List the causes of the various pupillary abnormalities which may assist in the differential diagnosis of the comatose patient
College Answer
|
ABNORMALlTY |
CAUSE |
NEUROANATOMICAL |
|
Miosis{<2mm in size) Unilateral |
Homer's syndrome |
Sympathetic paralysis |
|
Bilateral |
Pontine lesions |
Sympathetic paralysis |
|
Drug ingestion |
||
|
Organophosphate |
Cholinesterase inlnoition |
|
|
Barbiturate |
||
|
Narcotics |
Cetrtral Effect |
|
ABNORMALITY |
CAUSE |
NEUROANATOMICAL |
|
Mydriasis (>5mm in size) Unilateral fixed pupil |
Midbrain lesion |
3 'nerve damage |
|
. Bilateral fixed pupils |
Massive midbrain |
Bilateral 3rd nerve damage Mesencephalic damage |
|
Drugs |
||
|
Atropine |
Paralysis of athetics |
|
|
Tricyclics |
Prevent local reuptake of catecholamines by nerve |
|
|
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)
| 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 pupil. It 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 |
|
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.