CRANIAL NERVE III PALSY
Third nerve palsy results from damage to the oculomotor nerve anywhere in its course from the nucleus in the dorsal mesencephalon, its fascicles in the brainstem parenchyma, the nerve root in subarachnoid space, or in the cavernous sinus or posterior orbit. Damage to the third nerve nucleus results in an ipsilateral third nerve palsy with contralateral superior rectus under action and bilateral ptosis. Damage to the third nerve fascicles results in an ipsilateral third nerve palsy with contralateral hemiparesis (Weber's syndrome), contralateral intention tremor (Benedikt's syndrome), or ipsilateral cerebellar ataxia (Nothnagel's syndrome). Vascular infarct, metastatic disease and demyelinization are the common causes of brainstem involvement. Damage to the third nerve within the subarachnoid space produces an isolated third nerve palsy. The main causes are compression of the nerve by an expanding aneurysm of the posterior communicating artery or the basilar artery, and ischemic vasculopathy. There will always be pain in aneurysmal compression and pupillary involvement is typical, though there have been infrequent cases of aneurysmal compression that did not initially affect pupillary function. In ischemic vascular nerve third palsies, pain is frequent and the pupil is typically normal and reactive. Damage to the third nerve in the cavernous sinus, superior orbital fissure, or posterior orbit is unlikely to present as third nerve palsy due to the confluence of other structures in these areas. Cavernous sinus involvement may also include pareses of cranial nerves IV, VI and V-1, and an ipsilateral Horner's syndrome. The most common causes of damage in these areas include metastatic disease, inflammation, herpes zoster, carotid artery aneurysm, pituitary adenoma and apoplexy, and sphenoid wing meningioma.

MANAGEMENT
In complicated third nerve palsies where other neural structures are involved, have the patient undergo an MRI. In isolated third nerve palsies with no pupillary involvement where the patient is over 50, MRI scanning, an ischemic vascular evaluation, and daily pupil evaluation is indicated.

CRANIAL NERVE IV PALSY
The fourth cranial nerve nucleus is located in the dorsal mesencephalon. From here, the nerve fibers then decussate and exit the brain stem dorsally into the subarachnoid space. The nerve then courses around the brain to enter the cavernous sinus, superior orbital fissure, orbit, and innervate the superior oblique muscle. Damage to the fourth nerve nucleus or its fascicles within the brain stem will give a contralateral fourth nerve palsy, along with the associated signs of light-near dissociated pupils, retraction nystagmus, up-gaze palsy, Horner's syndrome, and/or internuclear ophthalmoplegia. Bilateral fourth nerve palsies are possible as well. The main causes of damage to the fourth nerve in this area are hemorrhage, infarction, trauma, hydrocephalus and demyelinization. The fourth nerve is especially prone to trauma as it exits the brain stem and courses through the subarachnoid space. In contrast to third nerve palsies within subarachnoid space, fourth nerve palsies are rarely due to aneurysm. The most common causes of damage to the fourth nerve in this region are trauma and ischemic vasculopathy. The most likely result from damage within subarachnoid space is an isolated fourth nerve palsy. Due to the large number of other neural structures that accompany the fourth nerve as it travels through the cavernous sinus and superior orbital fissure, it is unlikely that the patient will exhibit an isolated fourth nerve palsy due to damage within these areas. More likely, there will be an associated palsy of cranial nerves III and VI. Common causes of damage to the fourth nerve in these areas are herpes zoster, inflammation of the cavernous sinus or posterior orbit, meningioma, metastatic disease, pituitary adenoma, and carotid cavernous fistula. Trauma to the head or orbit can cause damage to the trochlea, resulting in superior oblique muscle dysfunction.

MANAGEMENT
A fourth nerve palsy often presents suddenly, but may additionally result from decompensation of a longstanding palsy. In order to differentiate these two types of palsies, examine old photographs of the patient. A patient with a decompensated longstanding palsy will present with a compensatory head tilt in old photos. Further, patients with decompensated longstanding fourth nerve palsies will have an exaggerated vertical fusional ability. Longstanding fourth nerve palsies typically are benign and no further management is necessary. In the case of complicated fourth nerve palsies, (i.e., those that present with other concurrent neurological dysfunction), the patient should undergo neuroradiological studies dictated by the accompanying signs and symptoms. In the case of isolated fourth nerve palsies caused by recent trauma, the patient should undergo an MRI or CT scan of the head to dismiss the possibility of a concurrent subarachnoid hemorrhage. If the fourth nerve palsy is not associated with recent trauma, investigate for a history of past trauma. If the fourth nerve palsy is due to previous trauma and has recently decompensated, you can manage the diplopia with vertical prisms. If the patient is elderly and has a fourth nerve palsy of recent origin, perform an ischemic vascular evaluation to search for diabetes and hypertension. If the palsy is caused by vascular infarct, it will spontaneously resolve over a period of three to six months and the patient will not require further management beyond periodic observation and either temporary occlusion or press-on prism therapy.

