• 
  

  THE TRIGEMINAL NERVE

  
  
  
  
  • 
    

    Neuroanatomy

    
    
  • 
    

    Clinical testing

    
    
  • 
    

    Symptoms of trigeminal nerve dysfunction

  
  
• 
  

  FACIAL NUMBNESS

  
  
• 
  

  CAUSES OF FACIAL PAIN AND HEADACHE

  
  
  
  
  • 
    

    Migraine

    
    
  • 
    

    Tension-type headache

    
    
  • 
    

    Cluster headache

    
    
  • 
    

    Ocular causes of pain

    
    
  • 
    

    Orbital and cavernous sinus disease

    
    
  • 
    

    Giant cell arteritis

    
    
  • 
    

    Intracranial disorders

    
    
  • 
    

    Carotid dissection

    
    
  • 
    

    Neurogenic pain

    
    
  • 
    

    Sinus and dental disease

    
    
  • 
    

    Pain of unknown origin

  
  
• 
  

  PHOTOPHOBIA

  
  
• 
  

  KEY POINTS

Given the ubiquitous (but mistaken) belief that headaches are usually caused by eye problems, ophthalmologists will see many patients with headache as the chief complaint. Therefore, the ophthalmologist needs to be familiar with common primary headache syndromes. The neurologist, on the other hand, has broad experience in the area of primary headaches, but needs to be well-versed in ocular, orbital, and other secondary causes of headache and face pain.

Pain and sensation from the eye and face, as well as intracranial structures, are mediated by the trigeminal nerve, which is directly or indirectly involved in all pain syndromes of neuro-ophthalmic interest. After exploring trigeminal neuroanatomy and its clinical implications, clinical conditions causing facial numbness, headache, and other head and ocular pain are discussed.

NEUROANATOMY

The trigeminal nerve (cranial nerve [CN] V) is a mixed cranial nerve: In addition to its major sensory function, CN V provides motor innervation to the muscles of mastication. At its origin CN V does not contain autonomic fibers; however, parasympathetic innervation to the lacrimal gland and sympathetic fibers to the pupillary dilator travel briefly with peripheral segments of the nerve.

The trigeminal system also innervates cranial blood vessels (the trigeminovascular system), and in this role has the unusual ability to act as an effector as well as a sensor. Trigeminally innervated cranial blood vessels dilate after a nociceptive stimulus by way of chemical mediators released from the same sensory terminals that were activated by the stimulus; in this circumstance trigeminal nerves are serving as both the afferent and efferent limbs of a reflex. It is referred to as the trigeminovascular reflex and is believed to play an important role in the pathogenesis of primary headache syndromes such as migraine.

Nuclear Complex

The sensory portion of the trigeminal nucleus is markedly elongated and stretches from the midbrain to the upper cervical segments in the spinal cord where it blends with the root entry zone of the spinal cord (Figure 13–1). The trigeminal nucleus is a major sensory integration area and receives input from more than CN V; it has afferent connections with the somatosensory cortex above, with spinal cord afferents below, from the reticular formation and red nucleus, in addition to sensory input from other cranial nerves (CNs VII, IX, and X).

In the mesencephalon, the upper aspect of the nuclear complex (rostral or mesencephalic nucleus) receives proprioceptive and deep sensation afferents from the tendons and muscles of mastication. The pons contains the section known as the principal (or chief) sensory nucleus, serving light touch from skin and mucous membranes of the face. The lower extension of the nuclear complex is the spinal nucleus that receives pain and temperature afferents. The spinal nucleus is divided into segments that correspond to dermatomes that are concentric around the mouth (see Figure 13–1A). This central representation of the face is different from the trigeminal nerve’s peripheral distribution that divides the face into three longitudinal regions (see Figure 13–1B). Thus, the distribution of facial numbness or paresthesia is helpful in determining whether sensory deficits are central or peripheral in origin. For example, brainstem ischemia or demyelination is likely to cause concentric perioral numbness or paresthesia, whereas lesions affecting the peripheral nerve cause a band of numbness in the trigeminal nerve’s cutaneous distribution.

Fibers from the chief sensory nucleus travel in the contralateral medial lemniscus to the ventral posteromedial nucleus of the thalamus. A smaller uncrossed projection also exists, as well as more diffuse pathways through the reticular formation.

The motor nucleus of CN V is medial to the chief sensory nucleus in the pons. Axons from the motor nucleus join the sensory fibers to form CN V and travel with the mandibular division of the nerve to innervate the muscles of mastication.

Peripheral Nerve

The trigeminal nerve enters the brainstem at the level of the pons from the gasserian ganglion. The gasserian (or trigeminal) ganglion is the point at which the ophthalmic, maxillary, and mandibular divisions of CN V converge (Figure 13–2). This structure relays sensory information from the three major divisions of CN V to the trigeminal nuclear complex. The gasserian ganglion is located in Mekel cave, a concavity in the base of the temporal bone, inferolateral to the sella turcica.

