Brainstem reflexes

Created 11/04/2023, last revision 18/09/2023

  • brainstem reflexes are involuntary motor responses originating in the brainstem
  • these reflexes are mediated by neural circuits that bypass higher cortical centers, allowing for rapid, automatic responses
  • they are used in clinical settings to assess the integrity of the brainstem and cranial nerves

Diencephalic level
Painful stimulus is applied to the neck, face, or upper trunk → rapid dilation of the ipsilateral pupil occurs (1-2mm from baseline)
The ciliospinal reflex is mediated by the sympathetic nervous system and is regulated by the ciliospinal center in the spinal cord. Stimulation of this center induces pupil dilation on the same side as the stimulus

  • dilation is relatively mild (1-2 mm) and should be checked with a magnifying glass
  • the reflex is extinguished in lesions of the brainstem, cervical spinal cord, and in lesions of the preganglionic and postganglionic fibers ⇒ areflexia is of little use in topical diagnoses
  • the reflex may help differentiate lesions at the cortico-subcortical and diencephalic levels
    • if present, the disturbance of consciousness is caused by a lesion at the cortico-subcortical level
    • when the reflex is absent and the other brainstem reflexes are preserved, the lesion extends to the diencephalic level
  • other factors influencing CS reflex:
    • reflex is mediated by the cervical sympathetic fibers and is thus absent in Horner syndrome
    • reflex is also absent during propofol-induced anesthesia
    • enhanced ciliospinal reflex is observed in asymptomatic patients with cluster headache
  • the presence of ciliospinal reflex after cardiopulmonary resuscitation may be a favorable prognostic sign for regaining consciousness

Pathway of the sympathetic innervation of the pupil

The sympathetic efferent pathway has three neurons and is ipsilateral

  • The first (central) neuron arises from the posterior hypothalamus and descends through the brainstem on the same side, close to the trigeminal nucleus. It descends dorsally and rostrally to the red nucleus and lies at the lateral tegmentum of the midbrain, pons, and medulla. It synapses at the ciliospinal center of Budge, located at C8 to T2 in the intermediolateral (IML) horn of the gray matter of the spinal cord.
  • The second (preganglionic) neuron originates from the ciliospinal center and goes to the superior cervical ganglion in the neck. It exits the spinal cord through the dorsal spinal root and enters the paravertebral sympathetic chain. The second-order neuron passes near the apex of the lung and may suffer damage in surgeries of the neck or bronchogenic carcinoma (Pancoast tumor), causing Horner’s syndrome.
  • The third (postganglionic) neuron passes on the surface of the internal carotid artery and joins the ophthalmic nerve at the cavernous sinus. The sympathetic postganglionic fibers pass through the nasociliary nerve and long ciliary nerve and reach the ciliary body and the dilator pupillae muscles

Diencephalic-mesencephalic level
Tapping the glabella and the supraorbital arches  → contraction of the upper parts of the orbicularis oculi muscle

  • the reflex has two integral components:
    • early – represented by the exteroceptive oligosynaptic trigeminofacial reflex with the center in the upper pons
    • late – a nociceptive multisynaptic reflex integrated within the meso-diencephalic reticular formation (RF)
  • the reflex is normally habituated through repeated stimulation (extinction phenomenon)
  • abnormal response:
    • in patients with cortical impairment, the reflex is not habituated due to a disturbance of the cortical inhibitory mechanism
    • minimal unilateral contraction indicates an incomplete lesion of the diencephalic-mesencephalic RF
    • reflex is affected by sedatives
    • reflex is absent in lesions of the diencephalic-mesencephalic junction

Diencephalic-mesencephalic level
Repeated movements of the patient’s head (anteflexion-retroflection) → conjugate movement of the eyeballs occurs in the opposite direction (baby-doll response)

