Supernumerary Abducens Nerves: A Comprehensive Review

BACKGROUND

Branching and/or replication of the abducens nerve is not an uncommon occurrence. Although numerous variations have been documented, the rarest forms are duplicated or triplicated nerves, where multiple nerve roots originate from the brainstem, travel intracranially, and attach to the lateral rectus as separate entities.

METHODS

We conducted a systematic literature search on the topic of supernumerary abducens nerve, using PubMed and Google Scholar.

RESULTS

After screening, 16 studies were included: 11 cadaveric studies and 6 case reports.

CONCLUSIONS

In this paper, we review the literature on variations found, discuss current hypotheses and clinical relevance, and propose future studies. Neurosurgeons should be aware of such nerve variants when viewing imaging or operating in the regions traversed by the abducens nerve.

An unusual variant of the abducens nerve duplication with two nerve trunks merging within the orbit: a case report with comments on developmental background

This study reports the first case of abducens nerve duplication along its entire intracranial course, ending within the orbit. A distinct abducens nerve duplication reaching the common tendinous ring (annulus of Zinn), as well as another split within the intraconal segment of the nerve have been revealed. Additionally, two groups (superior and inferior) of abducens nerve sub-branches to the lateral rectus muscle were visualised using Sihler's stain. The analysed anatomical variation has never been reported before and it seems to be in the middle of the spectrum between the cases of duplication occurring only within the intracranial segments of the abducens nerve found in the literature and those continuing throughout the whole course of the nerve. Abducens nerve duplication may be treated as a relic of early stages of ontogenesis. Such a variant might result from alternative developmental pathways in which axons of the abducens nerve, specific for a given segment of the lateral rectus muscle, run separately at some stage, instead of forming a single stem.

Cervico-Oculo-Acoustic Syndrome in a Male with Consanguineous Parents

Background:

The cervico-oculo-acoustic syndrome comprises Klippel-Feil anomaly, sensorineural deafness and Duane's retraction syndrome. Polygenic, autosomal dominant, and X-linked inheritance have been hypothesized. The disorder has rarely been reported in males.

Case Report:

A 42-year-old male, born of consanguineous parents, presented with Duane's syndrome, mixed hearing loss, C2-C3 fusion, neck stiffness, and right facial palsy. A variety of cardiac, neurological and urogenital anomalies occurred in his relatives. The electro-oculographic studies showed impaired abduction and adduction of the right eye and impaired abduction of the left eye. Vergence, vertical eye movements and peripheral vestibular responses were normal. Somatosensory evoked potentials showed absence of the N13 peak and brainstem auditory evoked potentials bilateral delay of the I-III interpeak latencies.

Conclusions:

Consanguinity of the patient's parents, not previously reported, suggests autosomal recessive inheritance, but autosomal dominant inheritance is indicated by the family history. The pattern of the oculomotor deficit is consistent with bilateral dysplasia of the abducens nuclei with preserved internuclear neurons in the right abducens nucleus. Neurophysiological investigations revealed lower brainstem and cervical cord involvement.

Human CHN1 mutations hyperactivate alpha2-chimaerin and cause Duane's retraction syndrome

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes alpha2-chimaerin, a Rac guanosine triphosphatase-activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase alpha2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance alpha2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant alpha2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that alpha2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Bilateral incyclotorsion, absent facial nerve, and anotia: fellow travelers in Möbius sequence or oculoauriculovertebral spectrum?

We report a patient with bilateral incyclotorsion, asymmetric facial palsy, monocular depression deficiency, and unilateral Type 3 Duane syndrome, along with contralateral anotia and a midline cleft palate. A dysplastic middle ear cavity and unilateral absence of the facial nerve were noted on magnetic resonance imaging scanning. The differential diagnosis includes Möbius sequence, oculoauriculovertebral spectrum, and a congenital cranial dysinnervation disorder.

