facial nerve- pathophysiology, electrodiagnostic and imaging

1. FACIAL NERVE:
PATHOPHYSIOLOGY,
ELECTRODIAGNOSTIC TESTS
&
IMAGING
Moderator: Dr Prachi Mene
Consultant ENT and Head & Neck Surgery
Presented by Dr Ranjeet kumar Lal
Secondary DNB

2. • INTRODUCTION
• PATHOPHYSIOLOGY
• TOPODIAGNOSTIC TESTS
• ELECTRODIAGNOSTIC TESTS
• IMAGING

3. • It’s the 7th cranial nerve
• It is a mixed nerve
• 10,000 Motor , Sensory , parasympathetic fibers
• Motor root – 7000 special visceral efferent fibers
• Sensory & Parasympathetic – 3000 fibers carried by
“NERVUS INTERMEDIUS” (Nerve of Wrisberg).
• The facial nerve innervates 14 of the 17 paired muscle
groups of the face.

4. Structure of nerve
• Each of the nerve fibers
consists of:
• a central protoplasmic
process of parent
neuron—the axon
• Axon is surrounded by
– myelin -an insulating layer
and
– Schwann cells
-- by the thin protoplasmic
cytoplasm cells that
constitute the
neurilemmal sheath. .

5. Components of a Nerve fiber
• Endoneurium
– Surrounds each nerve fiber
– Provides endoneural tube for
regeneration
– Much poorer prognosis if
disrupted
• Perineurium
– Surrounds a group of nerve
fibers
– Provides tensile strength
– Protects nerve from infection
– Pressure regulation
• Epineurium
– Surrounds the entire nerve
– Provides nutrition to nerve

7. Pathophysiology of facial nerve
Degree of nerve injury.
Seddon in 1943 described
three types of nerve
injuries:
1. Neurapraxia
2. Axonotemesis
3. Neurotmesis

8. Neuropraxia
• Compression or stretch
• Anoxic, physiologic blockage of both axoplasmic transport and
ion channel functions along the affected axon.
• Loss of function is usually temporary
• Release of compressive agent results in rapid and complete
recovery of function.
• compression or traction resulting in prolonged ischaemia --
demyelination.

9. Neuropraxia
• “The term neurapraxia is best reserved for
those situations where electrodiagnosis has
conclusively shown that demyelinating
conduction block is solely responsible for the
neural lesion”

10. Axonotmesis
• -occurs due to blunt injury to a nerve
• resulting in Wallerian degeneration distal to
the injury.
• the connective tissue layers (epi-, peri- and
endo-neurium) remain intact.
• Conduction ceases throughout the distal
extent of the nerve within a few days of injury.

11. Neurotmesis
• nerve is completely divided or
• so badly disorganized by injury that recovery without
some form of surgical intervention is impossible.
• All the connective tissue layers as well as the axons are
disrupted at the site of injury
• Spontaneous axonal regeneration will be imperfect and
disorderly, and may not occur at all.
• Distance is a significant factor in recovery.

12. Sunderland grading (1951)
Sundeland seddon Pathology
Grade 1 neuropraxia compression causing endoneurial oedema but no significant
morphological changes.
Substantial and prolonged mechanical compression- focal
demyelination
Grade 2 axonotmesis Axons degenerate distal to the site of the lesion.
Endoneurium, perineurium and epineurium remain intact
Grade 3 axonotmesis Endoneurium disrupted, axons degenerate distal to the site of
the lesion
Grade 4 axonotmesis Perineurium disrupted, axons degenerate distal to the site of
the lesion
Grade 5 nuerotmesis Epineurium disrupted, axons degenerate distal to the site of
the lesion

13. Grade 6
• In 1988, MacKinnon and Dellon added a Grade
6 injury to Sunderland’s classification’
• Better reflect the common clinical scenario of
a mixture of crush and transection injury
occurring within a damaged nerve.

