Trigeminal Neuropathy

Excerpt

Trigeminal neuropathy refers to dysfunction in sensory or motor functions involving cranial nerve V, the trigeminal nerve. Trigeminal neuropathy (TNO) typically presents with numbness in the region innervated by the trigeminal nerve, sometimes associated with paresthesias, pain, or masticatory weakness. TNO can present from the involvement of the fifth cranial nerve (CN V) anywhere in its course, from the nuclei in the brain stem to its peripheral branches. It can be caused by known etiologies such as traumas, tumors, rheumatologic diseases, demyelinating, or idiopathic ones. Sometimes, TNO can be the initial manifestation of an underlying tumor or a relapse from a known neoplastic process. Multiple sclerosis, glioma, and infarct are the most commonly reported abnormalities in the brain stem that cause trigeminal neuropathy. Neurovascular compression, schwannomas, and meningiomas are common cisternal causes. Unilateral TNO could be secondary to focal lesions, while bilateral, symmetric presentations can be seen with connective tissue diseases or idiopathic etiology. TNO can cause facial pain in the distribution of one or more trigeminal nerve branches. The pain in trigeminal neuropathy is usually continuous, described as burning or squeezing, and is often accompanied by allodynia and cold hyperalgesia. While the symptoms of TNO can be concerning, it is most important to rule out potentially harmful underlying conditions such as malignancy, vascular malformation, and autoimmune diseases.

Trigeminal neuralgia should not be confused with trigeminal neuropathy. Trigeminal neuralgia is a paroxysmal, electric shock-like pain that is abrupt in onset and termination, without sensory loss or motor weakness. Classic trigeminal neuralgia can be due to neurovascular compression of CN V. Secondary trigeminal neuralgia is caused by various conditions like multiple sclerosis, vascular malformation, or autoimmune disease.

Anatomy of CN V

The trigeminal nerve is the largest cranial nerve and has both sensory and motor components with complex anatomy. The nerve course can be divided into four segments: brainstem, cisternal, Meckel's cave/cavernous sinus, and extracranial.

1) Sensory and motor nuclei

The brainstem contains the 1 motor and 3 sensory nuclei of the trigeminal nerve. The sensory nuclei include the mesencephalic nucleus, principal sensory nucleus, and spinal nucleus. Their functions are as follows:

1. Mesencephalic nucleus—in the midbrain. It receives and transmits proprioception from the orofacial region and is responsible for unconscious proprioception from muscles of mastication and other head and neck muscles. It also serves as an afferent limb of the jaw jerk reflex.

2. Principle sensory nucleus—in the upper pons. It is responsible for conscious proprioception, fine touch, vibration, and two-point discrimination.

3. Spinal nucleus—lower pons to cervical cord up to the C2-C4 level. It is composed of the oralis, interpolaris, and caudalis subnuclei. It is responsible for crude touch, pain, and temperature sensations. It also receives afferent fibers from cranial nerves VII, IX, and X.

4. Motor nucleus —upper pons on the lateral surface of the pontine tegmentum. It supplies the small motor root, which joins V3 (the mandibular division of the trigeminal nerve), bypassing the trigeminal ganglion.

2) Nerve entry and intracranial anatomy

1. The nerve root entry zone is the cisternal segment where the cranial nerve V enters the prepontine cistern. This area can be affected by vascular compression, infections, and neoplasms.

2. The nerve then travels through the prepontine cistern to Meckel's cave by an opening called Porus trigeminal or dural foramen.

3. Within the Meckel's cave (also called the trigeminal cave or Meckel cavern) lies the ganglion of the sensory branch, which is called the trigeminal, semilunar, or Gasserion ganglion. This ganglion consists of the nerve bodies of pseudounipolar neurons, which have one axon directed to the periphery (skin, tissue, or muscles) and one axon directed to the nuclei in the brainstem.

4. The trigeminal nerve then divides into three subdivisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves.

5. The ophthalmic (V1) and the maxillary (V2) divisions cross the cavernous sinus on its lateral wall, inferior to the abducens nerve. The ophthalmic nerve (V1) exits the skull via a superior orbital fissure. The maxillary (V2) nerve exits the skull via foramen rotundum. The mandibular (V3) nerve exits the skull via foramen ovale.

6. The motor root passes under the ganglion in Meckel's cave. Fibers of the motor root are distributed to the V3 division of the trigeminal nerve.

3) Trigeminal nerve divisions after the trigeminal ganglion

1. Ophthalmic (V1) is the smallest division and is a pure sensory nerve. It has 3 terminal branches (frontal, lacrimal, nasociliary). It supplies the eye (upper eyelid, conjunctiva, cornea), scalp, forehead, frontal sinus, ethmoid sinus, and dorsum of the nose. It also provides a parasympathetic supply to the lacrimal gland.

2. The maxillary nerve (V2) has 14 terminal branches, supplying the lower eyelid and its conjunctiva, cheeks, maxillary sinus, superior palate, upper lip, upper molar, incisor, canine teeth (and the associated gingiva). It also provides a parasympathetic supply to the lacrimal gland and mucous glands.

3. The mandibular nerve (V3) is the largest division and has 4 terminal branches: auriculotemporal nerve, buccal nerve, inferior alveolar nerve, and lingual nerve. It provides the only motor functions of CN V.

4) Trigeminal nerve function

1. Sensory supply includes most of the face from mid-cranium to the lower chin, including the eye and external ear, and the general sensation of anterior 2/3rds of the tongue.

2. Motor branches innervate four muscles of mastication (temporalis, master, medial pterygoid, lateral pterygoid). They also supply 4 other muscles—mylohyoid, anterior belly of digastric, tensor tympani, and tensor veli palatini muscles.

3. CN V also innervates parts of the sympathetic nervous system via V1 and the parasympathetic system via V1, V2, and V3, including the submandibular, sublingual, and parotid glands.