Perineural spread
Perineural spread from skin and head-and-neck malignancies can occur along the trigeminal and facial nerves. Its presence has marked implications for staging and treatment of otolaryngological malignancies, automatically upstaging tumours to T3 in the most recent eighth edition of American Joint Committee Cancer staging manual [12]. Maxillary nerve (V2) perineural spread occurs from primary tumours in the midface skin, maxilla, upper lip and palate. Mandibular nerve (V3) perineural spread occurs from tumours in the lower face, mandible, masticator space and parapharyngeal space. Trigeminal perineural involvement may also occur from spread along nerves communicating with facial nerve branches such as the greater superficial petrosal/vidian nerves near the pterygopalatine fossa and auriculotemporal nerve near the temporomandibular joint. Squamous cell carcinoma is the most common cause of perineural spread given its large prevalence, but adenoid cystic carcinomas of the minor salivary glands have the highest incidence [17] (Figs. 2 and 3). Brainstem tumours can also rarely spread anteriorly through CN V (Fig. 4).
Trigeminal neuralgia
Although trigeminal neuralgia is a clinical diagnosis, neuroimaging may be performed for confirmation in viral/idiopathic aetiologies and to assess for treatable neurovascular compression. Viral aetiologies, including herpes zoster and simplex viruses, involve the Gasserian ganglion, where they can lie dormant. Mild enhancement of the ganglion is non-specific and difficult to distinguish from normal perineural vascular plexus; however, cisternal/root entry zone enhancement is specific. Herpes rhombencephalitis asymmetric enhancement in the clinical context of reactivation has characteristic imaging findings (Fig. 5).
Neurovascular compression can be suggested in the appropriate clinical context with trigeminal cisternal/root entry zone deformation from a vascular loop such as the superior cerebellar artery or anterior inferior cerebellar artery [9]. Persistent trigeminal artery, the most common persistent fetal carotid-basilar anastomosis, normally runs through a dural foramen located immediately medial to the Meckel’s cave (Fig. 6). Rarely, a vascular loop may be associated with trigeminal neural arteriovenous malformation, where symptoms may be from the malformation itself or nerve compression/deformation from enlarged feeding and draining vessels [11] (Fig. 7). Microvascular decompression is an effective treatment for these cases, although stereotactic radiosurgery has also been used, especially in the context of arteriovenous malformation (AVM). Non-vascular/idiopathic causes of trigeminal neuralgia are treated with anticonvulsants, antispasmodics and botox. Radiosurgery (gamma knife) is reserved for medically refractory symptoms. Enhancement without expansion can be transient or persistent following stereotactic radiosurgery [16].
Neoplastic, inflammatory and other
Predictably, the most common neoplasm of Meckel’s cave is a trigeminal nerve sheath tumour, schwannoma and neurofibroma. Nerve sheath tumours result in nerve and foraminal enlargement, demonstrate T2 hyperintense signal with moderate-to-intense heterogeneous enhancement. A dumbbell shape provides specificity, with the waist at constricting foramina (Fig. 8). In contrast, meningiomas often display T2 hypointense signal and show uniform, avid enhancement. Nerve sheath tumours may be isolated or syndromic, in phacomatoses such as neurofibromatosis (Fig. 9). Neurofibromatosis should be considered in cases of multiple nerve sheath tumours and dural ectasia (Fig. 10).
Leptomeningeal metastases, most frequently from breast and lung malignancies, result in linear segmental enhancement, usually in the setting of disseminated disease [22]. Lymphoma can result in neural involvement from either perineural invasion or leptomeningeal disease [2]. Lymphoma can cause dural tail, but absence of hyperostosis helps differentiate it from meningioma (Fig.11). Sequences that highlight CSF such as T2 SPACE or CISS play an important part in helping detect CSF disseminated malignancies. The normal high T2 signal of CSF maybe replaced by low signal from malignant lesions.
Inflammatory aetiologies such as sarcoidosis have more nodular enhancement than other leptomeningeal diseases due to granulomas and can involve the trigeminal nerve. Symmetrical involvement of Meckel’s cave is rarely reported [13] (Fig. 12). Neurosarcoidosis is rare without pulmonary manifestations and facial nerve involvement is more common [6, 19]. Additionally, involvement of pituitary hypothalamic axis can help point towards the correct diagnosis.