14. • Type of nerve injury can help to assess the prognosis
of recovery of facial nerve functions
Gerade 1 and 2
injuries
Bell’s palsy and
herpes zoster
cephalicus
Complete recovery
Grade 3 injuries Trauma, tumour or
cholesteatoma
Recover partially
Grades 4 and 5
injuries
Trauma, tumour or
cholesteatoma
require surgical
intervention

15. NERVE REGENERATION
Defined as a complex
interaction of neurons,
Schwann cells, extracellular
matrix and neurotrophic
substances.
− Regeneration follows
degeneration, which is
usually of the Wallerian
type.
− there is sprouting at the
axonal end
− one regenerating axon may
produce as many as 20
sprouts
− sprouting ends usually
restore the nerve continuity,
which is accompanied by
reformation of myelin

17. NERVE REGENERATION
May cause three major changes in
the axon:
1. The distance between the node
of Ranvier is altered.
2. The myelin covering the axons is
much thinner than normal.
3. splitting and crossing of axons
that reinnervate the denervated
muscle groups.
4. It may not correspond to the
cell-body unit arrangement that
was present prior to
degeneration.

18. The various complications of regeneration such
as
– synkinesis,
– crocodile tears,
– Facial myokimias,
– hemifacial spasms and
– stapedius muscle contraction

19. Nerve regeneration
Factors affecting regeneration are:
• Site of lesion:
• Higher the site of the lesion poorer is the prognosis.
• Closer the lesion to the neuron, poorer is the quality of regenartion
• Duration of the injury:
– Longer the duration of the injury,
– poorer is the prognosis and quality of regeneration.
• Age:
– Regeneration is better in children due to a better neural plasticity seen in
children
• Nutrition
• Blood supply
• Associated injury or infection

20. FACIAL PALSY

21. Approach
HISTORY
onset
duration
Progression
h/o head injury
Recurrence
PHYSICAL FINDINGS
U/l or b/l
pain over mastoid /
preauricular area
vesicles on the pinna /
EAC
s/s/o ASOM / CSOM
Head & neck examn
Showing mass or
involvement of CN
INVESTIGATIONS
Topodiagnostic
Tests,
Electrodiagnostic tests
&
Imaging

22. HOUSE BRACKMANN FACIAL NERVE PALSY
GRADING SYSTEM

23. SUPRA NUCLEAR VS INFRANUCLEAR
LESION

24. TOPODIAGNOSTIC TESTS

25. TOPODIAGNOSTIC TESTS
• SCHIRMER TEST- decrease in lacrimation of 75% or
more as compared to normal side. Or < 10mm for both
sides at 5 min.
• STAPEDIAL REFLEX TESTING - if absent , site of lesion
between geniculate ganglion and stapedius muscle. If
present then site of lesion is distal to stapedius muscle.
• TASTE TESTING – conc. Sweet, salt, sour and bitter
solution tested along lateral margin of anterior 2/3 of
tongue towards tip / electrogustometry ( EGM )
• SUBMANDIBULAR GLAND FLOW- compared by
sialometry using 6% citric acid.
• TESTING FACIAL MOVEMENT

26. TOPODIAGNOSTIC TESTS

27. ELECTRODIAGNOSTIC /
ELECTROPHYSIOLOGOCAL
TESTS

28. ELECTRODIAGNOTIC TESTS
• They have been developed to:
– evaluate the degree of dysfunction and
– assumed viability of the facial nerve
– in the anticipation that results would provide prognostic
information for recovery.
• These tests are not normally used in the assessment of
incomplete paresis, which has a higher probability of
full, functional recovery.
• They are only used in patients :
– with complete established paralysis,
– when they can aid management decisions concerning
facial reanimation.

