Category: Neuroradiology, Region: Head-Brain and brain nerves, Plane: MPR
Summary: Conjoined nerve root is a type of developmental anomaly involving nerve root. It is common in spinal nerve roots, commonly seen in lumbar region and never reported in cranial nerve roots. The incidence of conjoined lumbar nerve root (CLNR) up to 14% has been reported in cadaveric studies 1 and up to 6% in recent prospective MRI study 2. Since first described in 1949, more than 120 cases of CLNR have been reported 2. We also found a few reports of conjoined cervical nerve root 3, however conjoined cranial nerve root (CCrNR) has never been reported in world literature till date. We report the first case of CCrNR diagnosed with magnetic resonance (MR) imaging using high resolution steady state free precession (SSFP) sequence. In this, bilateral trigeminal (CN5), facial (CN7) and vestibulocochlear (CN8) nerves emerge as a conjoined nerve root from lower lateral pons, then courses in cisternal segments and subsequently divides into individual nerves to enter into the internal acoustic meatus (IAM) and Meckel’s cave. CCrNR were diagnosed using MR imaging using high resolution SSFP, because of its ability to generate a strong signal in tissues that have a high T2/T1 ratio such as cerebro-spinal fluid (CSF). Thus, it is particularly useful in visualizing the cisternal segments of the cranial nerves as they provide excellent contrast resolution between CSF and nerves, as well as high spatial resolution4. Association of CLNR with vertebral anomalies has been described in literature5. Congenital block vertebrae at C2-C3 and C4-C5 levels were also noted in our case of CCrNR. Case history: Clinical presentation: A 32 yr male presented with neck pain and hedache. There was no radiation of pain to the upper limbs. There was no history of tingling or numbness in upper limbs. Central nervous system and systemic examination were normal. Laboratory tests were normal. Imaging findings and diagnosis: First standard MRI brain with spin echo sequences was performed, which was normal. Then high resolution MR imaging with the SSFP sequence of the skull base region was conducted in 60 contiguous 1-mm-thick slices in the plane of the optic chiasm and major cranial nerves. Axial SSFP at the level of lower pons showed bilateral sensory roots of CN5 emerging from lower lateral pons along with CN7 and CN8 nerves as a common trunk. Further, these common trunks were seen coursing through the cerebellopontine angle (CPA) cisterns and subsequently divided into individual nerves just before IAM to enter into the Meckel’s cave and IAM (Fig 1). Coronal reformation of SSFP showed both common trunks including the sensory root of CN5 lying infero-lateral to and far away from motor nerve roots of CN5 which exits at its normal mid pons level (Fig 2). Motor root normally runs inferolateral with the sensory root (Fig 3), whereas in this case reversal of relation of sensory and motor nerve roots of CN5 was noted. Continuous scrolling of SSFP axial sequence from above downward demonstrated bilateral oculomotor nerve (CN3) roots emerging on medial aspect of the cerebral peduncles of midbrain, then motor nerve roots of bilateral CN5 emerging at the mid Pons level, then bilateral common trunk of the sensory roots of CN5 and the roots of CN7 and CN8, and then the roots of the abducens nerve (CN6) in that order [Video]. The findings were suggestive of bilateral CCrNR involving sensory roots of CN5, CN7, and CN8. Screening of spine with T2 sagittal sequences showed congenital block vertebrae at C2-C3 and C4-C5 levels and secondary disc degeneration and posterior disc bulges at C3-C4, C5-C6, and C6-C7 (fig2). These findings explained the symptoms of neck pain and giddiness. Patient was further referred to orthopedic department and started on medications for neck pain. Discussion: Embryology and etiology: Cranial nerve roots are arranged strictly segmentally. Deviations can be observed and are considered to be congenital in origin, arising during the fetal development of brain structures. Exact etiology of conjoined nerve root is not yet established. Genetic mechanism has been proposed. The rhombencephalon is composed of the cerebellum, pons and medulla. With the exception of the oculomotor nuclei, cranial nerve nuclei are derived from rhombencephalic neuronal precursors. In early development, the hindbrain is segmented into five compartments called rhombomeres. Neuronal populations within individual rhombomeres display limited intermixing with neighboring compartments. As a result, the location and position of a neural progenitor during hindbrain segmentation determine its contribution to adult brainstem structures and axonal connections. Due to their compartmental identity, these neuronal progenitors display programmed migratory behaviors and send axons along defined trajectories toward their peripheral targets. While a neural cell progenitor’s position along the anteroposterior axis determines its identity, the cell’s position along the dorsoventral axis appears to influence its sensory or motor function 6. Investigations into the molecular mechanisms of hindbrain development have yielded insights into the potential relationship between human developmental disorders and the molecular signals that determine neuronal identity and axonal guiding in the brainstem 7. Proposed mechanism is alteration in neuronal progenitors resulting in alteration in programmed migratory behavior and causes several developmental disorders including conjoined