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The cranial nerves (TA: nervi craniales) are the twelve paired sets of nerves that arise from the cerebrum or brainstem and leave the central nervous system through cranial foramina rather than through the spine. The cranial nerves are numbered one to twelve, always using Roman numerals, i.e. I to XII. Most have cranial nerve nuclei located in the brainstem.
The third and fourth cranial nerves originate from the midbrain:
The middle four cranial nerves originate from the pons:
The final four cranial nerves originate from the medulla oblongata:
See mnemonic for cranial nerves.
Visualization of cranial nerves in routine clinical practice can be challenging as most are quite small and difficult to resolve. Intracranially, the cisternal portion is most readily appreciated as the nerves are surrounded by CSF. As the nerves exit the skull, their location can be implied by knowledge of normal anatomy but, due to the complicated anatomy of the base of the skull and head and neck, actually visualizing these small structures is challenging on all modalities.
With the exception of the largest cranial nerves (e.g. optic nerves and trigeminal nerves) CT is limited in its ability to directly visualize nerves, although pathology can be identified in a number of scenarios:
if the nerve is enlarged it may be directly visualized of adjacent structures including bony foramen may be distorted or remodeled
if there is abnormal contrast enhancement such as due to perineural tumor spread
Directly imaging of cranial nerve fibers within the substance of the brain is not readily achievable in clinical practice. Involvement is generally deduced from a priori knowledge of their expected location and trajectory from the nucleus to the nerve root entry zone.
In some instances, the intraparenchymal course of cranial nerve fibers can be assessed using diffusion tensor imaging, although this is only practical for the largest nerves 6.
Imaging the cranial nerves as they pass from the brain to their respective foramina is best achieved with high-resolution high-T2-contrast sequences that can, ideally, be reformatted in other planes (e.g. various implementations of steady-state sequences: steady-state free procession (SSFP), fast imaging employing steady-state acquisition (FIESTA) or constructive interference in steady-state (CISS) 4. The cranial nerves appear as dark structures surrounded by high-signal CSF.
Diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) can also be used to evaluate fiber integrity in the cisternal portion of nerves although again only the largest nerves can be investigated in this manner. The vast majority of studies published use diffusion-weighted imaging (ADC, fractional anisotropy and mean diffusivity) on the trigeminal nerves, most frequently in the setting of trigeminal neuralgia 6. Tractography can be used to try and evaluate the location of nerve fibers in relation to adjacent posterior fossa tumors 6.
Once out of the dura, imaging is more challenging due to the need to distinguish these small tortuous nerves from adjacent soft tissues (muscle, fat, vessels, connective tissue, etc.). A number of sequences are being developed with more or less utility in the routine clinical setting. These are generally discussed under the umbrella term high-resolution high-contrast magnetic resonance neurography (HRHC-MRN). Although these are also T2-weighted scans, as the nerves are relatively high in T2 signal compared to surrounding soft-tissues, they generally aim to visualize the nerves as high signal structures on a dark (suppressed) background 3.
Both intracranially and extracranially T1 weighted scans are useful in delineating surrounding anatomy and, with the administration of contrast, pathological enhancement 4.
Diffusion tensor tractography (DTI) has also been used peripherally to assess nerve integrity, for example in the setting of inferior alveolar nerve damage following dental procedures or involvement of branches of the mandibular nerve by salivary gland tumors 7,8.
History and etymology
Thomas Willis (1621-1675) was responsible for the original numbering of the cranial nerves, as well as his famous anatomical circle in the brain.
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