29. ELECTROPHYSIOLOGICAL NERVE
TESTING
• NERVE EXCITABILITY TEST (NET)
• MAXIMUM STIMULATION TEST (MST)
• NERVE CONDUCTION VELOCITY TEST (NVT)
• ELECTROMYOGRAPHY (EMG)
• ELECTRONEURONOGRAPHY (EnoG)
• TRANSCRANIAL MAGNETIC STIMULATION

30. NERVE EXCITABILITY TEST (NET)
• Compares transcutaneous current threshold
required to elicit minimal muscle contraction
between two sides
• A difference of 3.5 milliamperes (mA) or more in
thresholds between the two sides - a reliable
indicator of progressive degeneration:indicator
for surgical decompression
• If the paralysis becomes total, the test can
determine whether a pure conduction block
exists or whether degeneration is occurring, as
indicated by progressive loss of excitability

31. MAXIMAL STIMULATION TEST (MST)
• Instead of measuring threshold, however, maximal stimuli is
employed. (current levels at which the greatest amplitude of facial
movement is seen)
• strength and duration of stimulation is gradually increased from 1mA
to 5mA.
• Degree of facial contraction is subjectively assessed as either
– Equal (100%),
– mildly decreased (75%),
– markedly decreased (50%), or
– without response (0%) ,
compared with that on the normal side.
• Useless <72 hrs.
• Symmetric response within first ten days – complete recovery in
>90%
• No response within first ten days – incomplete recovery with
significant sequelae

32. Comparison of NET and MST
TEST TECHNIQUE OUTCOME MEASURE PROGNOSTIC VALUE
NET Compares transcutaneous current
threshold required to elicit minimal
muscle contraction between two sides
> 3.5 mA difference is
considered significant
Indicate poor
prognosis
MST Compares muscle contraction at
maximal nerve stimulation (~5 mA)
between two sides
Equal response,
reduced response or
absent response
Loss of response
within 10 days is
associated with
incomplete recovery

33. NERVE CONDUCTION VELOCITY TEST
• To test the latency response of a muscle on electrical
stimulation.
• The latency for each compound action potential
(CMAP) is defined
– as the time between onset of stimulus and onset of
response.
• The least reliable prognostic test-
– due to the fact that there is a variable transmission of
stimulus across the neuromuscular junction.
• It correlates with the degree of myelination and hence
low velocity or high latency indicates demyelination.

34. ELECTROMYOGRAPHY
• 1st used by Weddell and
colleagues (1944) for facial
paralysis.
• The recording of spontaneous and
voluntary muscle potentials by
needles introduced into the
muscle is called
electromyography(EMG).
• Records motor unit potentials of
the orbicularis oculi & orbicularis
oris muscle during rest &
voluntary contraction
• In a normal resting muscle
biphasic / triphasic potentials are
seen every 30-50msec

35. • Polyphasic potential indicate regenrative process & surgical intervention
is therefore not indicated
• Fibrillation indicate lower motor neuron denervation but viable motor end
plates, so surgical intervention needed(to achieve nerve continuity)
• Electrical silence indicates atrophy of motor end plates & need for muscle
transfer procedure

36. ELECTROMYOGRAPHY
EMG can be used to determine:
1. If a nerve in question is in fact in
continuity (volitional activity
recorded)
2. Evidence of degenration (
fibrillation after 10-14 days)
3. If there are early sign of
reinnervation (polyphasic
innervation potentials after 4-6
weeks)
• Fibrillation potentials
typically arises 2-3 weeks
following injury
• With regeneration of
nerve after injury,
polyphasic reinnervation
potential replaces
fibrillation potential
• Reinnervation potentials
may precede clinical signs
of recovery by 6-12 weeks

37. EVOKED ELECTROMYOGRAPHy(EEMG) OR
ELECTRONEURONOGRAPHY (EnoG)
• Records compound muscle action potential (CMAP) with
surface electrodes placed transcutaneously in the nasolabial
fold (response) and stylomastoid foramen (stimulus).

38. EVOKED ELECTROMYOGRAPHy(EEMG) OR
ELECTRONEURONOGRAPHY (EnoG)
• Described by Esselen (1977),
• popularized by Fisch (1981) as electroneuronography
(ENoG)
• May et al. (1981) called it as evoked electromyography
(EEMG).
• Action potentials developed
– is measured and expressed as % of degeneration
compared to normal side.
• Surgical intervention in case of immediate paralysis
with> 90% degeneration.
• It differs from EMG in that bipolar electrodes are used
for stimulating as well as recording.