Abducens-oculomotor synkinesis associated with internuclear ophthalmoplegia and acquired abducens nerve palsy

We report a case of acquired abducens-oculomotor synkinesis, internuclear ophthalmoplegia, and abducens nerve palsy following brainstem hemorrhage. Weakening surgery on the medial rectus muscle did not eliminate the synergistic movement. The mechanism responsible for the abducens-oculomotor synkinesis is discussed.

Normal abduction in a patient with duplicated abducens nerve

PURPOSE

To our knowledge, there has been no report of ophthalmologic findings related with a duplicated abducens nerve in the ophthalmic literature. This study reports such findings.

METHODS

An ophthalmologic examination was performed in one patient with a duplicated abducens nerve, revealed with thin-sectioned magnetic resonance imaging (MRI) across the brainstem level.

RESULTS

The MRI disclosed a duplicated left abducens nerve. The patient was orthotropic in five cardinal positions, and her ductions and versions were full.

CONCLUSIONS

One patient with a duplicated abducens nerve showed orthotropia and normal ocular movement.

Obstetrical and Pediatric Anesthesia Anesthetic management of children with Moebius sequence

BACKGROUND

Moebius sequence is a rare congenital absence of the sixth and seventh cranial nerves, although there may be additional congenital cranial neuropathies. Developmental delay, cardiac and musculoskeletal abnormalities may also co-exist. Oro-facial manifestations include bilateral facial nerve palsy resulting in a mask like facies, drooling, incomplete eye closure, and strabismus secondary to the extra-ocular muscle imbalance. This condition has multiple implications for anesthetic care.

METHODS

We reviewed 111 anesthesia records of 46 patients with Moebius sequence for anesthesia technique and related complications.

RESULTS

Facial nerve palsy was universally present and bilateral in 44 (93.6%) patients. Thirty-two (68%) had concomitant sixth nerve palsy. Oro-facial and limb abnormalities were present in 16 (35%) and 18 (39%) of patients respectively. Endotracheal intubation, when attempted, was easy in 76 of 106 cases. Tracheal intubation was consistently difficult in seven patients and intubation failure occurred in a single patient only. Statistically significant factors associated with difficult tracheal intubation included structural abnormalities of the mandible and palate and abnormalities of four cranial nerves (IX, X, XI, XII).

CONCLUSION

We confirm that tracheal intubation may be difficult in patients with Moebius sequence. We identify disease features that might predict a difficult tracheal intubation and thus allow the anesthesiologist an opportunity to plan accordingly.

Hypoplastic Oculomotor Nerve and Absent Abducens Nerve in Congenital Fibrosis Syndrome and Synergistic Divergence with Magnetic Resonance Imaging

OBJECTIVE

There is only one neuropathologic report of congenital fibrosis of the extraocular muscles (CFEOM), and none of synergistic divergence. The aim of this report was to study the oculomotor nerve and the abducens nerve in 2 such patients with magnetic resonance imaging.

DESIGN

Observational case reports.

METHODS

Ophthalmologic examination and thin-sectioned magnetic resonance imaging across the brainstem level were performed in 2 patients with CFEOM and synergistic divergence. To confirm the accuracy of the procedure, we compared the results obtained with those of a control group of 40 individuals using the same technique.

MAIN OUTCOME MEASURES

The oculomotor nerve and the abducens nerve on magnetic resonance imaging.

RESULTS

Magnetic resonance imaging disclosed bilateral hypoplasia of the oculomotor nerve in both patients, and absence of the abducens nerve on the affected side of synergistic divergence. The oculomotor and abducens nerves were observed in 80 of 80 eyes (100%) screened as controls.

CONCLUSIONS

In both patients with CFEOM and synergistic divergence, the oculomotor nerve was hypoplastic bilaterally, and the abducens nerve was absent on the side exhibiting synergistic divergence.

Duplication of the abducens nerve at the petroclival region: an anatomic study

OBJECTIVE

During its course between the brainstem and the lateral rectus muscle, the abducens nerve usually travels forward as a single trunk, but it is not uncommon for the nerve to split into two branches. The objective of this study was to establish the incidence and the clinical importance of the duplication of the nerve.

