Cranial Nerve Dysfunction Evaluation

Introduction to Cranial Nerves

The 12 cranial nerves emerge from specific regions of the brain and pass through distinct foramina at the skull base, each serving specialized sensory and motor functions essential for head and neck innervation, as noted by the American College of Radiology 1, 2
Cranial nerves provide sensory and motor innervation to the head and neck region, with each nerve having a unique function, as described in the table below:

Component	Description
Central Nervous System (CNS)	Comprises the brain and spinal cord, functioning as the command center for processing information
Peripheral Nervous System (PNS)	Consists of nerves and ganglia outside the CNS
Autonomic Nervous System	Divided into Sympathetic, Parasympathetic, and Enteric Divisions, controlling various bodily functions
Cranial Nerves	12 pairs of nerves emerging from the brain and brainstem, providing sensory and motor innervation to the head and neck region [2, 1]

The American College of Radiology recommends contrast-enhanced MRI as the gold standard imaging modality for evaluating cranial nerve dysfunction, with protocols tailored to the specific cranial nerve(s) affected and covering the entire pathway from brainstem nuclei to end organs 2
High-field strength magnets (1.5-3.0T) with phased-array head coil are essential for cranial nerve imaging 2
Essential sequences for cranial nerve imaging include T1-weighted (pre-contrast), T2-weighted, T1-weighted with contrast enhancement, and fat-suppressed T1-weighted post-contrast 2
Specialized 3D sequences, such as constructive interference in steady state, 3D-balanced fast field echo, and 3D-driven equilibrium radio frequency reset pulse, are useful for better visualization of cranial nerves 2
CT is complementary to MRI, superior for evaluating bony foramina and skull base, and useful in trauma cases and for patients who cannot undergo MRI 2

For olfactory nerve (CN I) evaluation, contrast-enhanced MRI tailored to the anterior cranial fossa is recommended, while CT is useful for sinus inflammatory disease and trauma 2
For trigeminal nerve (CN V) evaluation, contrast-enhanced MRI and MR angiography for vascular compression evaluation are recommended 2
For facial nerve (CN VII) evaluation, dedicated contrast-enhanced MRI of the orbit, face, and neck, with a protocol tailored to the temporal bone and parotid area, is recommended 2
For glossopharyngeal nerve (CN IX) evaluation, focused contrast-enhanced MRI of the posterior fossa is recommended 2
For vagus nerve (CN X) evaluation, contrast-enhanced MRI or neck CT, with imaging extending to the upper chest (aortic pulmonary window), is recommended 2
For hypoglossal nerve (CN XII) evaluation, neck MRI covering the entire nerve pathway is recommended 2

Incomplete imaging coverage can lead to missed diagnoses, and always imaging the entire nerve pathway from nucleus to end organ is essential 2
Inadequate contrast can lead to missed diagnoses, and intravenous contrast is imperative for MRI evaluation of cranial neuropathy 2
Insufficient resolution can lead to partial-volume effects, and using thin-section imaging is recommended 2
Missing multiple nerve involvement can lead to missed diagnoses, and mass lesions may affect multiple cranial nerves due to their close proximity 2
Overlooking subtle signs, such as nerve enhancement, enlargement, foraminal expansion, or muscle volume loss, can lead to missed diagnoses 2
Assessment of cranial nerves includes evaluating vision (special sensory) through visual acuity, visual fields, and fundoscopic examination, as well as facial sensation, jaw movement, and specific cranial nerve functions, such as facial expression, taste, and parasympathetic functions 2

Vascular lesions, such as compressing vascular loops, aneurysms, and vertebrobasilar dolichoectasia, can affect cranial nerves, as well as neoplastic processes, such as perineural spread of tumor, which can occur along CN V and VII 2, 1
Inflammatory/infectious conditions, such as meningitis, encephalitis, sarcoidosis, and granulomatosis with polyangiitis, can affect cranial nerves, and traumatic injuries, such as fractures affecting foramina, can disrupt cranial nerves, with CN I being most commonly disrupted by trauma 2
Treatment depends on the underlying cause, with options including corticosteroids for Bell's palsy, carbamazepine or other anticonvulsants for trigeminal neuralgia, and surgical resection or stereotactic radiosurgery for schwannomas, meningiomas, and paragangliomas, emphasizing the need for thorough evaluation and proper imaging techniques 2