Brainstem reflexes

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)

→ more about caloric reflex

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)
Avoid 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

• 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

• 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

• 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