Page 275 - Atlas of Small Animal CT and MRI

Cranial Nerves 265

internal acoustic meatus, courses through the facial In our experience, ocular granulomatous meningoen-
canal in the temporal bone, and exits through the cephalitis can often have subtle MR imaging features.
stylomastoid foramen.
The origin and intracranial path of cranial nerve VIII, Infectious inflammatory disorders
the vestibulocochlear nerve, is similar to that of the Infectious cranial neuritis can be viral, bacterial, mycotic,
facial nerve, emerging from the trapezoid body adjacent or protozoal. Imaging descriptions of infectious inflam-
and dorsal to the emergence of the facial nerve. The matory cranial neuropathies are sparse, but expected
nerve also exits the cranial cavity through the internal features would include nerve enlargement, variable T1
acoustic meatus. Because of the close proximity of and T2 intensity on unenhanced MR images, and some
intracranial parts of these nerves, disorders affecting one degree of enhancement following contrast administra-
can often also affect the other. tion. Mass lesions may also be seen when a suppurative
or granulomatous inflammatory response is present
Inflammatory and idiopathic disorders (Figure 2.10.4).
Seventh and eighth cranial neuropathy often occurs
Noninfectious disorders from intracranial extension of bacterial otitis media/
Idiopathic cranial neuropathy interna, and neuritis may be accompanied by regional
Idiopathic trigeminal neuropathy is peripheral, often meningitis and abscess formation (see Chapter 1.2,
bilateral, and is the most common cause of masticatory Figure 1.2.10). 12
muscle paralysis in the dog. This disorder causes
dropped jaw from dysfunction of the mandibular branch
motor innervation of the masticatory muscles. Variable Neoplasia
facial sensory deficits may also be present. 7 Optic nerve meningioma
Idiopathic facial paralysis is the most common cause
of acute facial nerve neuropathy in the dog and is also Because the meninges cover the optic nerves, retrobul-
seen in cats. This disorder is peripheral in origin and is bar meningiomas can occur either in situ or by
most often unilateral but can be bilateral. Clinical signs expansion of an intracranial meningioma through the
include: palsy of the external ear, lips, and cheek; a lack optic canal. Imaging features of meningiomas are
of palpebral closure; and ptyalism. 8 described in Chapter 2.8. Optic nerve meningiomas
Idiopathic trigeminal neuropathy imaging charac- can have both extracranial and intracranial compo-
teristics include diffuse nerve enlargement that is T1 nents and cause exophthalmos. Owing to location,
isointense and T2 iso‐ to hyperintense on MR images. dural tails are not a feature of extracranial meningi-
Affected nerves consistently enhance following con- omas (Figures 2.10.5, 2.10.6).
trast administration (Figure 2.10.2). 9 Peripheral nerve sheath tumor
Imaging diagnosis of idiopathic facial neuropathy
may be more challenging. The nerve is visible on unen- Peripheral nerve sheath tumors most commonly affect
hanced images, but features may be unremarkable. the origin and branches of the trigeminal nerve. These
Affected nerves variably enhance following contrast tumors may be benign or malignant. Clinical signs
administration. Contrast‐enhanced ultrafast gradient associated with cranial nerve V nerve sheath tumor are
8
echo sequences have been reported to increase sensitiv- unilateral and include atrophy of the temporalis and
ity of detection of nerve enhancement in dogs with facial masseter muscles.
nerve neuropathy. 10 Trigeminal nerve sheath tumors appear as an isoat-
tenuating extraaxial mass, usually in the region of the
Ocular granulomatous meningoencephalitis origin of the nerve lateral to the pons. The ophthalmic,
The ocular form of granulomatous meningoencephalitis maxillary, and mandibular branches of the nerve can all
(GME) is uncommon compared to disseminated and be involved. Trigeminal nerve sheath tumors appear T1
focal forms. Features of GME are more fully described in isointense and T2 iso‐ or hyperintense on MR images.
Chapter 2.6, but the ocular manifestation is character- These tumors generally intensely, and uniformly con-
ized clinically by blindness and optic neuritis and may trast enhance on both CT and MR images (Figures 2.10.7,
occur as a component of more widespread disease. 2.10.8). Affected cranial nerve V branches are enlarged
Ocular and optic nerve involvement is most often bilat- and have a similar enhancement pattern as the central
eral. Magnetic resonance imaging features have been tumor mass. The trigeminal canal, the orbital fissure,
reported to include T1 and T2 isointensity with enhance- and the round and oval foramina are often enlarged as a
ment following contrast administration (Figure 2.10.3). result of bone resorption resulting from expansion of the
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