Diffuse cranial nerve marked enlargement can be seen in chronic inflammatory demyelinating polyneuropathy (CIDP), neurofibromatosis and hereditary sensory motor neuropathies (HSMNs). HSMN type I (Charcot-Marie-Tooth disease) demonstrates no significant enhancement nor leptomeningeal disease [3] (Fig. 13). Diagnosis is frequently known from genetic testing of the autosomal-dominant characteristic clinical history of distal weakness and absent reflexes beginning in the second decade. CIDP demonstrates diffuse enhancement and neurofibromatosis demonstrates more defined mass lesions and numerous additional findings such as plexiform fibromas and sphenoid wing dysplasia.
Non-trigeminal disease
Meckel’s cave, being composed of dura, is subject to meningiomas. Meningiomas may originate in the dura mater in or around Meckel’s cave (Fig. 14). Rarely, the tumours can be confined within the cave and arise from the trigeminal nerve [8]. Additionally, the cave may also be invaded by non-neural processes such as posterior extension of pituitary macroadenoma and orbital inflammatory disease. Lesions of adjacent bone and other structures may extrinsically compress the canal, best assessed on thin coronal T2-weighted imaging. Examples include petrous apex, petroclival fissure and clival diseases, osseous expansion from ocular nerve sheath tumours and internal carotid artery (ICA) aneurysms.
Thin, high-resolution, three-dimensional constructive interference in steady state imaging can distinguish the second most common primary neoplasm in Meckel’s cave, meningioma, from nerve sheath tumour. Meningiomas are peripheral with enhancing dural tail, arising from the dural reflections comprising the cave’s margins, while nerve sheath tumours will be more central within the cave, growing along the course of the nerve. Calcifications and T2-hypointensity in meningiomas are additional distinguishing findings (Fig. 15).
Pituitary macroadenomas can be invasive, contiguously extending from the sella, through the cavernous sinus, to Meckel’s cave. Such large tumours are associated with sellar expansion, ICA encasement without extrinsic compression, sphenoid sinus extension, and are relatively homogeneous and moderately T2-hyperintense.
Posterior extension of pathology to Meckel’s cave can also be seen in Tolosa Hunt, a variant of orbital inflammatory disease (orbital pseudotumour) involving the orbital apex that extends posteriorly into the cavernous sinus [7]. Patients present with painful ophthalmoplegia and cavernous fullness, asymmetric enhancement and ICA narrowing (Fig. 16).
Osseous processes compressing Meckel’s cave involve the petrous apex, petroclival fissure and clivus. Petrous apex cephalocele is usually an incidental finding, reflecting benign ballooning of the arachnoid space communication with Meckel’s cave. Fluid distended Meckel’s cave has an enlarged porus trigeminus notch and a smooth expansile cystic space in the anteromedial petrous apex [20] (Fig. 17). Findings may reflect intracranial hypertension, similar to empty sella, and is associated with spontaneous CSF leaks due to dehiscence [1]. Signal characteristics follow CSF, with FLAIR suppression. Petrous apex mucoceles have a similar appearance but do not connect to Meckel’s cave; rather, they compress it, resulting in symptoms. Cholesterol granulomas are T1 hyperintense, demonstrate susceptibility and no FLAIR suppression. Congenital cholesteatomas and epidermoids both demonstrate restricted diffusion, incomplete FLAIR suppression and no enhancement, but congenital cholesteatomas are localised to the petrous apex, while epidermoids are in the cerebellopontine angle/prepontine cistern and are much more proliferative, extending into multiple cisterns and encasing the basilar artery [14] (Fig. 18). Petroclival/petrooccipital fissure chondroid lesions demonstrate characteristic imaging features on CT with rings and arcs calcifications (Fig. 19). Clival chordomas demonstrate extensive bony destruction, marked T2-hyperintensity, haemorrhagic and calcific susceptibility, and honeycomb enhancement pattern. They can be distinguished from pituitary macroadenoma by lack of sellar mass, sparing of the sphenoid sinus and signal characteristics. There are isolated case reports of intradural chordomas of the Meckel’s cave and paraganglioglioma.
Aneurysms of the petrous and cavernous segments of the ICA can result in mass effect on Meckel’s cave given the close proximity (Fig. 20). These segments are demarcated by the petrolingual ligament. Ruptured aneurysms in either location do not cause subarachnoid haemorrhage as they are extradural, but cavernous ruptured aneurysms and dissections may cause carotid-cavernous fistulas.