CRANIAL NERVE VI PALSY
Cranial nerve VI arises in the pons, in close association with the facial nerve and paramedian pontine reticular formation (PPRF). Due to this arrangement, damage to the sixth nerve within the brain stem will produce a sixth nerve palsy as well as a facial nerve palsy or an internuclear ophthalmoplegia. Associated findings may also include leg paralysis with sixth nerve palsy (Raymond's syndrome), or leg paralysis, facial paralysis and sixth nerve palsy (Millard-Gubler syndrome). These additional findings identify the location of damage as the pons, where ischemic infarct, tumor and demyelinization are the common causes. The sixth nerve travels through the subarachnoid space where it ascends the clivus and enters the cavernous sinus. Within the subarachnoid space, the sixth nerve may be stretched against the clivus as the brain stem herniates through the foramen magnum due to increased intracranial pressure. This will give a bilateral sixth nerve palsy (which is often intermittent) and papilledema. As the sixth nerve passes over the petrous apex of the temporal bone, damage here can result in a sixth nerve palsy, facial pain and hearing loss. This occurs due to inflammation of the temporal bone (Gradenigo's syndrome) or nasopharyngeal carcinoma. Within the cavernous sinus, the sixth nerve is joined by the oculosympathetic nerves, and cranial nerves III, IV and V-1. Damage here will yield a sixth nerve palsy and Horner's syndrome, as well as a concurrent CN III and IV palsy. The etiology may be aneurysm, meningioma, pituitary adenoma, inflammation, or fistula. The sixth nerve is also vulnerable to ischemic infarct from diabetes and hypertension; this remains a prime cause of isolated sixth nerve palsy.

MANAGEMENT
A sixth nerve palsy combined with any of the above mentioned neurological signs indicates a need for MRI of the appropriate area. In children, sixth nerve palsy often occurs from a presumed viral cause and has an excellent prognosis. However, if the palsy does not recover, or worsens over several weeks, the child should be examined for a pontine glioma. In the adult under 50 years, obtain MRI studies of the brain. Adult over 50 with an isolated sixth nerve palsy require a workup for ischemic vascular diseases such as diabetes and hypertension. If the patient is over the age of 65 years, order an erythrocyte sedimentation rate (ESR) to rule out giant cell arteritis. If no etiology is discovered on MRI or hematology studies, monitor the patient monthly for several months until resolution (or until other signs develop which would indicate an etiology). The vast majority of CN VI palsies due to ischemic vasculopathy (or idiopathic etiology) will resolve without treatment in three to six months. Fresnel prism correction or unilateral occlusion will temporarily alleviate the diplopia.

CRANIAL NERVE VII (FACIAL NERVE) PALSY
The muscles that close the eyes and wrinkle the forehead are bilaterally innervated. A unilateral lesion in the cortex or supranuclear pathway spares eyelid closure and forehead wrinkling but results in contralateral paralysis of the lower face. Since the area of the cortex associated with facial muscle function lies near the motor representation of the hand and tongue, weakness of the thumb, fingers and tongue ipsilateral to the facial palsy is not uncommon. The facial nucleus contains four separate cell groups that innervate specific muscle groups. Lesions of the fibers of the superior salivatory and lacrimal nuclei (parasympathetic preganglionic fibers supplying the sublingual, submandibular and lacrimal glands) include temporal bone fractures and infections, schwannomas, neuromas (cerebellopontine angle tumors) and vascular compression, producing deficits in hearing, balance, tear production and salivatory flow. Lesions that involve the ganglion include geniculate ganglionitis (Ramsey-Hunt syndrome: zoster oticus). Lesions such as acoustic neuroma that also involve cranial nerve VIII can impair hearing, facial nerve function and produce corneal hypoesthesia (CN V). Lesions of the zygomatic and lacrimal nerves impair reflex tear secretion. Middle cranial fossa disease is indicated when defective tear production accompanies CN V (muscles of mastication) or CN VI palsy. Lesions of the facial nerve disable the ability to dampen sound, producing hyperacusis. Lesions to sensory afferent fibers that transmit taste (fibers that also innervate the salivary glands) cause an interruption in salivatory flow and an inability to sense taste from the anterior two-thirds of the tongue. The portion of the facial nerve that contains the motor fibers that innervate the muscles of facial expression exits the stylomastoid foramen and enters the substance of the parotid gland before distribution. Therefore, investigate lesions of the parotid gland also as part of the work up. Lesions that occur within the cortical, extrapyramidal or brainstem levels are known as central lesions. Lesions outside the brain are referred to as peripheral. The common causes of peripheral CN VII palsy include cerebellopontine angle tumor (7 percent), trauma (21 percent), otitis media, herpes zoster oticus (Ramsey-Hunt syndrome), Lyme disease, sarcoidosis, parotid neoplasm, syphilis, diabetes mellitus, pregnancy and HIV.

MANAGEMENT
First obtain a complete history. Perform a cursory evaluation of the 12 cranial nerves as well as a comprehensive ocular examination with dilated fundus and optic nerve evaluation. Pay close attention to the affected eyelid's posture, corneal wetting (tear break up time), blink posture, tear quality (sodium fluorescein staining) and tear quantity (Schirmer tear testing). In cases where diagnosis is questionable, ask the patient to close both eyes while you try to open the lid. If one lid is significantly easier to open than the other, suspect CN VII palsy. You can manage exposure keratopathy with ocular lubricating drops and ointments. Moisture chamber patches (e.g. Guibora eye patch) or eyelid taping are also possible solutions. Moisture chamber shields can be attached to spectacle temples to create a moist ocular environment and lessen tear evaporation. Since idiopathic facial nerve palsy is a diagnosis of exclusion, order laboratory testing (Lyme titer, rheumatoid factor, erythrocyte sedimentation rate, antinuclear antibody, echocardiogram, fluorescent treponemal antibody absorption test, HIV titer, chest X-ray), lumbar puncture (in patients with suspected neoplasm), CT and MRI and/or appropriate referrals (otolaryngology, neurology, neurosurgery).