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Experimental Study on Facial Nerve Regeneration With or Without Geniculate Ganglionectomy
Zhengmin Wang, MD;
Chunfu Dai, MD, PhD;
Yuhai Zhang, MD
Arch Otolaryngol Head Neck Surg. 2001;127:422-425.
ABSTRACT
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Objective To investigate regeneration of the distal facial nerve following nerve
grafting within the tympanic segment with geniculate ganglion preservation
or dissection.
Design Randomized controlled trial.
Subjects Twenty-three adult New Zealand albino rabbits were used in this study.
Interventions A 2-mm tympanic segment of the facial nerve was removed, and the greater
auricular nerve was harvested for grafting in all animals. In group 1 (10
rabbits), the geniculate ganglion was preserved. In group 2 (13 rabbits),
the geniculate ganglion was dissected. Mastoidal and extratemporal segments
of the facial nerve were harvested 3 months postoperatively for histological
examination by electron microscopy.
Results The number of myelinated axons in normal facial nerves was 1819.6 ±
535.6. In group 1, the number of myelinated axons was 123.6 ± 31.1,
and, compared with normal facial nerves, the diameter of the regenerative
axons was decreased and the sheath thickness in the regenerative fiber was
diminished. In group 2, the number of myelinated axons was 515.1 ±
103.1, while the myelin sheath thickness was proportionate to axon diameter.
(Data are given as mean ± SD.)
Conclusion Geniculate ganglionectomy may improve motor axon regeneration.
INTRODUCTION
SEVERAL ATTEMPTS have been made at morphological and quantitative analysis
of the geniculate ganglion.1, 2, 3
Previous studies2 indicated that the total
number of ganglion cells in a single temporal bone ranged from 589 to 4183
(mean, 2162 cells). In 88% of patients, most of these cells were found in
the geniculate ganglion.2 However, in 8% of
patients, most of these cells were in the internal acoustic meatus; in 4%,
the meatus and geniculate ganglion contained an equal number of cells.2 There was no correlation between total ganglion cell
number and age or sex of the patient.1 The
ganglion cell bodies were aggregated at the apex of the genu, close to the
origin of the greater superficial petrosal nerve.2
These findings suggested a possible therapeutic benefit from geniculate ganglionectomy
in patients with facial paralysis.
Relative to other cranial nerves, the facial nerve is particularly prone
to injury because of the long distance it traverses intratemporally and extratemporally.
Studies4, 5, 6, 7, 8
have been conducted that focus on axon regeneration and functional recovery
following facial nerve injury. Nerve regeneration involves a complex interaction
of neurons, Schwann cells, elements of the extracellular matrix, and a host
of neurotrophic substances. With respect to surgical repair, suturing the
severed nerve ends and nerve grafting have remained the procedures of choice.4 The immediate neural environment plays an important
role. The success of neural regeneration depends on the cellular matrix components
(lamina, type IV collagen, neural adhesive molecular, and others) and on neurotrophic
factors, such as nerve growth factor, that are produced by the denervated
target nerve.5, 6, 7
A recent study8 showed that electromagnetic
stimulation enhances early regeneration and facial movement. Most investigations
have focused on motor fiber regeneration and facial movement.
It is poorly understood how the secretomotor fibers and gustatory fibers
regenerate after facial nerve injury and whether their regeneration affects
motor fiber regeneration. The purpose of the present study was to observe
regeneration of the distal facial nerve following nerve grafting within the
tympanic segment with geniculate ganglion preservation or dissection.
MATERIALS AND METHODS
ANIMALS
Twenty-three adult New Zealand albino rabbits of both sexes weighing
2.5 to 3.2 kg were randomly assigned to 2 groups. All animals underwent removal
of a 2-mm tympanic segment of the facial nerve, and the greater auricular
nerve was harvested for grafting. In group 1 (10 rabbits), the geniculate
ganglion was preserved. In group 2 (13 rabbits), the geniculate ganglion was
dissected. Mastoidal and extratemporal segments of the facial nerve were harvested
3 months postoperatively for histological examination.
SURGICAL PROCEDURE
All operations were performed on the left side with an operating microscope
(Carl Zeiss, Inc, Oberkochen, Germany). The right side served as a control.
All surgical procedures were performed under aseptic conditions. The animals
were anesthetized with inhalation through intubation of 0.5% enflurane with
equal parts of nitrous oxide and oxygen. The otic vesicle was exposed postauricularly.
