Cranial Nerve Pathology

The cranial nerves in feline and canine patients are similar to those in humans; there are 12 pairs or nerves, many of which serve similar functions and have similar naming conventions as compared to humans. This page will not be an in depth discussion of cranial nerve anatomy or specific pathology, but will focus on concise and applicable information for identifying pathology and selecting and using appropriate imaging techniques.

Most of the cranial nerves in veterinary patient are quite small and can be difficult to detect, even on fairly high resolution MRI scans. The nerves best visualized directly on MRI include the optic nerves, the trigeminal nerves, and the vestibulocochlear nerves / inner ear.

The optic nerves are paired nerves extending from the optic chiasm, just rostral to the pituitary gland, toward the eyes in an oblique dorsal plane. The optic nerves are not as commonly affected as the other cranial nerves, but do require more special imaging when it is necessary to assess them. To best assess the optic nerves, consider a few things: The nerves are small, surrounded by CSF, cushioned in fat, well vascularized, and take a tortuous and oblique path between the eyes and the brain. Fat saturation and oblique angles will be required, FLAIR may be helpful to suppress CSF, and 3D Fast Spin Echo will help suppress the signal from blood vessels if available.

Useful Sequences:

• Dixon T1 or T2 FSE
• T2 FLAIR with Fat Saturation
• 3D FSE with Fat Saturation

1. Acquire normal sagittal slices, and ensure slice coverage and FOV includes the entire eye and brain.
2. Acquire normal axial slices, and ensure slice coverage includes the entire eye and brain.
3. On the Sagittal images, scroll back and forth and identify where the optic nerve exits the eye caudally, and follow it through multiple slices until reaches the optic chiasm. Choose an approximat angle here, roughly 45-ish degrees from the pituitary.
4. On the axial images, identify where the optic nerves exit the eyes, and follow the nerves back toward the brain, select a position about halfway, and adjust the left/right rotation that may be needed.
5. increase slice coverage to include the entire orbit

See GIFs below to get a sense of the anatomy.

Dorsal Oblique T2 FLAIR Fat Sat

Dorsal Oblique 3D PD FRFSE

The Sagittal Oblique is fairly straightforward. While looking at a dorsal image, plan about a 30-45 degree oblique from straight sagittal and expand slice coverage to include the entire orbit.

Sagittal Oblique Reformat 3D PD FRFSE

The Trigeminal nerves are 3 paired nerves extending from the brainstem rostrally. The three branches include the ophthalmic, maxillary, and mandibular, and serve sensory and motor functions for the face. Due to their relatively small size and oblique course, they can be tough to visualize much beyond their origins without high resolution imaging. When trigeminal nerve pathology is present, downstream effects of denervation may be seen such as muscle wasting or inability to close the mouth. These changes can readily be seen as an asymmetry in the muscle size on either side of the head, and sometimes hyperintensity on T2 weighted imaging. While there are no special views needed to visualize the nerves, high in-plane resolution and thin slices are helpful, and slice coverage from the eyes through the brain is required.

Useful Sequences:

• T1 or T2 FSE
• DWI b600-1000
• T1 Post Gd with Fat Saturation

All three branches visualized on a DWI b1000

Right sided trigeminal tumor seen on DWI b1000

In the slide below, notice the asymmetrical muscle size (blue arrow) and how fat saturation improves image quality after contrast administration.

The vestibulocochlear and facial nerves are located closely together in the inner ear and their origins can be well visualized on MRI with appropriate slice planning. Some common findings in animals with pathology in this area can include head tilt, and may be due to infection, inflammation, or neoplasia. A common source of pathology is infection within the bulla, air filled spaces just ventral to the inner ear. In certain cases, further high resolution imaging can be helpful.

Useful Sequences:

• T1 FSE with Fat Saturation
• 3D T2 or PD FSE
• 3D FIESTA-C (GE)/CISS (Siemens)