The fifth cranial nerve is routinely tested in the unconscious ICU patient as the sensory component of the corneal reflex.In the conscious patient, one may also test sensation in the major territories (forehead, cheek, chin) as well as the power of the temporalis and masseter. In the unconscious or uncooperative hot case patient, the presence of a bite block or Guedel airway jammed into the oral cavity alongside the endotracheal tube alerts the keen-eyed candidate of the possibility that the muscles of mastication are very much innervated, and capable of forecful chewing actions.
Lesions of the trigeminal nerve
- Absent corneal reflex
- Absent sensation in the sensory distribution
- Weakness/wasting of the muscles of mastication (masseter and temporalis)
- The motor supply is carried by the mandibular branch.
- The mandible will deviate towards the paralysed side when the mouth opens.
- Chronic unilateral lesions will present with unilateral temporalis atrophy.
Causes of a unilateral lesion of the trigeminal nerve
A higher central lesion (eg. cerebral or thalamic) will have to be contralateral to the clinical findings.
- A hemispheric infarct of the MCA territory may produce sensory loss in the trigeminal distribution on the same side as the hemiparesis.
- With hemispheric lesions, masseter strength is usually preserved. The supranuclear control of trigeminal nerve motor functions is bilateral, so a hemispheric infarct is never going to produce a unilateral lesion (although voluntary control of the masseter will be lost).
Brainstem lesions would be ipsilateral.
- Pontine stroke (lateral rostral pons or above) - with ipsilateral body sensory loss
- Medullary stroke (lateral medulla) - with contralateral body sensory loss
- Mid-pontine stroke (ipsilateral pons) - if only the face is affected
- Isolated masticatory motor failure suggests that the lesion is actually limited to a small area of the mid-pons.
- Raised intracranial pressure (a "false localising sign")
- Pontine tumours
A peripheral lesion may be specifically affecting a single branch.
- A sensory lesion confined to a branch distribution suggests the lesion cannot be nuclear - i.e. it must be somewhere beyond the trigeminal ganglion:
- Craniofacial trauma
- Base of skull fracture
- Maxillary sinusitis
- Aneurysm of the internal carotid artery
- Cavernous sinus thrombosis
Causes of an isolated bilateral lesion of the trigeminal nerve
Bilateral lesions are freakishly rare in isolation
- Diabetic neuropathy
- Large pontine tumours
- Cavernous sinus thrombosis (extensive)
Obviously, bilateral absence of the corneal reflex is to be expected in the context of braindeath.