Sixth Nerve Palsies
What is the Anatomy of the Sixth Nerve?
The paired abducens nuclei are located in the dorsal lower portion of the pons, separated from the floor of the fourth ventricle by the genu of the facial nerve (facial colliculus). The nucleus contains motor neurons for the lateral rectus muscle and interneurons traveling via the medial longitudinal fasciculus (MLF) to the contralateral medial rectus subnucleus of the third nerve. The sixth nerve nucleus thus contains all the neurons responsible for horizontal conjugate gaze. The nerve fascicle leaves the nucleus and travels within the substance of the pontine tegmentum, adjacent to the medial lemniscus and the corticospinal tract. The sixth nerve leaves the brainstem in the horizontal sulcus between the pons and medulla (lateral to the corticospinal bundles). It enters the subarachnoid space, ascends along the base of the pons in the prepontine cistern, courses nearly vertically along the clivus, and travels over the petrous apex of the temporal bone where it is tethered at the petroclinoid (Grüber’s) ligament in Dorello’s canal beneath. It enters the substance of the cavernous sinus lateral to the internal carotid artery and medial to the ophthalmic division of the trigeminal nerve (VI) to enter the orbit via the superior orbital fissure. In their course from the pericarotid plexus to the ophthalmic branch of the trigeminal nerve, the pupil’s sympathetic fibers join the abducens nerve for a few millimeters.
Based on this topographic anatomy, sixth nerve palsies (SNPs) may be categorized as isolated or nonisolated. The criteria for the diagnosis of an isolated SNP are listed in Table 13–1. We define six types of SNP in Table 13–2. These types help to differentiate etiology and guide the management of SNP. The localization of nonisolated SNPs is outlined in Table 13–3.
Is the Nonisolated SNP Due to a Pontine (Lower Pons) Lesion?
Sixth nerve nuclear lesions cause a horizontal gaze palsy, rather than an isolated abduction deficit. An ipsilateral facial palsy may occur because of the close proximity of the facial and abducens nerve in the pons. Nuclear lesions are usually associated with other brainstem signs (e.g., hemiparesis, hemisensory loss, a central Horner’s syndrome). Likewise, lesions of the sixth nerve fascicle involve adjacent structures (e.g., cranial nerves V, VII, and VIII; cerebellar ataxia; a central Horner’s syndrome; or contralateral hemiplegia). Patients with a presumed nuclear or fascicular SNP should undergo neuroimaging (usually magnetic resonance imaging [MRI]) directed to the pons. The etiologies of nuclear or fascicular lesions in the pons are listed in Table 13–4.
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Ipsilateral abduction deficit Incomitant esodeviation that is typically worsened with gaze into the field of the weak lateral rectus muscle (may become comitant over time) Exclusion of Duane’s retraction syndrome, spasm of the near reflex, and other causes of abduction deficits that can mimic an SNP, and exclusion of patients with signs of the following: Orbital disease (e.g., Chemosis, proptosis, lid swelling, injection, and positive forced ductions) Myasthenia gravis (e.g., ptosis, Cogan’s lid twitch sign, orbicularis oculi weakness, muscle fatigue or variability) Multiple cranial nerve palsies (including bilateral SNP) or radiculopathy Brainstem signs (e.g., Horner’s syndrome, hemiplegia, cerebellar signs) Systemic, infectious, or inflammatory risk factors for an SNP (e.g., history of previous malignancy, giant cell arteritis, collagen vascular disease) Exclusion of patients with severe headache |
