Abductor nerve nucleus exists at the lower level of pons
Case 299
【Progress】
She is during observation under the reference neuro-surgeon clinic.
【Discussion】
Of 12 brain nerves, four nerves are relevant to eye; optic nerve (II) undertakes visual sensation: other three nerves of oculomotor nerve (III), trochlear nerve (IV), and abducens nerve (VI) undertake motor nerve, movement of eyeballs. The disorder of each motor nerve, therefore, causes diplopia. Oculomotor nerve nucleus and trochlear nerve nucleus are present at midbrain (tegmentum), while abducens nerve nucleus is present at pons (tegmentum). Pons looks the largest part of brain stem. The swelling area of pons composes of mainly cerebrum pyramidal fiber and middle cerebellar peduncle fiber. Then, there are only two brain nuclei of trigeminal nucleus and abducens nucleus in pons. Namely, middle brain and pons own two brain nerve nuclei in each, while nuclei of brain nerves of VII to XII are present at medulla oblongata (1).
Abducens nerve nucleus exists at pons and abducens nerve comes out from the boundary between pons and medulla oblongata to prepontine cistern. It passes from prepontine cistern to cavernous sinus via Dorello’s canal which exists at the most internal edge of petrous bone and between Meckel’s cave and internal carotid artery (2-5).
Meckel’s cave includes trigeminal nerve ganglion and persistent trigeminal artery in case of the presence of the remnant artery (5). Internal carotid artery is largely categorized into supraclinoid ICA (C1, C2), cavernous ICA (C3, C4) and petrous ICA (C5). Supraclinoud ICA is subcategorized into C1 from bifurcation of middle and anterior cerebral arteries to posterior communicating artery and C2 from posterior communicating artery to ophthalmic artery. Cavernous type is subcategorized into clinoid portion (C3) and cavernous sinus to petrous bone.
Clinically, abducens nerve palsy can occur internal carotid aneurysm, petrous bone tumor (chondrosarcoma) and trigeminal nerve tumor (neurinoma) or inflammation (herpes zoster) (2-5). Brain MRI and/or CT contributes to identify the pathogenesis of abducens nerve palsy.
In our case, brain CT showed a lesion with high attenuation at the bottom of pons, indicating small hemorrhage. Our patient experienced abducens palsy twice, due to the same lesion of small hemorrhages at pons. It is considered that small cavernous hemangioma causes repeated hemorrhage at pons.
【Summary】
We presented 56-year-old female presented in our hospital with diplopia yesterday following the previous diplopia episode occurring around 8 months before. Brain CT depicted a lesion with high attenuation at bottom of the pons, corresponded to abducens nerve nucleus. It is suspicious of latent presence of hemangioma because of repeated hemorrhage at the same site. It is borne in mind that of the three brain nerve relevant with eyeball movement, oculomotor nerve nucleus and trochlear nerve nucleus are present at midbrain tegmentum, while abducens nerve nucleus, at pons tegmentum. Abducens nerve comes out from the boundary between pons and medulla oblongata, runs prepontine cistern and enters cavernous sinus via Dorello’s canal which situates at the most internal edge of petrous bone. Namely, cavernous abducens nerve runs between cavernous internal carotid artery and Meckel’s diverticulum which include trigeminal nerve ganglion and persistent trigeminal artery in case of the presence of the primitive artery remnant.
【References】
1.Mamata Y, Muro I, Matsumae M, et al. Magnetic resonance cisternography for visualization of intracisternal fine structures. J Neurosurg 1998;88:670–678
2.Ono K, et al. Detailed MR Imaging Anatomy of the Abducent Nerve: Evagination of CSF into Dorello Canal. American Journal of Neuroradiology April 2004, 25 (4) 623-626
3.Haim E, et al. 2012). "Dorello's Canal for Laymen: A Lego-Like Presentation". Journal of Neurological Surgery. Part B, Skull Base. 2012; 73 (3): 183–189.
4.Felix U et al. "Dorello's canal: a microanatomical study". Journal of Neurosurgery. 1991; 75 (2): 294–298. doi:10.3171/jns.1991.75.2.0294. PMID 2072168.
5.Malhotra A, et al. Neuroimaging of Meckel's cave in normal and disease conditions. Insights Imaging. 2018 Aug;9(4):499-510
2023.5.18