The anatomy of the trigeminal nerve is complex. It consists of both motor and sensory components.
There are three sensory nuclei located within the brainstem:
- Midbrain: Mesencephalic Nucleus– proprioceptive fibres from the muscles od mastication terminate in this nucleus
- Pons: Called the principle sensory nucleus located within the pons. Like the dorsal columns they decussate at the level of the medial meniscus to ascend to the somatosensory cortex
- Medulla:Spinal Nucleus. Receives fibres conveying pain and temperature. These fibres than join the fibres with the spinothalamic tract from the lower limb. This is important because in a lateral medullary syndrome patients can have ipsilateral loss of pain and temperature and contra-lateral loss of arm/leg of pain and temperature because the fibres have already decussated at the level of the spinal cord.
The motor and sensory nerve roots emerge from the lateral brainstem. The Gasserian Ganglion lies on the apex of the petrous part of the temporal bone. Here the three divisions of the trigeminal nerve merge. Each passes through its own foramen and carries innervation from a specific area of the face.
The Opthalmic Branch: Superior Orbital Fissure
The Maxillary Division: Foramen Rotundum
The Mandibular Division exits foramen ovale. The anterior division incorporates the motor branch of CNV5 innervating the muscles of the mastication. The posterior trunk innervates the anterior 2/3 of the tongue (in conjunction with chorda tympani)
5th Nerve Lesions
Lesions at the Brainstem (infra-nuclear): Vascular Lesions, demyelination, tumours (eg tectal plate glioma)
Lesions at the CPA angle eg acoustic neuroma, trigeminal neuralgia
Lesions at the Cavernous Sinus e.g expanding pituitary adenoma
Skull Base: eg a petrous temporal bone (associated with 6th nerve called Gradenigo’s syndrome)
Trigeminal Neuralgia
If there is any question you will be asked about it will be regarding trigeminal neuralgia. I was asked about this in medical finals. It is important because it can cause severe pain and some patients have even committed suicide secondary to the pain.
Patients with unilateral severe facial pain often occurring the 2nd and 3rd branches of the trigeminal nerve. This pain is often triggered by speaking, shaving and brushing his teeth.
It commonly affects women and >50 years old.
Aetiology:
Microvascular decompression: Superior Cerebellar Artery causing compression of the Trigeminal Nerve
Tumours at the CPA: eg meningioma’s, acoustic neuroma
Demyelination: MS is the other key differential
Management:
- Drug therapy: carbamazepine
- Microvascular decompression: release the Superior cerebellar artery from the trigeminal nerve