Bony trigeminal nerve entrapment in autosomal dominant osteopetrosis: a rare cause of secondary trigeminal neuralgia

Technical Summary: Bony Trigeminal Nerve Entrapment in Autosomal Dominant Osteopetrosis

Problem Statement
Trigeminal neuralgia (TN) is typically idiopathic or caused by neurovascular compression. However, secondary TN can arise from structural lesions, including skeletal abnormalities. This paper addresses a rare etiology: autosomal dominant osteopetrosis type 1 (ADO1). In ADO1, gain-of-function mutations in the LRP5 gene lead to excessive bone formation, cortical thickening, and osteosclerosis. In this specific case, diffuse skull thickening and petrosal bone overgrowth caused medial displacement of the trigeminal nerve and bony entrapment at the foramen rotundum (FR) and foramen ovale (FO). This resulted in a 46-year-old male suffering from refractory left-sided TN (V2 and V3 distribution) despite medical therapy, accompanied by subtle hearing loss and signs of intracranial hypertension.

Methodology
The authors employed a left-sided pretemporal extradural transcavernous approach (TCA) to address the complex anatomy created by osteopetrosis. The surgical technique involved the following key steps:

• Approach: A frontotemporal craniotomy was performed with the patient in a supine position, head extended and rotated. The temporalis muscle was mobilized and reflected inferiorly to protect the facial nerve.
• Bone Work: Due to the dense, sclerotic bone characteristic of osteopetrosis, careful drilling using both sharp and diamond burrs was required under high-magnification microsurgery. The frontal bone and greater wing of the sphenoid were drilled to visualize the periorbita.
• Cavernous Sinus Dissection: The meningo-orbital band was divided. The lateral wall of the cavernous sinus (CS) was dissected from the temporal dura to identify the superior orbital fissure (SOF), FR, and FO. Hemostasis in the CS was achieved using tissue glue and compression of the sphenoparietal sinus to prevent retrograde embolization.
• Decompression Strategy:
  • Unroofing: The FR and FO were unroofed to decompress the V2 and V3 divisions and facilitate nerve manipulation.
  • Petrous Drilling: The Kawase rhomboid was identified, and the petrous apex was drilled to create a corridor to the lateral prepontine cistern.
  • Ligament Release: The calcified anterior petroclinoid fold and the trigeminal fibrous ring (petroclinoid ligaments) were released.
  • Tentorial Opening: The tentorium was coagulated and cut to expose the cisternal segment of the nerve and the antero-lateral pons.
  • Final Mobilization: The trigeminal nerve was freed from arachnoid attachments and surrounding bony overgrowth along its entire intracranial trajectory, from the extracranial exit to the root entry zone.

Key Results

• Immediate Outcome: The procedure was completed without complications. The patient reported complete resolution of trigeminal neuralgia immediately post-surgery.
• Long-term Follow-up: The patient remained pain-free at subsequent visits. Imaging confirmed the decompressed nerve.
• Neurological Status: No new neurological deficits, cerebrospinal fluid leaks, or other complications were observed.
• Anatomical Findings: Preoperative imaging revealed a trigeminal angle of 122° (significantly reduced from the normal ~170°), indicating inferior and medial displacement of the nerve. Postoperative imaging confirmed the nerve was free from bony constraints.

Key Contributions

• Surgical Technique for Rare Pathology: The paper details a specific, aggressive surgical strategy (pretemporal extradural TCA with petrous apex drilling) tailored for TN caused by ADO1. Unlike standard microvascular decompression which targets the root entry zone, this approach directly addresses the anterior bony entrapment and foraminal stenosis unique to osteopetrosis.
• Comprehensive Decompression: The authors demonstrate that unroofing the FR and FO, combined with releasing the petroclinoid ligaments and drilling the petrous apex, allows for full decompression of the nerve along its entire course, rather than just at the root entry zone.
• Case Documentation: This report adds to the extremely limited literature on adult-onset TN secondary to cranial osteopetrosis, distinguishing it from cases where posterior fossa decompression alone was used.

Significance and Claims
The paper claims that recognizing underlying osteopetrosis is crucial for managing secondary TN, particularly in patients presenting with macrocephaly, craniofacial abnormalities, or relevant family history. The authors emphasize that standard surgical approaches may be insufficient for the altered anatomy of osteopetrosis. They argue that a multidisciplinary approach—including genetic counseling, high-resolution imaging, and neurosurgical planning—is essential.

The significance of this case lies in demonstrating that an aggressive, extended decompression strategy (addressing the nerve from the skull base exit to the brainstem) is necessary to prevent recurrence in patients with diffuse skeletal hyperostosis. The authors suggest that while TN secondary to cranial osteosclerosis is exceedingly rare, individualized surgical strategies that account for the specific bony overgrowth and ligamentous calcification are vital for successful outcomes.