Reversible abducens nerve palsy following cranial vault expansion in the setting of multisutural craniosynostosis: illustrative case

Abstract

BACKGROUND

Posterior cranial vault distraction osteogenesis (PVDO) is a commonly used cranial expansion procedure in infants and children with syndromic craniosynostosis. To date, there have been no reports of cranial nerve (CN) palsies in patients undergoing univector PVDO.

OBSERVATIONS

In this article, the authors describe the case of a 27-month-old female with Muenke syndrome who underwent long-distance (> 30 mm) PVDO and developed bilateral abducens nerve (CN VI) palsy after 40 mm of distraction. Following partial reversal of the distraction during the activation phase, the authors observed complete resolution of this palsy.

LESSONS

This report demonstrates that CN palsies are a potential complication for which the patient should be monitored, even when undergoing univector PVDO. Most notably, this report illustrates that a gradual reduction in the distraction distance can result in complete resolution of a CN VI palsy while also maintaining a significant degree of intracranial expansion.

https://thejns.org/doi/10.3171/CASE24762

Posterior cranial vault distraction osteogenesis (PVDO) has become an increasingly common technique for treating craniosynostosis. In most instances, it has been described for use in syndromic, multisutural cases. PVDO allows for a greater volumetric expansion than would be possible with conventional expansion, given soft tissue constraints. Its other purported advantages are decreased operative times, decreased blood loss, and repeatability if necessary.^1–3 The associated complications can be relatively common and are related to pin site infection, hardware exposure/failure, and dural injury.⁴ There has been one prior report of 2 patients who developed unilateral abducens nerve (cranial nerve [CN] VI) palsy and lateral rectus weakness while undergoing cranial distraction, which was treated with removal of all hardware. In this paper, we report the case of bilateral CN VI neurapraxia during long-distance PVDO that was successfully treated by partially reversing the distraction process.

Illustrative Case

This otherwise healthy 15-month-old girl was diagnosed with hereditary bicoronal craniosynostosis (Fig. 1) and had a similarly affected mother. She was promptly scheduled for surgery but missed that appointment and was lost to follow-up until she presented in the custody of the Department of Child Services/foster care at the age of 27 months. At this visit, there was evidence of severe cranial growth restriction and developmental delay. The patient’s genetic workup revealed Muenke syndrome (FGF3R exon 7 mutation).

FIG. 1.

Preoperative 3D CT scan. A: Vertex view demonstrating brachycephaly, bifrontal retrusion, and bilateral coronal suture fusion. B: Lateral view.

Her preoperative ophthalmological examination showed no papilledema and normal extraocular movements. Her preoperative head circumference (HC) was 47 cm. Preoperative surgical planning indicated a cranial index (CI) of 1.08. Plans were designed to normalize CI to 0.8. This would require a total distraction length of 45 mm. Although the total anteroposterior distance is typically described as around 30 mm,⁵ complete (i.e., extension of osteotomies to the foramen magnum) PVDO has previously been safely performed to a distance of 45 mm in patients with concomitant craniosynostosis and Chiari type I malformation.⁶ Therefore, 45 mm was assessed to be a safe target. A 50-mm distractor was planned to accommodate the expansion. On the day of the surgery, the patient underwent an uncomplicated posterior craniotomy and distractor placement (50-mm devices, KLS Martin Group). The distraction protocol included a 3-day latency period with subsequent distraction at 1.5 mm/day. Her operative and immediately postoperative courses were uncomplicated.

Clinical follow-up was performed on a weekly basis. Furthermore, guardians were provided with emails for sending pictures and receiving immediate analysis of any issue. On postoperative day (POD) 30, after 40 mm of distraction, the patient was noted to have bilateral CN VI palsy with evident lateral rectus palsy (Fig. 2) of an approximate duration of 1 day (according to the guardian). A CT scan obtained that day showed no evidence of hematoma, mass effect, or other pathology that could adequately explain her symptoms (Fig. 3). Three-dimensional reconstruction confirmed adequate device placement with successful distraction (Fig. 4). The pediatric ophthalmology team formally evaluated the patient and confirmed the diagnosis.

FIG. 2.

Photograph demonstrating right-sided lateral rectus palsy (red arrow). The child was affected on both sides.

FIG. 3.

Comparison of sagittal (A) and axial (B) intracranial CT scans before distraction and on POD 27.

FIG. 4.

Postoperative 3D CT scans obtained after CN VI palsy was noted. A: Vertex view. B: Lateral view. PVDO was accomplished using two 50-mm devices placed in a low lateral position.

Possible options for the management of this complication were considered. Removal of all distractors was the first option considered, due to reported success in the literature. It was hypothesized that the mechanism involved the reduction of stretch on the abducens nerve. A similar reduction in tension could be achieved by reversing the distraction to a given length. In our case, the distraction was reversed for 3 days (approximately 5 mm), after the report of gaze palsy, to determine if that was sufficient to reduce the stretch. This was done in a single setting. Two weeks later, her palsy was noted to have improved but had not completely resolved. It was unclear whether this indicated continued tension on the nerve or if it was the time course of continued neurapraxia resolution. In an effort to be cautious, the distractors were then reversed an additional 5 mm in one office visit. Resolution of the palsy was achieved 4 weeks following the initial diagnosis. The total distraction distance at the time of resolution of the palsy was 30 mm. The patient’s HC postdistraction was 55 cm. She had a consolidation period of 80 days after which the distractors were removed without incident.

Informed Consent

The necessary informed consent was obtained in this study.