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Type 1: nonisolated SNP are considered nonisolated in the presence of the exclusionary conditions outlined in Table 13–1 Type 2: traumatic Isolated unilateral SNPs that have a clearly established temporal relationship to significant previous head trauma and do not progress are considered traumatic in origin; patients with SNP following minor head trauma are excluded. Type 3: congenital Patients born with SNP Type 4: vasculopathic Vasculopathic SNPs occur in patients older than age 55 or those with known vasculopathic risk factors (e.g., hypertension or diabetes) Type 5: nonvasculopathic Patients without vasculopathic risk factors defined above are considered to have nonvasculopathic SNP Type 6: progressive (nonisolated) or unresolved SNPs that worsen after the acute stage (more than 2 weeks) as defined by a significant increase in the measured ocular deviation or who develop new neurologic findings are considered progressive or nonisolated; patients without resolution in the measured horizontal deviation after 12 to 16 weeks are considered unresolved |
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Structure Involved |
Clinical Presentation |
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Nuclear lesions |
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Abducens nucleus |
Horizontal gaze palsy |
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Möbius syndrome (gaze palsy with facial diplegia) | |
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Duane’s retraction syndrome (gaze palsy with globe retraction and narrowing of palpebral fissure with adduction) | |
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Dorsolateral pons |
Ipsilateral gaze palsy, facial paresis, dysmetria; occasionally with contralateral hemiparesis (Foville syndrome) |
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Lesions of the abducens fascicle |
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Abducens fascicle |
Isolated CN VI palsy |
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Anterior paramedial pons |
Ipsilateral CN VI palsy, ipsilateral CN VII palsy, contralateral hemiparesis (Millard-Gubler) |
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Prepontine cistern |
May have contralateral hemiparesis |
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Lesion of abducens nerve (subarachnoid, petrous) |
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Petrous apex (Dorello’s canal) |
CN VI palsy, deafness, facial (especially retro-orbital) pain (Gradenigo) |
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Cavernous sinus |
Isolated CN VI palsy; CN VI palsy plus Horner’s syndrome; also may affect CN III, IV, VI |
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Superior orbital fissure syndrome |
CN VI palsy with variable affection of CN III, IV, VI; proptosis |
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Orbit |
CN VI palsy; visual loss; variable proptosis, chemosis, lid swelling |
Source: Modified from Brazis, 2001, with permission from Lippincott Williams & Wilkins.
Is the SNP Due to a Subarachnoid Space Lesion?
Lesions of the subarachnoid space may result in unilateral or bilateral SNP. This SNP is a nonlocalizing finding because any cause of increased intracranial pressure may result in an SNP (see Table 13–3). Patients with a subarachnoid space lesion should undergo neuroimaging directed to this location followed by a lumbar puncture (LP) as needed (class III–IV, level B).
Is the SNP the Result of a Lesion of the Petrous Apex?
Lesions of the petrous apex causing SNP are associated with other neurologic findings, including involvement of other cranial nerves (e.g., fifth, seventh, and eighth) or facial pain. Neuroimaging should be directed toward the petrous apex (MRI or computed tomography [CT] for bone involvement) (class III–IV, level B).
