MRI of the head:
T2 weighted sagittal, transverse
Gradient echo T2* weighted transverse
Precontrast T1 weighted transverse
Postcontrast T1 weighted transverse, sagittal
Postcontrast T1 weighted dorsal with fat saturation
Postcontrast VIBE transverse – 3D T1 weighted high-resolution gradient echo sequence with fat saturation
Best seen on transverse post contrast T1 weighted images and VIBE images, there is a small circular, moderately primarily peripherally enhancing structure at the base of the cranial vault, shown here caudal to the pituitary fossa (figure). This structure is less conspicuous on T2 weighted images where it appears mildly hyperintense, and it is poorly distinguished on T1 weighted images appearing isointense to brain. Contrast enhancement of the structure is relatively focal allowing limited tracing of its course.
Enlargement and contrast enhancement of the left oculomotor nerve (cranial nerve III) consistent with idiopathic oculomotor neuropathy.
The oculomotor nerve innervates the extraocular muscles of the ipsilateral eye, specifically the dorsal rectus, medial rectus, ventral rectus, ventral oblique, and the levator palpebrae superioris muscles. The parasympathetic component of the nerve controls pupil constriction. Neurologic signs of oculomotor dysfunction consist of lateral strabismus, ptosis, and mydriasis. The nuclei of the motor portion of the oculomotor nerve are located within the rostral mesencephalon. The nuclei of the parasympathetic portion are located within the rostral colliculus of pretectal nuclei. The axons exit the brainstem and course next to the cavernous sinus lateral to the hypophysis before exiting the skull through the orbital fissure.
A recent publication (1) describes the MRI findings in 14 dogs with idiopathic oculomotor neuropathy. In their series, an underlying cause for nerve enlargement, contrast enhancement, and neurologic signs could not be determined ruling out to the best of their abilities endocrine, infectious, and neoplastic changes. Varying degrees of nerve enlargement and varying degrees and extent of contrast medium enhancement were described, ranging from non-enhancing to intensely enhancing and from focally enhancing to more diffuse enhancing.
2. Evans HE, De Lahunta A. Cranial nerves. In: Evans HE, De Lahunta A (eds): Miller’s anatomy of the dog, 4th ed. Missouri: Saunders Elsevier, 2013;708–730.
3. Couturier, L., Degueurce, C., Ruel, Y., Dennis, R., & Begon, D. (2005, 9). Anatomical Study of Cranial Nerve Emergence and Skull Foramina in the Dog Using Magnetic Resonance Imaging and Computed Tomography. Veterinary Radiology & Ultrasound, 46(5), 375-383.