25th June 2023

Anatomical Relevance

In general, isolated disorders of the cranial nerve itself, i.e. lower motor neuron lesions of the cranial nerve, are less common than pathology affecting cortical/supranuclear pathways contributing to the relevant cranial nerves or pathology that will generally affect several nerves, e.g. generalised neuropathy. There are several common aetiologies that can affect any distribution of nerves and also affect supranuclear contributions to cranial nerves.

In general:

• Any physical impediment along the course of a cranial nerve can result in loss of function; this is commonly due to traumatic causes or neoplastic causes
• Neuroinflammatory conditions, e.g. multiple sclerosis, Lyme disease can affect any distribution of nerves and, while less common, can cause isolated cranial nerve palsies.
• Ischaemia and infarction; a macrovascular infarction, i.e. stroke, can cause supranuclear or lower motor nuclear manifestations.
  • As per the rule of 4, infarctions above the level of the involved cranial nerve nuclei would result in an upper motor neuron manifestation of cranial nerve dysfunction, while lesions at the level of the nuclei cause lower motor neuron manifestations.
  • For example, a pontine stroke due to posterior circulation infarction can cause an ipsilateral complete facial palsy (lesion at level of CN 7 nuclei ) with possible pseudobulbar palsy (above the level of CN 9, 10 and 12 nuclei), while a cortical anterior circulation infarction would cause a contralateral forehead sparing facial palsy (being above the level of CN 7 etc.)
• Microvascular changes, e.g. in diabetes mellitus, can cause ischaemia to any nerve, including cranial nerves.