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Nuclear (horizontal gaze) Congenital (Carr, 1997) Möbius syndrome (Carr, 1997; Pedraza, 2000) Demyelinating Infarction or ischemia Neoplasm (pontine and cerebellar) Glioma Metastasis Histiocytosis X Trauma Wernicke-Korsakoff syndrome Fascicular Demyelination (Thömke, 1997, 1998) Infarction (King, 1995; Lopez, 1996; Thömke, 1998) Neoplasm (Balcar, 1999; Landolfi, 1998; Thömke, 1998) Trauma Hematoma (Thömke, 1998) Migraine (Lee, 2002) Subarachnoid Aneurysm or vascular abnormality (King, 1995) Persistent primitive trigeminal artery Posterior inferior cerebellar aneurysm Vertebral artery, including elongated vessel (Narai, 2000; Ohtsuka, 1996) Carcinomatous or leukemic meningitis (Wolfe, 1994) Chiari malformation or basilar impression (Chavis, 1998; Hirose, 1998; King, 1995) Following procedures Cervical traction Lumbar puncture (Thömke, 2000) Myelography (Bell, 1990) Postvaccination Radiculography (Dinakaran, 1995; Lloyd, 1995) Shunting for hydrocephalus Spinal or epidural anesthesia (De Veuster, 1994) Intrathecal glucocorticoid injection (Dumont, 1998) Inflammatory Retropharyngeal space inflammation (Fanous, 1992) Necrotizing vasculitis Sarcoidosis (Frohman, 2001; Sachs, 1990) Systemic lupus erythematosus Wegener’s granulomatosis Fisher’s syndrome (Nagaoka, 1996) Infectious Lyme disease (Lesser, 1990; Mastrianni, 1994) Syphilis (Slavin, 1992; Stepper, 1998) Tuberculosis Cryptococcal meningitis Cysticercosis (Keane, 2000) HIV-CMV encephalitis Neoplasm (Hashimoto, 1998) Abducens nerve tumor (Ichimi, 1997; Okada, 1997; Tung, 1991) Cerebellopontine angle tumor Clivus tumor (e.g., chordoma, chondrosarcoma, plasmacytoma) (Balcar, 1999; Forsyth, 1993; Harada, 1997; Mekari-Sabbagh, 2001; Movsas, 2000; Volpe, 1993b) Leukemia (Averbuch-Heller, 1994) Metastatic (O’Boyle, 1992) Skull base tumor (Keane, 2000; Volpe, 1993a) Nasopharyngeal carcinoma (Keane, 2000) Trigeminal nerve tumor Capillary hemangioma of Meckel’s cave (Brazis, 2000) Nonlocalizing sign of increased intracranial pressure (Aroichane, 1995) Pseudotumor cerebri (Krishna, 1998; Patton, 2000) Meningitis or meningeal irritation of any type Intracranial tumor Venous sinus thrombosis (Biousse, 1999) Spontaneous cerebrospinal fluid leak with intracranial hypotension (Apte, 1999; Case Records of the Massachusetts General Hospital, 1998; Ferrante, 1998; Horton, 1994; Mokri, 1997; O’Carroll, 1999; Schievink, 1996) Trauma (excluding surgical) (Hollis, 1997; Holmes, 1998; Lepore, 1995) Epidural hematoma of clivus (Mizushima, 1998) Petrous apex Neoplasm (e.g., nasopharyngeal carcinoma) Infection Complicated otitis media (Homer, 1996) Mastoiditis (Gradenigo’s syndrome) (Dave, 1997) Thrombosis of inferior petrosal or transverse/sigmoid sinus (Kuehnen, 1998) Trauma (Antoniades, 1993; Mutyala, 1996) Basilar skull fracture Inflammatory Cavernous sinus (Keane, 1996) Cavernous sinus thrombosis (Kriss, 1996) Cavernous sinus fistula (Eggenberger, 2000; King, 1995; Lee, 1998; Uehara, 1998) Superior ophthalmic vein thrombosis (Polito, 1996) Neoplasm (Eisenberg, 1999) Nasopharyngeal carcinoma (Keane, 1996) Pituitary adenoma Plasmacytoma (Bachmeyer, 1997) Lymphoma (Keane, 1996; Liu, 1993; Roman-Goldstein, 1998) Hodgkin’s disease (Kasner, 1996) Hemangioma (Brazis, 2000; Lee, 1995) Hemangioendothelioma (Phookan, 1998) Meningioma (Kawase, 1996) Rhabdomyosarcoma (Arita, 2001) Sixth nerve tumors Sphenoid sinus tumors Skull base tumors Squamous cell cancer of pterygopalatine fossa Subarachnoid diverticulum Sphenoid sinus mucocele (Muneer, 1997) Ischemia Inflammatory or infectious Herpes zoster (Chang-Godinich, 1997; Mansour, 1997; Smith, 1993) Actinomycoses (Holland, 1998) Tolosa-Hunt syndrome Internal carotid artery diseases Aneurysm (FitzSimon, 1995; Hahn, 2000; Keane, 1996; Silva, 1999) Dissection (Kerty, 1999; Lemesle, 1998) Dolichoectasia (Blumenthal, 1997; Neugebauer, 2001) Balloon test occlusion (Lopes, 1998) Cisplatin infusion (Alderson, 1996; Wu, 1997) Post–radiofrequency rhizotomy for trigeminal neuralgia (Harrigan, 1998) Orbital lesions Neoplastic (orbital schwannoma) Inflammation (orbital inflammatory pseudotumor) Infectious Traumatic (Lazow, 1995) Localization uncertain  Stay updated, free articles. Join our Telegram channel
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