The ophthalmic division (V₁) extends through the cavernous sinus and the superior orbital fissure into the orbit, receiving sensory information from three branches: the frontal, lacrimal, and nasociliary branches (see Figure 13–2). The ophthalmic division and its branches carry sensation from the eye, upper eyelid, and forehead. The branches of the nasociliary nerve receive sensation from the eye, the skin on the tip and side of the nose, and the medial canthus. Vesicular eruption from herpes zoster ophthalmicus in these cutaneous areas is ominous because involvement of the nasociliary branch represents a serious threat to the eye.

A tentorial-dural branch joins the ophthalmic division in the cavernous sinus, receiving sensory innervation from much of the intracranial dura, arteries at the skull base, and major venous structures. It is understandable why pain from intracranial diseases is commonly referred to the eye and orbit, given the common sensory pathway through the ophthalmic division.

The maxillary division (V₂) passes through the inferior cavernous sinus for a variable extent, and extends through the foramen rotundum and then into the inferior orbital fissure, traveling in the infraorbital canal along the floor of the orbit (see Figure 13–2). The nerve and its branches receive sensory innervation from parts of the mouth, nasopharynx, teeth, and maxillary sinus, as well as the lower eyelid and cutaneous areas of the cheek. Blowout fractures of the orbital floor commonly damage the infraorbital nerve, resulting in numbness of the cheek and other areas supplied by this nerve.

The mandibular division (V₃) contains both sensory and motor components, exiting the skull ventrally through the foramen ovale in the floor of the middle cranial fossa (see Figure 13–2). Motor branches supply the muscles of mastication (Table 13–1). The sensory distribution includes the area of the mandible (but not the angle of the mandible), lower lip, tongue, and external ear.

CLINICAL TESTING

Testing pain and light touch over the face is best performed by having the patient compare one side of the face to the other, as discussed in Chapter 7. The corneal blink reflex consists of an involuntary bilateral blink in response to light touch to the cornea, and thus offers a more objective assessment of sensory function. The ophthalmic division of the trigeminal nerve provides the afferent limb. The efferent response is activation of both orbicularis oculi via the facial nerves. The test is performed by lightly touching the unanesthetized conjunctiva and corneal limbus of each eye in turn with a clean cotton-tip or tissue wisp, and by comparing the magnitude of the bilateral blink response (see Box 7–4).

The motor component of the trigeminal nerve is assessed by palpating and comparing the bulk of the masseter muscles on each side as the patient grits his or her teeth. Lateral pterygoid function is evaluated by having the patient push his or her jaw laterally against resistance, comparing each side. Weakness to one side suggests contralateral pterygoid weakness.

SYMPTOMS OF TRIGEMINAL NERVE DYSFUNCTION

Numbness and paresthesias are common symptoms of disorders that affect the trigeminal nerve or nucleus. Pain can also be caused by disorders of CN V, such as trigeminal neuralgia or herpes zoster ophthalmicus.

Facial numbness and paresthesias caused by disorders affecting the brain are usually accompanied by other significant neurological deficits. Thalamic lesions cause hemibody and hemifacial numbness on the same side, while medullary lesions such as Wallenberg syndrome cause a crossed sensory loss, with facial numbness on the side opposite the body numbness. As discussed above and in Figure 13–1, the distribution of numbness and paresthesias on the face will show whether the lesion is central or peripheral.

The location of a lesion affecting the trigeminal nerve may be evident by concomitant signs and symptoms, such as ocular motor palsies from cavernous sinus or orbital apex lesions. Table 13–2 lists potential causes of decreased sensation from trigeminal nerve disorders by division.

Decreased corneal sensation from lesions affecting the ophthalmic division of the trigeminal nerve can cause a neurotrophic cornea, a keratopathy that can progress to corneal ulceration, and ultimately to a blind, painful eye. The trigeminal nerve provides the afferent limb of the corneal blink reflex, and loss of this reflex can lead to corneal injury. In addition, trigeminal innervation is important in maintaining the tear film and corneal integrity. A neurotrophic cornea is notoriously difficult to manage, often requiring tarsorrhaphy, as treatment with topical lubricants alone is often not sufficient. This condition can occur in the setting of cerebellopontine angle surgery, or as a result of surgery to relieve trigeminal-mediated pain (such as a trigeminal rhizotomy for trigeminal neuralgia).

Numb chin syndrome (mental neuropathy) presents as isolated numbness of the chin and lower lip that respects the midline. This condition is caused by boney lesions affecting the mental foramen, which is the passage for terminal branches of the mandibular division of CN V. Numb chin syndrome is rare but deserves mention, because this syndrome is frequently caused by mass lesions, such as metastatic cancer.

Headaches are classified as primary (migraine, tension-type, and cluster) or secondary (eg, from ocular disease). The clues to a headache’s origin may be revealed during a neuro-ophthalmic history and examination, even when the headache is not secondary to eye and orbit disease. A working knowledge of both primary and secondary headache syndromes is necessary to develop a differential diagnosis and make appropriate referrals when indicated. The history is particularly important when pain is the patient’s chief complaint, because in many cases few physical findings are present (Table 13–3).