3.5 Cranial Nerve Monitoring

Question 1

When would you monitor cranial nerves (CN)?

Answer 1

1. Surgical
   * Intraoperatively during a surgical procedure
   to identify the nerve, preserve
   function, and prevent intraoperative injury.

Several neurosurgical procedures warrant routine monitoring
of cranial nerves depending on the
position and extent of surgery.

All cranial nerves except CN I can be monitored.

As most cranial nerves are motor, placing the
EMG electrodes on muscles supplied by the
nerve tests their function.

CN VIII, being a sensory nerve,
is tested by brainstem evoked auditory potential
(BAEP).

CN II is rarely monitored.

2. Neurophysiological

• Intraoperative nerve monitoring to
  assess the degree of neuromuscular block
  (commonly used technique is the Train of Four (ToF)).

Peripheral branches of the facial nerve are used
due to their ability to cause visible
muscular contractions and their proximity to the skin.

3. • Brainstem testing in ITU: to confirm brainstem death.

Question 2

What kind of surgery would warrant monitoring for various CN

Answer 2

1. • III, IV, and VI: removal of tumours at clivus or skull base
2. • V: microvascular decompression for trigeminal neuralgia
3. • VII: acoustic neuroma and parotid gland surgery
4. • VIII: cerebello pontine angle tumours
5. • IX: radical neck dissection
6. • X: thyroid or vocal cord surgery

7 * XI and XII: skull base (jugular glomus tumour) surgery

Question 3

What are the anaesthetic
implications of using cn
monitoring for neurosurgery?

Answer 3

1. • Choice of anaesthetic—
     use of inhalational agents and muscle relaxants
2. • Physiological factors that influence evoked potentials—
     temperature, acid-base, blood pressure, haematocrit, etc.
3. • Electrical artefacts—
     electrical interference can be a problem as SSEP is
     believed to simulate pacemaker spikes on ECG tracing

Question 4

Neurophysiological monitoring

Answer 4

Muscle relaxant Volatile agent

eMG:
Electromyogram &laquo_space;<−>

MeP:
Motor-evoked potential &laquo_space;«

BAeP:
Brainstem-evoked auditory potential <−> <−>

sseP:
Somatosensory-evoked potential <−> «

Question 5

Muscle Relaxant?

MEP

SSEP

Answer 5

The degree of muscle relaxation is the only
anaesthetic factor for concern
when myogenic (EMG) activity is used for monitoring purposes.

Special anaesthetic consideration is not required with BAEP.

• If MEP is monitored, then use of TIVA with Propofol and Remifentanil
  with no muscle relaxation is the anaesthetic of choice (as volatile limited
  to ≤ 0.5 MAC)
• With SSEP, a similar technique is used but without the need to restrict the
  use of neuromuscular blocker

Question 6

choose a cranial nerve and tell me about its origin and course.

Trigem

Function

Answer 6

Trigeminal nerve is the largest cranial nerve.

Function

• Motor: to the muscles of mastication
• Sensory: to the face, orbit, tongue, nose and anterior scalp

Nuclei of origin

• One motor: upper pons below the floor of IV ventricle
• Three sensory
  ° Mesencephalic nucleus (proprioception):
  midbrain

° Principal sensory nucleus (touch):
upper pons

° Nucleus of spinal tract (pain and temperature):
pons to spinal cord

Question 7

Trigeminal Course

Answer 7

course

• The sensory fibres decussate
  and emerge at the upper pons

as a larger sensory and smaller motor root

• Gasserian (trigeminal or semilunar) ganglion

is crescent-shaped swelling formed

by the sensory fibres situated at the apex

of the petrous temporal bone.

The ganglion is surrounded superiorly by the temporal lobe,
medially by the internal carotid artery and cavernous sinus,
and inferiorly lies the motor root

• The motor fibres bypass the ganglion and join the mandibular division

Question 8

Trigeminal Division and distribution

Answer 8

1. • Ophthalmic (V1): sensory only
     ° Emerges via superior orbital fissure

° Frontal, lacrimal and nasociliary nerves

2. • Maxillary (V2): sensory only
     ° Leaves the base of skull via foramen rotundum

° Gives off branches to supply the pterygopalatine fossa
and the face before it exits through the
infraorbital foramen as the infraorbital nerve

3. • Mandibular (V3): sensory and motor
     ° Via Foramen ovale
     ° Sensory: auriculotemporal, buccal, lingual, and inferior alveolar nerves

• Motor: muscles of mastication

Question 9

How is trigeminal nerve tested to confirm brainstem death?

Answer 9

It is tested by the interrogation of brainstem-mediated
V and VII cranial nerve reflexes.

1. • Corneal reflex
     ° Cornea is touched with a wisp of cotton wool.
     Blinking of the eyelids is the normal response

° No response should be elicited in brainstem death

° Reflex: Afferent fibres via ophthalmic branch of CN V
and efferent pathway via CN VII

2. • Deep central somatic stimulation

° Apply deep supraorbital pressure and look for a central motor
response in the distribution of the facial nerve (grimace)

° Reflex: afferent via CN V and efferent via CN VII

Question 10

What are the causes of trigeminal nerve injury?

Answer 10

Mainly surgical;

during neurosurgical decompression of trigeminal ganglion,
maxillofacial procedures, and dental injections.

Question 11

When can facial nerve be damaged?

Answer 11

Anaesthetic:

compression from facemasks and endotracheal tube ties,

stretching of the nerve because of faulty positioning,

direct injury due to nerve blocks.

Surgical:
direct surgical trauma.

The risk factors associated with incidence of nerve injury are
diabetes,
intraoperative hypotension,
hypoxia,
hypothermia, and
electrolyte imbalance.