Through the surgical fenestration, the incus was removed and the tympanic
fallopian canal was identified above the footplate of the stapes. The fallopian
canal was opened with a diamond burr, and 2.0 mm of the facial nerve was removed.
The nerve graft was carefully inserted in the opened canal between the cut
ends of the facial nerve. The donor nerve for grafting was the greater auricular
nerve.
In addition to the above procedures, the animals in group 2 had the
malleus head removed to expose the geniculate ganglion. The geniculate ganglion
and the greater superficial petrosal nerve were excised, and the main trunk
of the facial nerve was preserved. To prevent regeneration of the intermediate
nerve to the distal segment of the facial nerve, bone wax was replaced in
the geniculate ganglion. Antibiotics were administered to minimize the possibility
of infectious complications. Three months postoperatively, all animals were
placed under deep general anesthesia, the otic vesicle was opened, the parotid
gland was retracted forward slightly, the stylomastoid foramen was dissected,
and the mastoidal and extratemporal segments of the facial nerve were harvested
for electron microscopic evaluation. The methods and protocol of the study
were reviewed and approved by the Institutional Animal Care and Use Committee
of Shanghai Medical University, People's Republic of China.
HISTOLOGICAL EXAMINATION
For electron microscopic evaluation, tissues were immersed in 3% glutaraldehyde
for 2 hours and then in 1% osmium tetroxide for 1 hour. Dehydration in a series
of graded ethanols was followed by gradual infiltration with epoxy (Embed-812;
Electron Microscope Sciences, Tokyo, Japan). Semithin sections were made and
stained with toluidine blue O for light microscopic examination, and ultrathin
sections were stained with uranyl acetate and lead citrate for electron microscopy
(JEM-2000CX; JEOL Co, Tokyo, Japan). Axon count and size and distribution
of unmyelinated and myelinated fibers were determined by electron microscopic
examination.
Analyses to determine the statistical significance of the difference
in the axon count of the facial nerve between group 1 and group 2 were performed
using t tests with commercially available software
(STAT-VIEW, version 5.0; Abacus Concepts Inc, Berkeley, Calif).
RESULTS
In normal facial nerves, myelinated axons were evenly distributed. Sheath
thickness was proportionate to axon diameter (Figure 1), and the number of axons was 1819.6 ± 535.6.
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Figure 1. Electron microscopic examination
of a mastoidal segment of normal facial nerve shows myelinated axons that
are evenly distributed and whose sheath thickness is proportionate to the
axon diameter (original magnification x960).
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In group 1, regenerative myelinated fibers and unmyelinated fibers were
identified in the mastoidal segments. However, the number of myelinated axons
was 123.6 ± 31.1, which was much fewer than were found in normal facial
nerves. In addition, the diameter of regenerative axons was decreased, and
the sheath thickness in regenerative fibers was diminished (Figure 2). Extratemporal segments were almost completely composed
of connective tissue, with fewer myelinated axons than were found in the mastoidal
segments. Myelinated axons were diffusely distributed throughout the extratemporal
segments. It is estimated that only one eighteenth to one twelfth of myelinated
axons are used to innervate muscle.
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Figure 2. In a mastoidal segment of facial
nerve in group 1, myelinated axons (arrowhead) and a greater number of unmyelinated
axons (star) were noted under electron microscopy (original magnification
x480).
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In group 2, a large number of regenerative myelinated axons was found
in the mastoidal segments. The myelin sheath thickness was proportionate to
the diameter of the axons (Figure 3).
No unmyelinated axons were seen. In the extratemporal segments, myelinated
axons were evenly distributed. The number of myelinated axons was 515.1 ±
103.1, which is about one quarter to one third of that in normal facial nerve.
Proliferation of connective tissue was noted among axons. However, comparing
group 1 with group 2, the number of regenerative myelinated axons was increased
significantly (P<.001) in group 2.
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Figure 3. In group 2, electron miscroscopy
shows a large number of myelinated axons in extratemporal segments of facial
nerve following geniculate ganglionectomy. There were fewer myelinated axons
than were found on the control side. Fibers were evenly distributed, with
a proliferation of connective tissue (arrowhead) among the axons (original
magnification x12 000).
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Data are given as mean ± SD.