  • vertical and horizontal oculocephalic reflex (doll’s eyes reflex) is an application of the vestibular-ocular reflex (VOR) used for neurologic examination of cranial nerves III, VI, and VIII
  • vestibulo-proprioceptive-oculomotor reflex helps:
    • test for oculomotor paralysis in unconscious patients
    • assess the level of brainstem functional transsection
  • absent in conscious persons (inhibited by visual cortical centers)
  • vertical deviation of at least one eyeball indicates the presence of the reflex
  • usually, only head retroflexion is tested (repeated anteflexion may increase the risk of herniation)
    • upward movement can be tested by corneal reflex (Bell’s phenomenon)
  • reflex is absent in lesions at the diencephalic-mesencephalic junction ⇒ dissociation of the oculocephalic reflexes (vertical absent, horizontal present)
  • this reflex is also affected by lesions of the vestibulo-ocular structures

Mesencephalic-pontine level (middle mesencephalon)

  • the pupillary light reflex (PLR) regulates the diameter of the pupil in response to the intensity of light falling on the retina
  • illumination of one eye induces constricition in both pupils
    • direct response – miosis of the illuminated pupil
    • consensual response – miosis of the contralateral pupil
  • optic nerve lesion:  direct and indirect responses are absent if the affected eye is illuminated
  • oculomotor nerve lesion:  illumination of the damaged side induces only a consensual reaction; the direct reaction is absent (due to the damage of efferent pathways). Illumination of the healthy eye induces only a direct reaction
  • brainstem death: absence of pupillary reaction in both eyes

Mesencephalic-pontine level (upper pons)
Cornea touch from the side (to eliminate blink reaction triggered by the optic nerve) → bilateral eyelid closure + Bell’s phenomenon ( elevation and slight abduction of the eyeballs)

  • the intensity of the evoking stimulus must be proportional to the subject’s level of consciousness.
    • a gentle touch to the cornea in a fully conscious person
    • stronger or repeated stimulation in a coma
  • a nociceptive polysynaptic reflex, with the trigeminal nerve as the afferent pathway and facial and oculomotor nerves as the efferent pathway
  • the afferent part of the reflex arc has synapses with bulbar and pontine nuclei of the trigeminal nerve ⇒  corneal reflex is altered by pontine and bulbar lesions

Mesencephalic-pontine level (middle pons)

  • also called the jaw-jerk reflex
  • with the patient’s jaw slightly open, the reflex is elicited by tapping with a reflex hammer on the chin or on a tongue blade resting on the lower teeth or tongue
  • in a positive response, a sudden stretching of the masseter muscle causes a reflex contraction, moving the jaw upward
  • trigemino-trigeminal reflex
    • the afferent arc: trigeminal nerve (V3 branch)
    • the efferent arc: trigeminal nerve (V3 branch)
  • abnormal responses:
    • the reflex is extinguished in a primary pontine lesion or as a consequence of central craniocaudal deterioration syndrome reaching the level of upper pons (functional transsection)
    • the reflex ceases in a deep coma as a result of the muscular atonia
    • an exaggerated jaw jerk (sometimes with clonus) implies a bilateral lesion above the level of the pons (e.g., pseudobulbar palsy)

Mesencephalic-pontine level (middle pons)

  • verify an intact external auditory canal and tympanic membrane via otoscopy
  • keep the patient´s head at 20-30° anteflexion
  • instill 30-50 mL of cold saline into the external ear over 1 minute (both sides should be tested separately, with a 5-minute interval between tests. The normal response is represented by eye movement toward the side of the irritated ear canal and the fast component of the nystagmus beating away from the stimulated ear (the fast component is absent in coma)
  • afferent pathway: vestibular nerve
  • center: the vestibular and abducens nuclei in the pons, as well as oculomotor nuclei in the mesencephalon
  • efferent pathway: oculomotor and abducens nerves (to medial and lateral rectus muscles)

Mesencephalic-pontine level (lower pons)
Rotation of the patient’s head from side to side induces conjugate movement of the eyeballs in the opposite direction (doll’s eyes)
void this test in patients with head or neck injuries!

