Brainstem reflexes

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

• dilation is quite mild (1-2 mm) and should be checked with a magnifying glass
• it 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 level
  • 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:
  • because this reflex is mediated by the cervical sympathetic fibers, it is absent in Horner syndrome
  • reflex is also absent in propofol-induced anesthesia
  • enhanced ciliospinal reflex is present in asymptomatic patients with cluster headache
• the presence of ciliospinal reflex after cardiopulmonary resuscitation may be a positive prognostic sign for regaining consciousness

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 by 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 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 the 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) controls the diameter of the pupil in response to the intensity of light falling on the retina
• illumination of one eye causes both pupils to constrict
  • 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 produces only a direct reaction
• brainstem death: the 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)

• otoscopy is used to verify an intact external auditory canal and the tympanic membrane
  
• keep the patient´s head in 20-30° anteflexion
• instill 30-50 mL of cold saline into the external ear over 1 minute (both sides should be tested separately, 5 min apart). The normal response is represented by eye movement to the side of the irritated ear canal

• afferent pathway: glossopharyngeal nerve
• center: the vestibular and abducens nuclei in the pons and 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)
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 is probably at the cortico-subcortical or diencephalic level

• lesion of the homolateral pontine visual center

• failure to elicit during 15 rotations
• indicates a large pontine lesion (primary or secondary due to herniation)
• exclude the effect of barbiturates

• allows assessment of oculomotor nerve palsy
• e.g., paresis of adduction with intact contralateral abduction indicates oculomotor nerve palsy
• rarely, the one-and-half syndrome can be induced (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 abducens nerve nucleus and the ipsilateral medial longitudinal fasciculus (FLM)

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 in the first 20 seconds
• polysynaptic reflex ⇒ trigeminovagal reflex
  • afferent part – trigeminal nerve (V1 branch)
  • efferent part – vagal nerve
• the reflex may induce bradycardia, in extreme cases, even cardiac arrest

• when the bulbar level is reached during the craniocaudal deterioration syndrome in supratentorial lesions
• in primary caudal brainstem lesions
• in pontine lesions damaging bilateral trigeminus nuclei
• in orbital fracture and 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 by damaging the ciliary ganglion

Bulbar level
The test is performed by a deep tracheal irritation. The response in brainstem death is the absence of a cough

• vagal nerve – multiple nuclei forming in medulla oblongata – vagal nerve