COMMENT
It is well-known that there are somatic motor, gustatory, and secretomotor
fibers within the facial nerve. Early regenerative gustatory fibers in chorda
tympani and secretomotor fibers are unmyelinated. Mature regenerative gustatory
fibers and somatic motor fibers are myelinated. An anatomical study9 has indicated that the total number of myelinated
nerve fibers in the facial nerve varies from 7500 to 9370, depending on the
anatomical level of the nerve segment. The greatest number of nerve axons
was found at the level of the middle of the mastoidal portion. The peak diameter
of the facial nerve axon was between 4 and 6 µm. The number of facial
nerve fibers decreased with the age of the patient.9
Further investigation showed that a significant proportion (15%-20%) of the
fiber composition in the facial nerve trunk and its peripheral branch is nonmotor.10 Bruesch,11 using chromatolytic
technique in cats to trace the distribution of the afferent fibers through
the facial nerve branches, found that 20% of fibers traversed the greater
superficial petrosal nerve. The remainder was distributed in the chorda tympani
(45%), posterior auricular rami (21%), branches to mimetic muscle (8%), deep
cervical branch (5%), and nerve to the stapedius (1%). Somatic motoneurons
are in the facial motor nucleus, which is located in the brainstem. Secretomotor
neurons are situated in the superior salivatory nucleus. Sensory neurons of
the gustatory fibers localize in geniculate ganglia via the chorda tympani
nerve, supplying taste buds in the tongue.
In this study, we demonstrated axonal regeneration following facial
nerve grafting with or without geniculate ganglionectomy. With geniculate
ganglion preservation, an increase in regenerative gustatory fibers was identified
in mastoidal segments following facial nerve transection. There were more
unmyelinated fibers than myelinated fibers. In contrast, with geniculate ganglionectomy,
a large number of myelinated axons was found in the mastoidal segments, the
number of regenerative axons was similar to that of the control side, and
there were no gustatory fibers. It is speculated that geniculate ganglionectomy
has a positive effect on motor axon regeneration. The results may be explained
by the fact that gustatory fiber is a C-type fiber and grows quickly following
injury, and that gustatory neurons are in geniculate ganglia. However, compared
with gustatory fibers, motor fibers must traverse a longer distance from the
facial motor nucleus to the injury site. Therefore, it is advantageous for
the regenerative gustatory fibers in the band of Büngner to regenerate.
After geniculate ganglionectomy, the band of Büngner gustatory fibers
served as guides for motor fiber regeneration.
Ylikoshi et al12 observed degenerative
changes in the distal stump of the human facial nerve. Their results showed
that the sensory component of the facial nerve had a normal appearance when
the facial nerve was severed at the internal acoustic meatus with the geniculate
ganglion left intact. When the geniculate ganglion was damaged, the normal-looking
sensory component of the facial nerve was absent from the distal stump. When
the entire tympanic portion of the facial nerve was interrupted, most of the
endoneural tube had thin unmyelinated fibers in the distal stump around the
foramen, and few myelinated fibers appeared intact. It may be that regenerative
sensory fibers interfere with the regeneration of the motor fibers. This observation
was consistent with the findings of our study.
Moreover, in group 2, no unmyelinated fibers were found in the mastoidal
segments following geniculate ganglionectomy. In addition, compared with group
1, the number of regenerative myelinated fibers was significantly increased
in the mastoidal segments. This suggests that, without the effect of nonmotor
fiber, more motor fiber may be regenerated.
From our experimental data, it seems reasonable to conclude that geniculate
ganglionectomy is associated with improvement of motor axon regeneration.
Earlier studies13, 14 proposed
geniculate ganglionectomy as a means of excising the cell bodies of aural
cutaneous pain afferents in patients with geniculate neuralgia. These reports
showed that this surgery benefited these patients. A histological basis for
the use of geniculate ganglionecomy as a treatment for geniculate neuralgia
was thereby established. It is recommended that geniculate ganglionectomy
be performed when facial nerve decompression or grafting is needed in patients
with facial paralysis, as it may enhance motor axon regeneration and improve
the recovery from facial palsy.
The present study only investigated motor axon regeneration following
geniculate ganglionectomy. Further studies should be undertaken to focus on
facial movement evaluation by electrophysiologic testing.
AUTHOR INFORMATION
Accepted for publication September 22, 2000.
We thank Christopher Lam, MD, and Dingding Xiong, MD, PhD, for their
helpful comments on the paper.
From the Department of Otolaryngology, Eye, Ear, Nose, and Throat Hospital,
Shanghai Medical University, People's Republic of China.
Corresponding author and reprints: Chunfu Dai, MD, PhD, Department
of Otolaryngology, Eye, Ear, Nose, and Throat Hospital, Shanghai Medical University,
Shanghai 200031, People's Republic of China (e-mail: daichf{at}online.sh.cn).
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