Discussion

Observations

PVDO is widely used for cranial expansion in syndromic craniosynostosis. Some groups have argued that the paradigm for surgical treatment of these patients has been fundamentally changed by this procedure.⁷ However, it is not without its complications. The most common complication reported for cranial distraction procedures is local infection (7.6% and 20.2% in nonsyndromic and syndromic patients, respectively), but CSF leaks, epidural abscesses, device malfunction, and soft tissue erosions have also been described with some frequency.⁸ With regard to PVDO specifically, local infection remains the most common complication, with one recent systematic review demonstrating a rate of 27.0%.⁹ Less common complications, such as sagittal sinus obstruction,¹⁰ are rarely reported. We present the rare complication of stretch-induced bilateral CN VI palsy related to long-distance (> 30 mm) PVDO and its complete resolution by partially reversing the process. Yoo et al. previously reported 2 cases of unilateral CN VI palsy during cranial distraction.¹¹ In their report, both patients underwent complex, multivector cranial distraction with four or five devices. The patient’s symptoms resolved, but this was accomplished by immediate removal of all hardware and relapse to the preoperative volumetric state. In contrast, our report demonstrates that CN VI function can be restored by partially reversing the distraction process.

CN VI is the longest CN in the skull and includes a long course from its nucleus in the pons along the cranial base. The segments of the nerve are categorized as subarachnoid, pertroclival, and intracavernous. The nerve exits the brainstem anteriorly at the junction of the pons and medulla and ascends along the cranial base where it pierces the dura at the clivus. The course of CN VI is described as somewhat tortuous and long, with one cadaveric study estimating a total overall length of approximately 50 mm in adults.¹² The nerve is bound to the cranial base along the petrous portion of the temporal bone in Dorello’s canal with tight fibrous attachments acting as the roof of this canal. The nerve then enters the cavernous sinus with CN III, CN IV, V1, V2, and the internal carotid artery. It enters the orbit through the superior orbital fissure and innervates the lateral rectus along its deep aspect.^12–14 The tight confinement of CN VI to the cranial base and its long course are thought to contribute to its vulnerability to injury in a variety of etiologies.

There are numerous documented etiologies for CN VI palsy. These include craniomaxillofacial trauma, lumbar puncture, skull base surgery, vasculopathy, mass effect (aneurysm, tumor), and autoimmune conditions.^15–17 In this case report, we have demonstrated that conventional, univector PVDO long-distance distraction (> 30 mm) can result in bilateral CN VI palsy with resultant lateral rectus palsy. Given the anatomy of the abducens nerve on the cranial base, stretch neurapraxia is the likely etiology of this finding. It remains unclear whether the absolute distance or the ratio of nerve length to distraction length is the critical factor in determining how much distraction is too much. Another factor that could have contributed to the traction injury is the rate of distraction. Usual rates of distraction range from 1 to 3 mm/day, with most protocols in the 1- to 2-mm/day range. In this case, activation occurred at 1.5 mm/day. It is possible that a slower rate than the one used could have allowed a larger distraction distance without jeopardizing CN VI.

Interestingly, our case also illustrates that a gradual reduction in the distracted distance can resolve CN VI palsy while maintaining a significant degree of intracranial expansion. In the report by Yoo et al., the neurapraxia was resolved by complete hardware removal and a return to the prior state.¹¹ While this option was seriously considered in our case, it was believed that reversal of distraction provided a similar benefit to distractor removal. Additionally, there was significant disagreement regarding the management of her craniosynostosis and developmental delay. While her current guardians understood the necessity of increasing her cranial volume, others who had previously been responsible for her care did not and had delayed her surgery for more than a year. It was possible that if the distractors were removed, the opportunity to treat her craniosynostosis would be lost during critical growth and development periods. Additionally, ophthalmological and neurosurgical input guided decision-making toward a more gradual reduction in cranial volume. Furthermore, we reasoned that reduction of the cranial expansion to a period before the palsy was observed would return the nerve to a length that would restore normal nerve function. This proved to be correct, as the neurapraxia resolved with a 1-cm loss of total distraction within a month. The consolidation period of 3 months was used to compensate for the possibility that the regenerate could have been compromised by the stretched soft tissue collapsing into the area of newly forming bone. After removal of the hardware, the bone was solid and maintained its expanded volume.

The relief of Chiari malformation after PVDO has been noted and provides further evidence for dural and cerebral traction during this process.¹⁸ The distance traveled in our case also raised concerns for stresses and strains on the patient that would have been centered on the midbrain and the junction of the brain-bone interface at the occiput. Apparently, none of these other locations of pressure were as sensitive as CN VI, as there were no cerebellar or midbrain-related symptoms.

Lessons

Our case has informed and changed the management of these patients at our institution. Currently, distraction is performed at a rate of 1.5 mm/day for 30 mm and then is reduced to 1 mm/day if further distraction is deemed necessary. Additionally, parents are counseled about lateral rectus palsy and are given instructions to take daily pictures of patients in right and left extreme lateral gaze when distracting past 30 mm. These pictures are then sent to the multidisciplinary cranial anomalies clinic for evaluation. It is believed that this increased vigilance and slower distraction should eliminate this potential complication in the future.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Ackerman, Haider, Tholpady. Acquisition of data: Ackerman, Boente, Tholpady. Analysis and interpretation of data: Ackerman, Gerety, Boente, Tholpady. Drafting the article: Gerety, Celie, Tholpady. Critically revising the article: Ackerman, Gerety, Chu, Cordes, Tholpady. Reviewed submitted version of manuscript: Ackerman, Boente, Chu, Celie, Cordes, Tholpady. Approved the final version of the manuscript on behalf of all authors: Ackerman. Statistical analysis: Tholpady. Administrative/technical/material support: Boente, Celie, Tholpady. Study supervision: Tholpady.

Correspondence

Laurie L. Ackerman: Indiana University School of Medicine, Indianapolis, IN. lackerma@iu.edu.