39. EVOKED ELECTROMYOGRAPHy(EEMG) OR
ELECTRONEURONOGRAPHY (EnoG)
• Waveform responses are analyzed
– to compare peak to peak amplitudes between normal and uninvolved
sides where
– the peak amplitude is proportional to the number of intact axons.
• It shown to be the most accurate prognostic indicator of all
electrodiagnostic tests.
• Its main indication: acute onset complete facial paralysis
• This method offers the potential advantage of an objective
registration of electrically evoked responses, and
• The amplitude of response of the paralyzed side (in mV) can be
expressed as a precise percentage of the normal side's response.
– Response <10% of normal in first 3 weeks -poor prognosis
– Response >90% of normal within 3 weeks of onset- 80-100%
probability of recovery

40. EVOKED ELECTROMYOGRAPHy(EEMG) OR
ELECTRONEURONOGRAPHY (EnoG)
• Not useful until 4th day of paralysis as it takes about 3 days for
degeneration to reach completion
• Also of less value after three weeks bcoz of nerve fiber
desynchronization
• Advantages: Reliable
• Disadvantages:
– Uncomfortable
– Cost
– Test-retest variability due to position of electrodes

41. ENoG & EMG
Study Measurement When to measure Use in acute onset
paralysis
Use in long-
standing
paralysis
ENoG Evoked CMAP
compared to
normal site
Between 3 days and
3 weeks
>90% of degenerated
fibres – poor prognosis
Not useful due to
desynchronisatio
n
EMG Active motor unit
potentials after
voluntary forceful
contraction
Complementary to
ENoG after 2 weeks
Presence of active motor
potentials in response to
voluntary contractions
indicates good prognosis
Fibrillation
potential suggest
Wallerian
degeneration
In long-standing
paralysis Polyphasic
potentials
suggest
reinnervation

42. TRANSCRANIAL MAGNETIC
STIMULATION
• Transcranial magnetic stimulation is able to stimulate
the facial nerve within the cranium.
• Magnetic impulses induce an electric current within
the brain and stimulate neuromuscular tissue.
• The depolarization of the facial nerve takes place at the
root entry zone (REZ) by a transcranial penetration.
• It helps to eliminate the delay in testing nerve function
due to its ability to stimulate the intratemporal
segment of the facial nerve.
• There is the decreased pain in testing compared to
electrical stimulation

43. ELECTROPHYSIOLOGICALS TESTS
• Electric impulse can stimulate only
normal/neuropraxic fibres and can’t
distinguish between axonotemesis or
neurotemesis
• Provides no useful information in cases of
incomplete facial paralysis
• It fails to provide information on the
immediate post paralysis period (first 72
hours).
LIMITATIONS

44. INTRATOPERATIVE FACIAL NERVE
MONITORING
• The common procedures :
– decompression,
– revision mastiodectomy,
– cochlear implant,
– parotidectomy,
– posterior and middle cranial fossa approach,

45. GOALS OF INTRAOPERATIVE NERVE
MONITORING
Early identification of facial nerve over the soft tissue,
tumor or bone
Mapping the course of the facial nerve in the temporal
bone or tumor
Warning the surgeon of an unexpected facial nerve in
the temporal bone or tumor
Reducing the mechanical trauma to the facial nerve
during re-routing or tumor dissection
Evaluation and prognosis of facial nerve function at the
conclusion of surgery

46. IMAGING OF FACIAL NERVE

47. IMAGING TECHNIQUES IN THE
EVALUATION OF FACIAL NERVE
• Imaging should be tailored to both the suspected
pathology and clinical localisation of the lesion along
the course of VII.
• MRI is indicated
– if a facial palsy is localised to the cisternal or
intracanalicular segments of the facial nerve or the
pontine nuclei
– The proximal extracranial portion of the facial nerve in the
parotid gland
• High resolution CT is indicated
– If the lesion can be localised to the mastoid, tympanic, or
labyrinthine segments

