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.