METHODS

The study was performed on 100 sides of 50 autopsy materials. In 10 of 11 cases of duplicated abducens nerve, colored latex was injected into the common carotid arteries and the internal jugular veins. The remaining case was used for histological examination.

RESULTS

Four of 50 cases had duplicated abducens nerve bilaterally. In seven cases, the duplicated abducens nerve was unilateral. In 9 of these 15 specimens, the abducens nerve emerged from the brainstem as a single trunk, entered the subarachnoid space, split into two branches, merged again in the cavernous sinus, and innervated the lateral rectus muscle as a single trunk. In six specimens, conversely, the abducens nerve exited the pontomedullary sulcus as two separate radices but joined in the cavernous sinus to innervate the lateral rectus muscle. In 13 specimens, both branches of the nerve passed beneath the petrosphenoidal ligament. In two specimens, one of the branches passed under the ligament and the other passed over it. In one of these last two specimens, one branch passed over the petrosphenoidal ligament and the other through a bony canal formed by the petrous apex and the superolateral border of the clivus. In all of the specimens, both branches were wrapped by two layers: an inner layer made up of the arachnoid membrane and an outer layer composed of the dura during its course between their dural openings and the lateral wall of the cavernous segment of the internal carotid artery. This finding was also confirmed by histological examination in one specimen.

CONCLUSION

Double abducens nerve is not a rare variation. Keeping such variations in mind could spare us from injuring the VIth cranial nerve during cranial base operations and transvenous endovascular interventions.

[New views on aberrant innervation of oculomotor muscles ]

Aberrant innervations of oculomotor muscles are a special group of clinical entities, the common sign of which is an abnormal innervation of the oculomotor muscles with nerve fibres which originally were not meant for this purpose. The majority of these anomalies develops during the embryonic period of lesions at the level of nuclei of cranial nerves or peripheral fibres. Congenital aberrations affect most frequently n. VI, less frequently n. III and rarely n. IV. Acquired affections are most frequent in n. III. Abnormal innervation connections concern not only oculomotor muscles but also muscles engaged in mastication and those of the upper eyelid. Clinically they are manifested as eyelid and facial synkinesias. Among aberrant innervations which lack an uniform terminology belong according to the more recent classification the following clinical units: Duan's syndrome, synergic divergence, double-elevator palsy, misdirection syndrome, Marcus Gunn phenomenon, some forms of congenital fibrous syndrome, trigemino-abducens synkinesia, pseudohyperfunction of the lower oblique muscle, crocodile tears syndrome, and aberrant affection of n. III. The authors describe the course, possible etiology of these anomalies and demonstrate, based on their own observations, clinical cases which can be explained by these mechanisms.

Duplicated abducent nerve and its course: microanatomical study and surgery-related considerations

OBJECT

The anatomy of the abducent nerve is well known; its duplication (ranging from 5 to 28.6%), however, has rarely been reported in the literature. The authors performed a microanatomical study in 100 cadaveric specimens (50 heads) to evaluate the prevalence of this phenomenon and to provide a clear anatomical description of the course and relationships of the nerve. The surgery-related implications of this rare anatomical variant will be highlighted.

METHODS

The 50 human cadaveric heads (100 specimens) were embalmed in a 10% formalin solution for 3 weeks. Fifteen of them were injected with colored neoprene latex. A duplicated abducent nerve was found in eight specimens (8%). In two (25%) of these eight specimens the nerve originated at the pontomedullary sulcus as two independent trunks: in one case the superior trunk was thicker than the inferior and in the other it was thinner. In the other six cases (75%) the nerve originated as a single trunk, splitting in two trunks into the cisternal segment: in two of them the trunks ran below the Gruber ligament, whereas in four specimens one trunk ran below and one above it. In all the specimens, the duplicated nerves fused again into the cavernous sinus, just after the posterior genu of the internal carotid artery.

