GENERAL NEUROLOGY

Pupillary response in consciousness disorders

David Goldemund M.D.
Updated on 14/03/2024, published on 22/11/2023

The primary goals of the neuro-ophthalmic exam are to determine the etiology, location, and severity of the consciousness disorder

Normal pupils

normal pupils are round and regular, equal in size (isocoric) under both light and dark illumination
- in adults, the typical pupil size ranges from 2 to 4 mm in diameter in bright light and 4 to 8 mm in the dark
normal direct and consensual response to light stimulus
- pupils constrict to direct illumination (direct response) and the illumination of the fellow eye (consensual response)
- in darkness, the pupils dilate
an intact near response (accommodation reflex)
- the test requires voluntary effort
- both pupils constrict when the eyes focus on a near object, accompanied by accommodation (lens thickening and eye convergence
absence of a RAPD

Pupillary light reflex

the pupillary light reflex (PLR) is easily tested in unconscious patients:
first, observe the pupil size in ambient and dim lighting to check for anisocoria
shine a bright light into one eye and then alternate between each eye, observing the reactivity of both pupils
note if the pupils react briskly, sluggishly, or show no response
a relative afferent pupillary defect (RAPD) can be detected by swinging the flashlight from one eye to the other

Abnormal pupils

abnormal pupil shape
- an oval pupil in a comatose patient is always indicative of a midbrain lesion or compression
- rule out other causes like ocular surgery, trauma, acute glaucoma, or congenital conditions
inadequate miosis or mydriasis
- bilateral, small but reactive pupils are nonspecific
  - may suggest a metabolic etiology, diencephalic lesion, or the effect of certain toxins/drugs
- bilateral nonreactive mydriasis can indicate midbrain lesions or bulbar coma
anisocoria
- unilateral mydriasis
  - uncal herniation – compression of the N III e at the Kernohan notch with the ipsilateral (rarely contralateral) nonreactive mydriasis (Hutchinson pupil) (Agrawal, 2020)
  - any compressive lesion along the third nerve (e.g., ruptured posterior communicating artery aneurysm) may cause ophthalmoparesis with nonreactive mydriasis
  - exclude pharmacologic dilation (administration of topical 1% pilocarpine will not constrict an atropinized, pharmacologically dilated pupil)
- unilateral, small but reactive pupil
  - Horner syndrome is caused by damage to the ipsilateral oculosympathetic pathway
    - the descending pathway from the hypothalamus to Budge´s ciliospinal center in the C8-T2 segments of the spinal cord
    - ascending sympathetic chain that returns to the internal carotid artery, the cavernous sinus, cranial nerve VI, and then via CN V1 to the eye
    - anisocoria worsening in the dark and mild ipsilateral ptosis (semiptosis) support the diagnosis of Horner syndrome (confirmation using cocaine drops is possible)
abnormal reaction to light
- failure to dilate in the dark
- failure to constrict to light or accommodation
- dissociation between direct and consensual reactions
- relative afferent pupillary defect (RAPD)
absent accommodation reaction
light-near reflex dissociation
- an impaired pupillary light reaction while the near reaction remains intact
- unilateral or bilateral (dorsal midbrain lesion)

diencephalon	homolateral miosis of 2 mm, hemianhidrosis, and often ptosis, i.e., central Horner syndrome due to a lesion of the subthalamic sympathetic center and the initial segment of the efferent pathway for pupillodilation light reflex is preserved bilateral miosis in thalamic hemorrhages is caused by a defect in the central inhibitory impulses to the Edinger-Westphal (E-W) nucleus
mesencephalon (midbrain)	tectum/pretectum lesion no light reaction with preserved accommodation response (light-near dissociation) preserved ciliospinal (CS) reflex isocoria with mydriasis 4-6 mm tegmental lesions damage to the E-W nucleus and sympathetic and parasympathetic pupillomotor pathways pupils are distorted, often anisocoric, 3-5 mm or mydriatic (depending on the preservation of sympathetic tracts) pupillary light reflex and ciliospinal reflex are absent unilateral tegmental lesion with mydriatic, reactive homolateral pupil, and contralateral hemiparesis may mimic the temporal cone. A distinguishing feature may be the preserved frontoorbicular reflex compressive lateral mesencephalic syndrome (uncal herniation) progressive homolateral mydriasis 5-9 mm (Hutchinson´s pupil) initially, the light reaction is preserved; later, the pupil becomes nonreactive and external oculomotor paresis with ptosis follows predominant pupillomotor involvement in the initial stage of CN III compression is due to anatomical predisposition – pupillomotor fibers, located on the upper dorsomedial side of the oculomotor nerve, are compressed against surrounding structures
pons	primary pontine lesions bilateral miosis ( pinpoint pupils) ~ 1 mm due to lesion of sympathetic pupillodilator pathways with facilitation of intact pupilloconstrictor fibers the light reaction is preserved as the pontine hemorrhage progresses, the pupils dilate and become nonreactive (probably due to the extension of the lesion into the midbrain) pontine stage of rostrocaudal deterioration syndrome pupils are nonreactive, diameter 3-5 mm
medulla oblongata	lateral bulbar lesions ipsilateral Horner syndrome with 1-2 mm miosis the light reaction is preserved *terminal stage of rostrocaudal deterioration* (bulbar coma)** bilateral nonreactive mydriasis (induced by systemic norepinephrine washout due to anoxia)