  • excludes direct destruction of the brainstem at the level of the oculomotor nuclei as a cause of coma
  • in unconscious patients, the lesion probably resides at the cortico-subcortical or diencephalic level
absent in one direction
  • lesion of the homolateral pontine or contralateral cortical visual center
  • failure to elicit during 15 rotations indicates a large pontine lesion (either primary or secondary due to herniation)
  • exclude the effect of barbiturates
dissociated horizontal movements
  • allows assessment of oculomotor disordres
    • oculomotor nerve palsy – paresis of adduction with intact contralateral abduction
    • internuclear ophthalmoplegia (INO)
      • impaired adduction of the ipsilateral eye with nystagmus of the abducting eye
      • caused by a lesion of the medial longitudinal fasciculus
    • one-and-half syndrome
      • a combination of ipsilateral conjugate horizontal gaze palsy (one) and ipsilateral internuclear ophthalmoplegia (INO) (a half)
      • only a swaying abduction movement of one eye is present, while the contralateral eye is immobile
      • the lesion is localized in the inferior pons and involves the paramedian pontine reticular formation (PPRF), abducens nucleus and the ipsilateral medial longitudinal fasciculus (MLF)

Ponto-bulbar level
The eyeball is compressed for 10-20 seconds; the test is positive if the pulse drops by > 8-10

  • oculocardiac reflex (OCR) tests the integrity of the caudal brainstem
  • the response is maximal within the first 20 seconds
  • polysynaptic (trigeminovagal) reflex
    • afferent part – trigeminal nerve (V1 branch)
    • efferent part – vagal nerve
  • the reflex may induce bradycardia and, in extreme cases, cardiac arrest
Absent or abnormal response:
  • when the bulbar level is reached during craniocaudal deterioration syndrome in supratentorial lesions
  • in primary caudal brainstem lesions
  • in pontine lesions affecting bilateral trigeminal nuclei
  • in orbital fracture or damage of the V1 branch of the trigeminal nerve
  • the response is increased in hypercapnia and hypokalemia, decreased after atropine administration
  • traumatic orbital hemorrhage may induce homolateral mydriasis and bradycardia due to damaging to the ciliary ganglion

Bulbar level
The test is performed by deep tracheal irritation or stimulation of the posterior pharyngeal wall or the laryngeal region using a soft-tip catheter.
Absent cough reflex suggests lesion in the afferent or efferent pathways (involving the vagus and phrenic nerves) or the medullary centers.
The response in brainstem death is the absence of a cough

  • cough reflex is a complex physiological mechanism that serves to protect the airways from aspiration and facilitate the removal of mucus and foreign matter.
  • vagal nerve (sensory receptors, mainly in the larynx and tracheobronchial tree) – multiple nuclei forming in the medulla oblongata (cough center) – vagal nerve
reflex afferent pathway center efferent pathway
Cilio-spinal (CS) cervical pain fibers (neck)
trigeminal nerve (face)
spinal cord
(intermediolateral column)
sympathetic fibers
Pupillary retina – optic nerve pretectal nucleus, Edinger-Westphall nucleus oculomotor nerve (miosis)
hypothalamus, cervical ganglion sympathetic fibers (mydriasis)
Fronto-orbicular (nasopalpebral)
trigeminal nerve (V1) pons facial nerve
Oculocephalic vertical vestibulocochlear nerve mesencephalon (midbrain) oculomotor nerve
Corneal trigeminal nerve (V1) pons facial nerve
oculomotor nerve
Masseter (jaw jerk)
trigeminal nerve (V3) pons trigeminal nerve (V3)
Vestibulo-ocular (caloric) reflex vestibulocochlear nerve mesencephalon
oculomotor nerve
abducent nerve
Oculocephalic horizontal vestibulocochlear nerve pons oculomotor nerve
abducent nerve
Oculocardiac trigeminal nerve (V1) pons
medulla oblongata
vagal nerve
Cough reflex vagal nerve
medulla oblongata vagal nerve
Gag reflex
glossopharyngeal nerve
medulla oblongata vagal nerve

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Brainstem reflexes