48. MRI scanning protocol
• MR scanning protocol for evaluation of VII
should include
– a heavily T2-weighted sequence (e.g. FIESTA, CISS)
– as well as thin-section T1-weighted postcontrast
sequences in axial and coronal planes.
– MRI allows evaluation of VII from the brainstem to
the fundus of IAC and
– particularly useful for determining the presence of
perineural spread from parotid malignancies

49. • On high-resolution T2-weighted images, the normal facial nerve
appears as a
– hypointense linear structure extending from the brainstem to the IAC,
anterior to the vestibulocochlear nerve, surrounded by T2-
hyperintense cerebrospinal fluid.
• Enhancement of the postgeniculate (postganglionic) segments of
the facial nerve
– can be a normal finding.
• However,
– asymmetric enhancement,
– nodularity,
– thickening, or
– extension to the cisternal/canalicular portions of the nerve
• are suggestive of disease.

50. Cisternal and canalicular course of the facial nerve. Axial heavily T2-weighted
CISS image through the pons shows intracisternal and intracanalicular component
of facial nerve (arrow) traversing the CPA cistern medial to vestibulocochlear (VIII)
nerve.

51. HRCT protocol
• High-resolution CT is the optimal modality for imaging the
intratemporal course of VII,
– from the fundus of the IAC to the stylomastoid foramen.
• The high-resolution acquisition
– with overlapping thin sections of 0.6 mm,
– reconstructed into 0.3– 0.4 mm sections, allows equivalent voxels of
0.3 mm3
• It allows assessment of
– the caliber and course of the IAC and bony facial nerve canal, as well
as
– integrity of the bony canal.
• In addition, CT has the advantage of demonstrating the relationship
of the facial nerve canal to normal anatomic landmarks such as the
ossicles, and otic capsule structures.

52. Normal facial nerve canal on CT. Axial and coronal high resolution temporal
bone CT images demonstrate the intracanalicular, labyrinthine (arrows in (A)), geniculate
ganglion (arrow in (B)), tympanic (arrow in (C)), and mastoid (arrow in (D)) segments of
the facial nerve.

53. Hypoplasia of facial nerve
Hypoplasia of the right facial nerve. Axial (A) and sagittal reformatted (B)
heavily T2-weighted image shows a narrowed right IAC (green arrow)
with hypoplastic VII and VIII cranial nerves (blue arrow and yellow arrow
respectively).

54. BELL’S PALSY
• The most common cause of facial paralysis, is
characterised by
– an abrupt onset of facial weakness.
• MR imaging using gadolinium may
demonstrate enhancement of
– the geniculate ganglion,
– labyrinthine segment, and
– proximal tympanic segment without significant
enlargement of the nerve.

55. Bell's palsy. Axial and coronal contrast-enhanced T1-weighted images reveal
marked, asymmetric enhancement of the geniculate (yellow arrows in (A and D)),
tympanic (yellow arrow in (B)), and mastoid (yellow arrow in (C)) segments of the right
facial nerve.
Normal enhancement of the postgeniculate (postganglionic) segments of
the left facial nerve (blue arrows) due to surrounding veins.
There should be no significant enlargement or nodularity of the nerve to confidently
make a diagnosis of Bell's palsy.

56. Ramsay Hunt syndrome

57. Axial fat-suppressed, contrast-enhanced, T1-weighted images through the temporal
bones of a patient who had a painful external ear vesicular eruption, otalgia and
acute right facial paralysis show intense enhancement of the right facial nerve in the
geniculate ganglion, canalicular, labyrinthine (blue arrow in(A)), tympanic (blue arrow
in (B and C)) and mastoid segments (blue arrow in (D))

58. CHOLESTEATOMA WITH FACIAL PASLY
• Subtle erosion of the facial nerve canal may be
impossible to detect, especially because the
canal wall is inherently thin.
• Gross invasion of the facial nerve canal is
usually demonstrable using HRCT.
• Cholesteatoma is typically associated with
high signal intensity on diffusion-weighted
b1000 sequences and low signal intensity on
ADC with newer DWI techniques

59. Cholesteatoma with right facial palsy. Axial (A, C) and coronal (B, D) CT images in a
patient presenting with recurrent, transient right facial paraparesis show a
multiloculated, smoothly marginated lesion eroding into the tympanic and proximal
mastoid segment of the right facial nerve (yellow arrow) and lateral semicircular canal.
Left facial canal (blue arrow) and lateral semicircular canal (black arrow) for
comparison.

