Cranial nerve disorders of dogs and cats (Proceedings)

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Cranial nerve disorders of dogs and cats (Proceedings)

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Apr 01, 2009

Trigeminal Nerve (Cranial Nerve V)

The trigeminal nerve is a large nerve that contains both motor and sensory components. The motor (efferent) portion is contained in the mandibular branch, and it innervates the muscles of mastication (temporalis, masseter, medial and lateral pterygoid, rostral digastricus and mylohyoid muscles). The sensory (afferent) portions of the trigeminal nerve are in the ophthalmic, maxillary and mandibular branches, and are responsible for nociception and proprioception to most parts of the head (face, eyelids, pinnae, cornea, oral cavity and mucosa of the nasal septum).

Testing the Trigeminal Nerve
A. Cutaneous Sensory Testing
1. Corneal reflex

  • Touch center of cornea (autonomous zone of the ophthalmic branch) with a moist cotton swab; observe for retraction of globe +/- eye blink
  • CN V afferent (ophthalmic branch only), CN VI +/- VII efferent

2. Palpebral reflex
  • Touch medial canthus of eye; observe for eye blink
  • CN V afferent (ophthalmic and maxillary branches), CN VII efferent

3. Trigeminofacial reflex
  • Touch area of maxillary vibrissae, observe for eye blink
  • CN V afferent, CN VII efferent

4. Pain responses
  • Require cortical processing, CN V is afferent

o Gentle, blunt stimulation of the nasal mucosa on the medial nasal septum causes retraction of the head (cortical response)
o Noxious stimulus to the lateral maxilla at the level of the canine tooth (autonomous zone for maxillary branch of CN V) causes ipsilateral curling of the lip (CN VII efferent part of this reflex) and withdrawal of the head away from the stimulus (cortical response)
o Noxious stimulus to the lateral mandible at the level of the canine tooth (autonomous zone for mandibular branch of CN V) causes withdrawal of the head away from the stimulus (cortical response)

B. Evaluating Motor Function
1. Paresis/paralysis. Ability to close the mouth and prehend food. May be difficult to detect if unilateral
2. Atrophy
3. Symmetry and tone of the muscles of mastication

Clinical Signs of CN V Dysfunction
A. Sensory dysfunction
1. Decreased (hypesthesia, anesthesia)

  • Decreased or absent reflexes (palpebral, corneal, trigeminofacial)
  • Reduced facial sensation
  • Corneal ulcer

2. Abnormal (paresthesia, hyperesthesia)
  • Rubbing or pawing at face

B. Motor dysfunction
1. Masticatory muscle paresis or paralysis

  • Unilateral—difficult to detect clinically
  • Bilateral—dropped jaw, inability to close mouth, drooling, difficulty in prehending food

2. Masticatory muscle atrophy
  • Severe neurogenic atrophy (unilateral or bilateral)

Lesion Localization
A. Intracranial
1. Pons and rostral medulla

  • Lesions in this area affect both sensory AND motor nuclei –Therefore, motor and sensory dysfunction is present
  • Other brainstem signs may also be present such as other cranial nerve deficits (particularly central vestibular signs), paresis, obtundation, conscious proprioception deficits

2. Caudal medulla and cranial cervical spinal cord
  • Lesions in this area only affect the sensory nucleus and tract
  • Therefore, sensory dysfunction is present WITHOUT motor dysfunction
  • Other brainstem signs may also be present

3. Intracranial but extramedullary
  • Lesions in this area include the root of the trigeminal nerve, trigeminal ganglion, and trigeminal nerve BEFORE it exits the foramen
  • Therefore, motor and sensory dysfunction is present
  • Other brainstem signs are usually present

B. Extracranial
Clinical signs vary with the location of the lesion along the trigeminal nerve and the branch(es) involved