nerves. Site predilection and types of conjoined nerve: Conjoined nerve root are common in lumbosacral nerve roots, most common being at L4–S1and usually unilateral. Bilateral CLNR are rare 8. Occurrence of conjoined nerve root in cervical nerve roots is rare and never reported in cranial nerves till date. Several authors have proposed classifications for conjoined nerve roots, but the classification proposed by Postacchini et al. 9 is most commonly used. According to this classification, type I refers to one or more roots that emerge at an abnormal cranial level; type II refers to one root that emerges at a more caudal level than normal; type III refers to two or more nerve roots that emerge through closely adjacent openings of the dura; type IV refers to two nerve roots that emerge from the dural sac in a common nerve trunk; and type V refers to an anastomotic branch that connects two nerve roots in their extrathecal course. Type II is the most common type 9. Normal appearances of CN5, CN7, and CN8 MR imaging using SSFP sequences 10: Heavily T2 weighted SSFP sequences (also known as, fast imaging employing steady-state acquisition, or FIESTA, and constructive interference steady state, or CISS) are capable of depicting the cisternal segments of all 12 cranial nerves10. The clinical utility of an SSFP sequence lies in its ability to generate a strong signal in tissues that have a high T2/T1 ratio, such as CSF and fat. SSFP sequences are particularly useful for visualizing the cisternal segments of cranial nerves because they provide excellent contrast resolution between CSF and nerves, as well as its high spatial resolution4. CN5 is the fifth paired largest cranial nerve associated with derivatives of the 1st pharyngeal arch. It has a large sensory root which originates from three sensory nuclei (mesencephalic, principal sensory and spinal nuclei of CN5) and innervates the face through its ophthalmic, maxillary, and mandibular branches and small motor root originates from motor nucleus and innervates the muscles of mastication. On MR imaging with SSFP sequence, large sensory root emerges from the lateral mid pons, travels anteriorly through the prepontine cistern and the porus trigeminus to the Meckel’s cave and forms the CN5 ganglion before giving three subdivisions. Small motor root passes inferior to the sensory root, along the floor of the Meckel’s cave and merges with the fibers of mandibular division (fig 4). CN7 emerges from the lateral aspect of the lower border of the pons and CN8 emerges at the groove between the pons and the medulla oblongata behind the facial nerve. These nerves traverse the CPA cistern at an oblique angle with a parallel course (Fig 5). Further the CN8 crosses the IAM and traverses through the internal auditory canal, where it divides into the cochlear, superior vestibular and inferior vestibular nerves. These three CN8 branches, along with the CN7, have a characteristic appearance on sagittal oblique SSFP cross-sectional images with CN7 lying in the anterosuperior quadrant, superior vestibular in the posterosuperior quadrant, cochlear lying in the anteroinferior quadrant and inferior vestibular in the posteroinferior quadrant (Fig 6) In our case bilateral sensory roots of CN5 are seen emerging from lower lateral pons along with CN7 and CN8 as conjoined nerve roots, lying infero-lateral to motor nerve roots of bilateral CN5 which exits at its normal mid pons level. In contrast to the normal anatomy where the motor nerve root of CN5 lies inferior to the sensory root, the exact reverse was found to be true in our case. Bilateral CCrNR then courses through the CPA cisterns and subsequently divides into individual nerves just before IAM to enter in Meckel’s cave and IAM. Using Postacchini et al classification, this CCrNR fits into type II in view of the lower than normal level of exit of the conjoined nerve root from the brain stem. Association of CLNR with vertebral anomalies has been described in literature 5. Common vertebral anomalies were absence of ipsilateral facet joints, vertebral arch defects, spina bifida and spondylolisthesis. In our case, associated congenital block vertebrae were noted at C2-C3 and C4-C5 levels. Learning points: • Several cases of conjoined nerve root are reported in relation to lumbosacral nerves and few in relation to cervical nerves; however to the best of our knowledge, a case of conjoined nerve root has never been reported in relation to cranial nerves in literature till date. We are reporting this case, as we wanted to introduce the concept of conjoined cranial nerve root, opening the scope for large scale studies in future. • Knowledge of CCrNR is essential preoperatively in cases of trigeminal neuralgia and common CPA tumors like acoustic schwannoma to avoid injury to normal nerves. • Associated presence of congenital block vertebrae in cervical spine further confirms the association of conjoined nerve root with vertebral anomalies. REFRENCES 1. Kaddish L, Simmons E. Anomalies of the lumbosacral nerve roots. An anatomical investigation of myelographic study. J Bone Joint Surg Br. 1984; 66: 411–416 2. Böttcher J, Petrovitch A, Sörös P, Malich A, Hussein S, Kaiser WA. Conjoined lumbosacral nerve roots: current aspects of diagnosis. Eur Spine J. 2004;13 (2): 147-51. 3. Chu CR1, Chesnut RM. Cervical conjoined nerve root variant: preoperative imaging and surgical conformation -Case report. J Neurosurg. 1994 Mar;80(3):548-51. 4. Schmitz B, Hagen T, Reith W. 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