Perspective

The 12 cranial nerves provide motor and sensory innervation to the head and neck. Some nerves serve purely motor functions (cranial nerves III, IV, VI, XI, and XII), some serve purely sensory functions (cranial nerves I, II, and VIII), and the remainder serve mixed motor and sensory functions (cranial nerves V, VII, IX, and X).

In addition to somatic and visceral sensory components, the cranial nerves provide the special sensory functions of sight, smell, hearing, taste, and balance. Understanding the functions of individual cranial nerves aids in recognition of patterns of the clinical syndromes classically associated with disorders of specific cranial nerves.

Epidemiology

Cranial neuropathies are a heterogeneous group of disorders with a variety of causes. Trauma is a common cause, and diabetes and hypertension are common comorbid conditions. Cranial nerves I, VI, and VII are the most frequently affected after minor head trauma.¹ Trigeminal neuralgia is a common cause of facial pain that affects approximately 4.5 per 100,000 individuals; women are affected twice as often as men, and it is more common in those older than 60 years.² Trigeminal neuralgia can be severely debilitating and has been termed the “suicide disease.”³ Bell palsy is the most common cause of acute facial paralysis worldwide. The peak age at incidence has been reported to be between 15 and 45 years,⁴ but other investigators have noted an increased incidence in individuals older than 70.^5,6 Pregnant women and patients with diabetes have an associated increased incidence of the disease. A familial association of Bell palsy is noted in 4% of cases,⁴ and it can cause both significant psychologic and physical morbidity.

Pathophysiology

Cranial Nerve III (Oculomotor Nerve)

Anatomy

The oculomotor nerve is a pure motor nerve that works in conjunction with cranial nerves IV and VI to coordinate extraocular movements. The oculomotor nerve controls the superior rectus (globe elevator), medial rectus (globe adductor), inferior rectus (globe depressor), and inferior oblique (globe elevator) muscles. It also controls the levator palpebrae superioris muscle (upper eyelid elevator) and the intrinsic visceral motor function of the sphincter pupillae muscles and the ciliary muscles, which perform pupillary constriction and accommodation, respectively.

Presenting Signs and Symptoms

The patient typically complains of double vision or difficulty seeing out of the affected eye. There may be mild photophobia in bright light. The patient may also complain of an inability to raise the eyelid (ptosis).

Cranial nerve III palsy is more common in patients older than 60 years and in those with diabetes or hypertension (Fig. 95.1).

Fig. 95.1 This 60-year-old man had diabetes mellitus, hypertension, coronary artery disease, chronic renal failure, and multiple myeloma. He sought medical care because of double vision (he described the images as “a little side by side but mostly up and down”), diplopia, ptosis, and papillary sparing. Findings on laboratory tests, magnetic resonance imaging, and magnetic resonance angiography were negative. The patient was evaluated by a neurologist and an ophthalmologist, and diabetic cranial nerve palsy was ultimately diagnosed. He was given an eye patch and scheduled for ophthalmologic follow-up.

Patients with herniation syndromes will have a history of trauma (Fig. 95.2), tumor, or other neurologic findings.⁷

Fig. 95.2 Ptosis and mydriasis suggest a cranial nerve III palsy. The appearance of these signs after a crush injury indicates that a skull fracture is impinging on the nerve canal.

(Reproduced with permission from Baker C, Cannon J. Images in clinical medicine. Traumatic cranial nerve palsy. N Engl J Med 2005;353:1955.)

Pain associated with unilateral mydriasis should alert the emergency physician (EP) to look for an aneurysm involving the terminal internal carotid artery. Computed tomographic angiography is more reliable than magnetic resonance angiography.⁸

Patients with an abscess or cavernous sinus thrombosis may have headaches, altered mental status, and seizures. This diagnosis should be considered in patients with signs and symptoms in the contralateral eye, previous sinus or midface infection, fever, chemosis, eyelid or periorbital edema, and exophthalmos. Extension of internal carotid artery dissection intracranially into the cavernous sinus can result in third, fourth, and sixth cranial nerve palsies.⁹

Treatment

Treatment is dependent on the cause. CT should be performed to exclude a herniating mass. Admission for magnetic resonance imaging (MRI) and neurology or neurosurgical consultation is indicated for acute-onset deficits.

Cranial Nerve IV (Trochlear Nerve)

Anatomy

The trochlear nerve innervates the superior oblique muscle of the eye and causes inward rotation and downward and lateral movement of the globe. It is the smallest cranial nerve but has the longest intracranial course.

Presenting Signs and Symptoms

Patients with a fourth cranial nerve palsy have double vision exacerbated by looking downward. The classic complaint is difficulty going down stairs. Most commonly, a history of trauma is reported. On physical examination the patient may unconsciously tilt the head away from the affected side (Fig. 95.3). Etiologic mechanisms are similar to those for the third cranial nerve and include inflammatory processes, trauma, and vascular causes.¹⁰

Fig. 95.3 A, Normal eye rotation. When the head tilts, both eyes rotate in the opposite direction. B, Cranial nerve IV palsy (right eye). The right eye is extorted and slightly elevated, which is causing double vision. The patient compensates by tilting the head to the left.

