third nerves are probably intact. Massive midbrain damage can be ruled out.
Failure of either pupil to constrict Either the tested optic nerve is damaged and light is not registering in the midbrain, or the midbrain is massively damaged.
Successful constriction of the tested pupil, but failure of conjugate constriction  

The reaction to swinging light
The pupils consensually constrict in the presence of light, and rapidly re-dilate when the light source is removed. Normally, with swinging light, the pupils of both eyes will constrict whenever light is directed at either pupil. This demonstrates normal optic nerve, 3rd nerve and midbrain function.
With rapid sequential light stimulus, the affected pupil will paradoxically dilate in response to light. This is an afferent pupilalry defect, or a Marcus Gunn pupilIt means that the tested optic nerve is damaged in the pre-chiasmal portion. During the swinging light test, there is a moment when the contralateral (healthy) pupil is again submerged in darkness, while the ipsilateral (affected) pupil has light shining upon it. With the darkness stimulating the dilation of both pupils, and the light stimulating nothing (owing to the optic nerve pathology on the tested side), the pupil exposed to light will dilate abnormally, until it is the same diameter as the unlit pupil.

Test of accomodation
The pupil dilates to observe distant objects, and constricts to regard near objects. This is a normal accomodation reflex.
The pupil accomodates to near and far objects,but fails to react to light This is an Argyle-Robertson pupil, and it is seen in various conditons:

Syphilis

Diabetes

Alcoholic midbrain degerenation

Parinaud syndrome

 

References

References

 

Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Chapter 58 The Pupils - by Robert H. Spector.

 

Broadway, David C. "How to test for a relative afferent pupillary defect (RAPD)."Community Eye Health 25.79-80 (2012): 58.

 

Fincham, Edgar F. "The accommodation reflex and its stimulus." The British journal of ophthalmology 35.7 (1951): 381.