60. Cholesteatoma with right facial palsy. Axial T2-weighted image (A) and periodically
rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) diffusion
weighted image (DWI) (B) in the same patient shows lesion to be of intermediate signal
intensity on the T2-weighted image and demonstrating high PROPELLER DWI signal
intensity, compatible with cholesteatoma

61. FRACTURE OF TEMPORAL BONE
• Fractures of the temporal bone may be associated with
facial paralysis in up to 50% of cases.
• Facial paralysis is usually delayed, incomplete, and
transient.
• Any fracture of the temporal bone can result in facial nerve
injury, it is more likely to occur with transverse (38%-50% of
cases) rather than with longitudinal (20%) fractures.
• Immediate facial paralysis usually indicates severe nerve
injury (transection).
• paralysis of delayed onset is usually due to an intramural
hematoma.
• Fractures are best demonstrated on HRCT

62. Trauma of the temporal bone with peripheral right VII palsy. Axial CT image
demonstrates a complex fracture with a transverse component through the
tympanic segment of the bony facial nerve canal (arrow). Note blood
products in the middle ear cavity.

63. Neoplasms with facial palsy
• Tumors of the facial nerve usually present
– with progressive facial paresis or paralysis.
• The facial nerve may be affected by both
– primary neoplasms, or
– secondarily involved -by tumours of the temporal bone or
– -by perineural spread of extratemporal malignancy
• Metastasis commonly affects the petrous apex and the internal auditory
meatus.
– CT typically shows erosion and destruction of cortical bone.
– HRCT can visualize mineralisation of ossifying haemangiomas, distinguishing
these tumours from facial nerve schwannomas.
– MRI demonstrating enhancement on contrast-enhanced T1-weighted images
– Margins may be hazy and meninges may be thickened in cases of metastatic
disease

64. Neoplasms with facial palsy
• MRI with gadolinium is useful for detecting
perineural spread from parotid and minor salivary
gland malignancies along the facial nerve
• MRI is useful for accurate identification of
schwannomas, lipomas and haemangiomas

65. CPA Lipoma

66. CPA lipoma. Noncontrast axial T1-weighted (A) and T2-weighted image (B)
demonstrates a nodular hyperintense lesion in the right CPA (arrow).

67. Facial nerve hemangioma

68. Facial nerve haemangioma. Axial CT image in a patient with a 2-month history of
progressive right VII palsy demonstrates a mass with fine trabecular bony matrix
causing widening of the genu of the facial nerve, labyrinthine segment and fundus
of the IAC

69. PERINEURAL SPREAD OF
MALIGNANCY

70. Perineural spread of adenoid cystic carcinoma of the parotid. Axial T1-weighted
(A, B) and T2-weighted MR images (C, D) in a patient with a 1-year history of progressive
right VII palsy demonstrate a thickened and oedematous right facial nerve in its parotid
(white arrow) and mastoid segments (blue arrow). Adenoid cystic carcinoma of the parotid
(yellow arrow) and incidental right CPA arachnoid cyst (asterisk).

71. Schwannoma of right facial nerve

72. Axial T1-weighted (A, B), coronal T2-weighted (C) and coronal postcontrast T1-
weighted (D) images in a patient with right peripheral facial nerve palsy demonstrates a
dumbbell-like lesion appearing hyperintense on T2-weighted MR images and
enhancing avidly with contrast, involving the extracranial parotid segment of the right
facial nerve. Note extension of the schwannoma to the mastoid segment of the facial
canal (arrow in (B)).