CONCLUSIONS

Although the presence of a duplicated abducent nerve is a rare finding, preoperative magnetic resonance imaging should be performed to rule out this possibility, thus tailoring the operation to avoid postoperative deficits.

Absence of the abducens nerve in Duane syndrome verified by magnetic resonance imaging

PURPOSE

To demonstrate that currently available magnetic resonance imaging techniques may verify the absence of the abducens nerve in Duane syndrome.

METHODS

We performed magnetic resonance imaging in a 36-year-old woman with left Duane syndrome, type 1, using spoiled gradient recalled acquisition in the steady state to obtain high-resolution T1-weighted images through the abducens nerve in its subarachnoid segment. Scans were obtained in the axial plane from the medulla to the midbrain and then reformatted along the plane of the abducens nerve.

RESULT

Unilateral absence of the left abducens nerve was verified using magnetic resonance imaging.

CONCLUSION

The absence of the abducens nerve in Duane syndrome can be verified by modern magnetic resonance imaging techniques.

[Crocodile tears and retraction syndrome]

BACKGROUND

An association between crocodile tears and Duane's syndrome has been described in more than 50 cases.

PATIENT

A 29-year-old man had tearing of his right eye whenever he ate something sour. At psychic weeping his right eye remained dry. In addition, he had Duane's retraction syndrome, also in the right eye.

CONCLUSION

In this case of congenital crocodile tears with an ipsilateral Duane's syndrome the normal function of the facial muscles suggests that the abnormal coupling between the gustatory nucleus and the lacrimal gland is not located in the facial nerve proximal to the Ganglion geniculi, as in most acquired cases. Rather, the abnormal coupling may be located in the brainstem. The association with Duane's syndrome indicates that abnormal innervational coupling is present also in the ocular motor domaine.

Medial rectus electromyographic abnormalities in Duane syndrome

We report on four patients from the botulinum toxin clinic at Moorfields Eye Hospital with Duane syndrome, who exhibited paradoxical medial rectus activity. EMGs were performed with a standard toxin needle and were qualitative in nature. Current physiodynamic theories as to the etiology of Duane syndrome are based on an aberrant innervation of the lateral rectus. Paradoxical electromyographic activity of the medial rectus may occur in Duane type I syndrome. A possible explanation for this finding is that the medial rectus may receive aberrant innervation but this may not be physiodynamically significant. We postulate that peripheral innervational anomalies may be secondary to, or coexist with, a brain stem anomaly. If an extraocular muscle possesses a dual innervation, then electromyographic abnormalities, without physiodynamic significance, may occur if the recording electrode comes into contact with muscle fibers innervated by a nerve that supplies a small segment of muscle.

Mobius syndrome with basal ganglia calcification

Basal ganglia calcification has not been described in Mobius syndrome. A family with two children with Mobius syndrome are reported. Bilateral basal ganglia calcification was seen on computed tomography in both. This is the first family where cerebral involvement has been clearly documented in this syndrome.

Möbius syndrome and transposition of the great vessels

A case of Möbius syndrome was associated with transposition of the aorta and pulmonary artery, as well as acheiria. This combination of anomalies supports the hypothesis that Möbius syndrome is caused by an intrapartum insult during the fourth to seventh week of gestation and is consistent with the vascular theory of embryopathogenesis.

Facial weakness and oligosyndactyly: ? independent variable features of familial type of the Möbius syndrome

A family consisting of two daughters, one with an isolated oligosyndactyly and the other with the Möbius Syndrome (VIth and VIIth nerve dysplasia) is presented. The majority of the individuals previously reported with an association of occulo-facial diplegia and limb anomalies have been sporadic. However, on examination of the parents in the family we report, the mother was found to have bilateral facial weakness. Isolated limb anomalies may, therefore, be a variable expression of a broad spectrum type of the Möbius Syndrome, with an autosomal dominant mode of inheritance.

