2.7 Trigeminal Nerve

Question 1

The Anatomy of the Trigeminal Nerve

Answer 1

The trigeminal (fifth cranial nerve, V) is the largest of the 12, and provides the
sensory supply to the face, nose and mouth as well as much of the scalp.

Its motorb branches include the supply to the muscles of mastication.

Question 2

Nuclei

Answer 2

It has a single motor nucleus and three sensory nuclei in the brain.

The motor nucleus is in the upper pons,
and lying lateral to it is the principal sensory nucleus,
which subserves touch sensation.

The mesencephalic nucleus is sited in the midbrain and subserves proprioception.

Pain and temperature sensation are subserved by the
nucleus of the spinal tract of the trigeminal nerve.

This lies deep to a tract of descending fibres which run from the pons to the substantia gelatinosa of the spinal cord.

Question 3

Ganglion

Answer 3

Sensory fibres pass through the trigeminal (Gasserian) ganglion.

It is crescent-shaped (hence its alternative description as the semilunar ganglion),

and lies within an invagination of dura mater near the apex
of the petrous temporal bone,

and at the posterior extremity of the zygomatic arch.

The motor fibres of the trigeminal nerve pass below the ganglion

Question 4

Divisions

Answer 4

From this ganglion pass the three divisions of the nerve:

the ophthalmic (V1), which
is the smallest of the three

; the maxillary (V2); and

the mandibular (V3).

(This division explains the name: ‘tri-gemini’; from the Latin for ‘triplet’).

Question 5

V1

Answer 5

Ophthalmic division V1:

this passes along the lateral wall of the cavernous sinus
before dividing just before the superior orbital fissure into the
lacrimal, nasociliary and frontal branches.

The frontal branch divides further into the supraorbital and supratrochlear nerves.

Question 6

V2

Answer 6

Maxillary division V2:

This runs below the ophthalmic division before leaving the
base of the skull via the foramen rotundum.

It crosses the pterygopalatine fossa, giving off
superior alveolar dental nerves, zygomatic nerves and sphenopalatine
nerves before entering the infraorbital canal and emerging through the infraorbital
foramen as the infraorbital nerve.

Question 7

V3

Answer 7

Mandibular division V3: this is the largest of the three branches and is the only
one to have both motor and sensory components.

Its large sensory root passes through the foramen ovale
to join with the smaller motor root, which runs beneath the ganglion.

Its branches include the sensory lingual, auriculotemporal and buccal
nerves; the inferior dental nerve, which is mixed motor and sensory; and motor
nerves to the muscles of mastication, the masseteric and lateral pterygoid nerves

Question 8

Trigeminal Neuralgia: Definition,

Answer 8

Definition: trigeminal neuralgia is a severe neuropathic pain with a reputation as one
of the worst pains in human experience.

Question 9

Clinical Features and

Question 10

Clinical Features and

Answer 10

The peak onset of the condition is in middle age. The pain typically
is intermittent, lancinating and of the utmost severity.

Attacks are spasmodic, lasting only seconds.

Patients are pain-free in the interim, but episodes may be very frequent.

Pain is limited usually to one (occasionally two) of the branches of the trigeminal nerve, which supplies sensation to the face.

It occurs least commonly in the ophthalmic division, which accounts for only around 5% of cases, and more frequently in the maxillary or mandibular divisions.

The distribution is always unilateral.

Paroxysmal pain can be precipitated by trigger points around the face
which react to the lightest of stimuli, such as a light breeze or touch, and by actions
such as chewing or shaving.

Question 11

Pathogenesis:

Answer 11

this remains speculative.

It may be caused centrally, with abnormal
neurons in the pons exhibiting spontaneous and
uncontrolled discharge in the nerve.

It may also be caused by peripheral factors:
due either to demyelination
(in younger patients, trigeminal neuralgia may be a first symptom of multiple sclerosis)

or to compression by abnormal blood vessels in the posterior fossa.

Question 12

Pharmacological treatment

Answer 12

: (in an anatomy oral you will probably not be asked
about this in great detail; it is included in the following for completeness.

This is adequate treatment for around 75% of cases,
although the effectiveness of medical
therapy does fade with time,
with up to 50% of patients eventually experiencing
breakthrough pain).

Question 13

Carbamazepine

Answer 13

is said to be effective in more than 90% of cases of true
trigeminal neuralgia (100 mg b.d. up to maintenance of 600–1,200 mg day1).
The full blood count must be monitored because the drug can cause bone
marrow suppression.

Question 14

Phenytoin

Answer 14

Phenytoin is effective in a smaller proportion (around 60%) and can be given
intravenously for acute intractable pain (the starting dose is 300–500 mg day1).

Question 15

Baclofen

Answer 15

Baclofen is an antispasmodic γ-amino butyric acid (GABA) analogue, which binds
to GABAB receptors (the dose is up to 80 mg day1).

Question 16

Gabapentin

Answer 16

Gabapentin is a GABA analogue, which does not, however, act on GABA receptors.
Its mechanism of action is unclear.

It is an anticonvulsant which clinicians
increasingly are using to treat neuropathic pain. It appears to be particularly
effective in patients whose trigeminal neuralgia is secondary to multiple sclerosis.
The dose is titrated against response to a maximum of 1,800 mg daily

Question 17

Non-Pharmacological Methods of Management
Destructive

RF ablation

Answer 17

Radiofrequency ablation: a needle is passed percutaneously and under X-ray control
through the foramen ovale to the trigeminal ganglion. The entry point of the needle
is below the posterior third of the zygoma.

Chemical ablation may also be used. This technique can be complicated by anaesthesia dolorosa, in which the patient loses not only the pain, but also most of the sensation to that side of the face, which feels dead and ‘woody’.

The patient needs to be awake and cooperative during part of the
procedure but needs to be ‘deeply sedated’ – transiently – for the ablation itself.

This
can be challenging.

Question 18

Gamma Knife surgery

Answer 18

Gamma knife surgery: this is stereotactic radiosurgery delivered by intensely
focused gamma radiation from an array of cobalt sources. It is as effective as radiofrequency
ablation, although full relief may take some weeks to develop.

Question 19

Surgical decompression:

Answer 19

Surgical decompression:

this is the most invasive therapeutic technique because
it requires formal neurosurgical exploration of the posterior fossa to identify the
aberrant vessel(s) which are compressing the nerve near its emergence from the pons.