Treatment

Treatment of isolated fourth nerve palsy is generally conservative, and the patient should be referred to neurology or neurosurgery as appropriate.¹⁰ CT, MRI, and neurology consultation are warranted if multiple cranial nerves are involved.

Cranial Nerve V (Trigeminal Nerve)

Anatomy

The trigeminal nerve is a mixed motor and sensory nerve. It provides motor innervation to the muscles of mastication, as well as sensation from the face, scalp, conjunctiva, globe, mucous membranes of the sinuses, tongue, teeth, and part of the external tympanic membrane.

The trigeminal sensory ganglion is located in the middle cranial fossa and branches into three divisions: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3).

Presenting Signs and Symptoms

Patients with trigeminal nerve dysfunction have either sensory or motor deficits. Sensory dysfunctions include paroxysmal pain, paresthesias (abnormal sensations such as burning, pricking, tickling, or tingling), dysesthesias (disagreeable, unpleasant, or painful sensations produced by ordinary stimuli), and anesthesia (loss of sensation). The motor dysfunction is usually described as difficulty chewing and difficulty swallowing.

Peripheral lesions cause loss of sensation or pain in only one division. Positive findings in two or more divisions (e.g., loss of light touch in one division and loss of sensitivity to pain, temperature, or pinprick in another division) should raise suspicion for a central cause.

The presence of associated cranial nerve deficits (III, IV, IV, or any combination of these nerves) suggests cavernous sinus involvement. In the setting of trauma, if a bruit over the orbit can be detected, a carotid–cavernous sinus fistula may be present. Associated involvement of cranial nerve VII or VIII or gait ataxia should raise suspicion for a cerebellopontine angle or lateral pontine tumor (Table 95.1).

Table 95.1 Clinicoanatomic Correlation of Localization of Lesions of Cranial Nerve V

Associated Horner syndrome may indicate a cervical or lateral brainstem lesion.

The main categories of trigeminal nerve dysfunction are trigeminal neuralgia and trigeminal neuropathy. A sudden onset of symptoms should raise suspicion for a vascular, traumatic, or demyelinating cause, whereas a more indolent course suggests tumor or inflammation (Table 95.2).

Table 95.2 Selected Specific Causes Associated with Trigeminal Nerve Disorders

ETIOLOGIC CATEGORY	SELECTED SPECIFIC CAUSES
Structural Disorders
Developmental	Brainstem vascular loop, syringobulbia
Degenerative and compressive	Paget disease
Hereditary and Degenerative Disorders
Chromosomal abnormalities, neurocutaneous disorders	Hereditary sensorimotor neuropathy type I, neurofibromatosis (schwannoma)
Degenerative motor, sensory, and autonomic disorders	Amyotrophic lateral sclerosis
Acquired Metabolic and Nutritional Disorders
Endogenous metabolic disorders	Diabetes
Exogenous disorders (toxins, illicit drugs)	Trichloroethylene, trichloroacetic acid
Nutritional deficiencies, syndromes associated with alcoholism	Thiamine, folate, vitamin B12, pyridoxine, pantothenic acid, vitamin A deficiencies
Infectious Disorders
Viral infections	Herpes zoster, unknown
Nonviral infections	Bacteria, tuberculous meningitis, brain abscess, Gradenigo syndrome, leprosy, cavernous sinus thrombosis
HIV infection, AIDS	Opportunistic infection; abscess, herpes zosterStroke, hemorrhage, aneurysm
Neurovascular Disorders
Neoplastic Disorders
Primary neurologic tumors	Glial tumors, meningioma, schwannoma
Metastatic neoplasms, paraneoplastic syndromes	Lung, breast; lymphoma, carcinomatous meningitis
Demyelinating Disorders
Central nervous system disorders	Multiple sclerosis, acute demyelinating encephalomyelitis
Peripheral nervous system disorders	Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathyTolosa-Hunt syndrome, sarcoidosis, lupus, orbital pseudotumor
Autoimmune and Inflammatory Disorders
Traumatic Disorders	Carotid-cavernous fistula, cavernous sinus thrombosis, maxillary/mandibular injury
Epilepsy	Focal seizures
Headache and Facial Pain	Raeder neuralgia, cluster headache
Drug-Induced and Iatrogenic Neurologic Disorders	Orbital, facial, dental surgery

AIDS, Acquired immunodeficiency syndrome; HIV, human immunodeficiency virus.

From Goetz CG, editor. Textbook of clinical neurology. 2nd ed. Philadelphia: Saunders; 2003.

Trigeminal Neuropathy

Causes include compression by an extrinsic mass, trauma, and vascular, inflammatory, or demyelinating disorders.

Symptoms include neuralgia or paresthesia (or both) involving half of the face. Unlike trigeminal neuralgia, the pain with trigeminal neuropathy is more constant. Loss of the corneal reflex is evident. The patient’s mouth may become more oval and oblique in appearance, and because of loss of masseter muscle strength, the chin may be deviated toward the affected side.

Until proved otherwise, neuropathies of cranial nerve V, the chin (numb chin; V3), and the suborbital region (numb cheek) should be presumed to be due to malignancies.¹¹

Tic Douloureux

The term tic douloureux was coined by Nicolaus André, a French surgeon, in 1756. Its mechanism is probably compression of the trigeminal nerve root within millimeters of entry into the pons.¹²

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