Bilateral congenital absence of the abducens nerve

A patient with lifelong, bilateral horizontal gaze palsies is presented. The anatomical findings of bilateral absence of the abducens nerve as it affected the brainstem, the course of the cranial nerves to the extraocular muscles, and muscle innervation are described. The possible relationship of these findings to Duane's syndrome and Möbius syndrome is discussed.

Unilateral Duane's retraction syndrome (Type 1)

Duane's retraction syndrome (DRS) is a congenital eye movement disorder characterized by adduction deficiency, abduction limitation, globe retraction, and palpebral fissure narrowing on attempted adduction. In a case of unilateral DRS, the right side of the brainstem, cavernous sinus, and orbit were completely normal. The left abducens nucleus contained no cell bodies from motor neurons, but in its rostral portion, it contained several small cell bodies believed to be compatible with internuclear neurons. The left abducens nerve was absent. The left lateral rectus muscle was partially innervated by branches from the inferior oculomotor nerve.

Familial, congenital paralysis of horizontal gaze

Eye movements were studied in a sister and brother with familial, congenital paralysis of horizontal gaze. Horizontal and vertical eye movements were recorded with DC electro-oculography and analyzed with a laboratory computer. All horizontal, conjugate eye movements were absent (saccades, pursuit, optokinetic nystagmus, vestibulo-ocular response, and visual-vestibular responses). Voluntary vergence eye movements were preserved and were used to track visual targets. An involuntary, horizontal, pendular nystagmus was found to represent disconjugate, smooth, vergence eye movements. Vertical saccades and vestibulo-ocular responses were normal. However, vertical pursuit, optokinetic nystagmus, and suppression of the vestibulo-ocular response by fixation were impaired. A developmental anomaly affecting motor neurons and interneurons in the abducens nuclei is suggested to be the cause of the absence of conjugate, horizontal eye movements.

Occurrence of cyclopia, myelomeningocele, deafness, and abducens paralysis in siblings

As holoprosencephaly without chromosome defect may be associated with other CNS-related anomalies such as mental retardation, mental illness, facial paralysis, endocrine disorders, deafness, spina bifida, and myelomeningocele, we present a family in which one girl had a myelomeningocele, a brother had orbital hypotelorism, facial and cerebral asymmetries, cerebral palsy, abducens paralysis, and inner ear deafness. A 3rd pregnancy was terminated at 16 weeks; the fetus had cyclopia. A common cause is discussed in these cases and in those families in which holoprosencephaly and additional malformations occur among different generations.

Bilateral Duane's retraction syndrome. A clinical-pathologic case report

Duane's retraction syndrome (DRS) is a congenital eye movement disorder characterized by abduction deficiency, adduction limitation, globe retraction, and palpebral fissure narrowing on attempted adduction. Although data from surgical and autopsy studies suggest a neurogenic cause for this disorder, only one well-documented case of DRS has been studied pathologically, and that report was incomplete. We describe the intracranial and orbital pathology of a clinically documented case of bilateral DRS. Both abducens nuclei and nerves were absent from the brainstem, and the lateral rectus muscles were partially innervated by branches from the oculomotor nerves. This report presents evidence that a cranial nerve anomaly may cause DRS. The reason for this neurogenic abnormality is briefly discussed.

[Unilateral Moebius syndrome associated with Poland's abnormality in a 25-year-old woman (author's transl)]

The authors present a case of unilateral Moebius syndrome associated with Poland abnormality in a 25-year-old woman. The patient had a complete left peripheral facial paralysis associated with involvement of both VI. In addition, there was microdactyly, syndactyly of the left hand, absence of the pectoralis major, of the mammary gland and nipple adding up to Poland syndrome. Surgery was envisaged with the aim of performing a hypoglossofacial anastomosis. Operation revealed muscles of normal appearance with a somewhat thin digastric and total absence of the facial nerve both at the point of its emergence through the stylomastoid foramen as well as in the parotid.

Anomaly of the abducens nerve

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