Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4504_Библиотеки_им_академика_М_И_Перельмана

The intracranial extraaxial segment or cisternal segment of the cranial nerves course between the brainstem and the skull base neuroforamina and ssures. They are surrounded by cerebrospinal uid (CSF), and therefore submillimetric heavily T2WI are optimal to demonstrate the nerves as grey/ black surrounded by high signal intensity white CSF. Submillimetric 3D-DRIVE/FIESTA/SPACE T2WI can be used for this purpose but their range is limited, and thus dif­cult to cover the desired eld of view. The 3D balanced­FFE sequence can acquire submillimetric isotropic images with a high resolution and a large range, covering all the CNs in an acceptable acquisition time. This sequence is however more sensitive to banding artifacts and motion. The banding artifacts result in black lines running through the high signal intensity uid of the orbits, semicircular canals, cochlea, and CSF surrounding the olfactory bulbs. Hence, these periph­eral regions and structures are better examined with 3D TSE T2W sequences. Another drawback of the 3D b-FFE sequence is that the pulsation of the basilar artery can cause dephasing or loss of the high signal intensity in the CSF sur­rounding this artery and in turn will frequently cause non­visualization of the abducens nerve. Visualization of this nerve can be accomplished with 3D TSE T2WI.A recent improvement of 3D b-FFE T2W sequence to 3D b-FFE XD T2W sequence eliminates these banding artifacts and there­fore this new sequence can be used in all areas, including olfactory bulbs, inner ear, orbits, and for the abducens nerve. Gadolinium-enhanced T1WI or black blood (BB) images are used to detect abnormal nerve enhancement and are the most sensitive images to detect pathology on the cisternal seg­ment. However, it should be cautioned that 3D T1W gradient­echo sequences are less sensitive to gadolinium enhancement and that subtle/weak enhancement can be missed on these images. TSE, SE and BB sequences, 3D or 2D, are more sensitive for gadolinium enhancement with the BB sequence being the most sensitive.

Subarachnoid CSF encapsulation or cyst formation can also cause displacement of or compression on CNs. They are isointense to CSF on all sequences, including DWI and 3D T2W GE sequences. On heavily T2W 3D sequences, the typically thin wall of the cyst can be demonstrated, conrm­ing the diagnosis, and resolving the cyst dimensions. Surgical cyst fenestration is performed in cases of causative CN or other neurological symptoms.

Pituitary adenomas and craniopharyngiomas can com­press the optic chiasm and cause bitemporal hemianopsia. Rathke cleft cysts are most often an incidental nding but may rarely cause vision loss.

CN disorders may be the rst signs of underlying menin­geal disease. Often multiple CNs are involved on both sides, but it can also be an isolated nerve, for example, non-specic abducens palsy (CN VI). The meningeal enhancement is dif­cult to detect on 3D T1WI as all meninges show enhance­ment even in normal patients and is even more difcult to see on post-contrast CT. 3D FLAIR is more sensitive but today post-contrast 3D BB imaging is the sequence of choice. In normal patients, no meningeal enhancement is seen on these images, making it easy to identify abnormal meningeal enhancement in the case of meningitis. In leptomeningitis, the leptomeninges, on the surface of the brain and the brain­stem, are thickened and enhancing. In pachymeningitis, the thickened enhancing meninges are contiguous to the skull. Pyogenes and tuberculous meningitis are two common causes of the brainstem and skull base leptomeningitis.

Repetitive trauma with bleeding can also result in menin­geal supercial siderosis. This haemosiderin deposition occurs also on the CNs and can result in degeneration and disorders of the involved CN. CN VIII is most frequently involved with resulting sensorineural hearing loss. The hae­mosiderin deposit can be seen as a hypointense rim around the CN on T2* images or SWI and atrophy can be visualized on submillimetric heavily T2WI (e.g. DRIVE, b-FFE). Similar siderosis can also be caused by haemorrhagic sur­gery, subarachnoid haemorrhage, slow growing tumours and in cerebral amyloid angiopathy.

Unilateral involvement of several CNs is suggestive of skull base disease. CT and contrast-enhanced MR together with the clinical history and laboratory data are used to obtain the correct diagnosis. In case of osteomyelitis, the infectious cause must be sought in the paranasal sinuses, middle ear and/or mastoids with different CNs affected, depending on where the infection is located. Cranial nerve VI runs in Dorello’s canal passing through the venous basilar plexus behind the clivus. Aggressive fungal infections originating in the sphenoid sinus can destroy the posterior wall of the sinus and enter this plexus, thus provoking a unilateral abducens palsy. In this case, CN VI will enhance and no longer be vis­ible as a black dot inside the enhancing basilar plexus on axial high-resolution post-contrast T1WI.

Aneurysms of the parasellar ICA can also be responsible for palsies of CNs III, IV, V1, and VI.However, due to its location deep in the cavernous sinus, the abducens nerve is often the rst to be affected by these aneurysms. The same is true for pituitary adenomas and craniopharyngiomas that extend in the parasellar region. Although the overall most frequently involved CN inside the cavernous sinus, regard­less of the cause, is CN III followed by CN VI.

Isolated peripheral CN VII dysfunction, with intact lacri­mal function and stapes reex and taste in the anterior 2/3 of the tongue, may be provoked by parotid and peri mastoid lesions.