CAPÍTULO V RESULTADOS
6.1. Recomendaciones
The abducens nucleus is located in the lower pons at the level of the facial collicu-lus. Like other somatic motor nuclei, the abducens nucleus is situated close to the midline. The nucleus is composed of two kinds of cells: lower motor neurons, whose axons constitute the abducens nerve, and internuclear neurons, whose axons ascend via the medial longitudinal fasciculus (MLF) to project to the medial rectus lower motor neurons in the contralateral oculomotor nucleus at the level of the superior colliculus (Fig. VI–5 inset). These are important for coordination of the lateral gaze (see Chapter 13). Axons of the lower motor neurons in the abducens nucleus course ventrally through the pons to emerge from the ventral surface of the brain stem at the pontomedullary junction (see Figs. VI–1 and VI–5).
FIGURE VI–4 Apex of the right orbit illustrating the tendinous ring and the somatic motor com-ponent of cranial nerve VI.
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CASE HISTORY GUIDING QUESTIONS
1. Why was Grace having nosebleeds?
2. Why did Grace have double vision only when she was looking to the left?
3. How did the doctor know that the problem was within the cavernous sinus?
4. Why did Grace not have a numb chin?
5. Where along the course of CN VI could damage occur?
FIGURE VI–5 Abducens nuclei in the brain stem.
A. Branchial motor axons from the facial nucleus loop over the abducens nucleus thereby creating an eleva-tion (bump) in the floor of the fourth ventricle, called the facial colliculus. Because of this close anatomic association, lesions of the facial colliculus affect both cranial nerve VI and cranial nerve VII.
B. Insert illustrates the internuclear neurons ascending and crossing over to the contralateral oculomotor nucleus.
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1. Why was Grace having nosebleeds?
The veins of the nose, including those of the highly vascular nasal mucosa, drain posteriorly into the cavernous sinus. The mass in Grace’s posterior pharynx was in-vading her cavernous sinus, interfering with blood fl ow, and causing venous conges-tion in her nasal mucosa. The increased blood pressure within the delicate mucous membranes caused the small blood vessels to rupture and thus caused her nose to bleed.
2. Why did Grace have double vision only when she was looking to the left?
Horizontal eye movements require the coordinated action of the medial and lateral rectus muscles. The doctor noticed that when Grace attempted to look to the left, her left eye was unable to abduct (Fig. VI–6). This led him to conclude that Grace’s left lateral rectus muscle was weak. On attempting leftward gaze, Grace was unable to align both eyes toward the same target. Therefore, the visual fi elds were projected onto different areas of the right and left retinae and two images were seen.
3. How did the doctor know that the problem was within the cavernous sinus?
The fi rst step to solving this problem was to identify the nerves involved. Grace was unable to abduct her left eye. The lateral rectus muscle, which is responsible for eye abduction, is innervated by CN VI. Therefore, either her left lateral rectus muscle or her left CN VI was involved. Grace also had numbness over her left forehead and cheek. The ophthalmic division of the trigeminal nerve (V1) supplies sensation to the forehead, the maxillary division of the trigeminal nerve (V2) supplies sensation to the cheek, and the mandibular division of the trigeminal nerve (V3) supplies
FIGURE VI–6 A. On attempted left lateral gaze, Grace was unable to abduct her left eye due to paralysis of her left lateral rectus muscle; therefore, she experienced double vision. B. When looking to the right, Grace was able to direct both eyes toward the
same object.
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sensation to the chin (see Chapter V). Therefore, the left V1 and V2 were also af-fected but the left V3 was spared.
The second step to solving the problem involves a knowledge of the course of the CNs. It is reasonable to assume there is only one cause for Grace’s problem and, therefore, that the site of the lesion is located where CN VI and CNs V1 and V2 are in close proximity to each other. The only anatomic place where this occurs is within the cavernous sinus (see Fig. VI–3). Therefore, disease processes of the cav-ernous sinus can affect these nerves.
4. Why did Grace not have a numb chin?
The mandibular division of the trigeminal nerve (V3) passes just outside the cavern-ous sinus and, therefore, was not involved by the infi ltrating mass.
5. Where along the course of CN VI could damage occur?
CN VI can be damaged anywhere along its course from its nucleus in the pons to its target muscle in the orbit (the lateral rectus muscle).
■ Within the Brain Stem
The nucleus and its axons can be affected by infarction, tumor, or demyelina-tion.
■ Between the brain stem and the cavernous sinus
The nerve itself can become ischemic (this is most commonly seen in patients with diabetes or hypertension). The nerve can be affected by meningeal infec-tion. Occasionally, cerebellopontine angle tumors can involve CN VI as well as CNs V, VII, and VIII (see Chapter VIII, Fig. VIII–3 check later). Mastoiditis (infl ammation of the mastoid process of the temporal bone) can involve CN V as well as CN VI (Gradenigo’s syndrome).
■ Within the cavernous sinus
As CN VI passes anteriorly through the cavernous sinus, the nerve can be in-volved in any disease process within the sinus (carotid aneurysm, sinus throm-bosis, infection, infl ammation, or tumor). Other CNs are commonly affected at this site (III, IV, V1, V2) owing to their close proximity within the cavernous sinus. Grace was found to have a nasopharyngeal carcinoma invading the cav-ernous sinus involving the left CN VI, V1 and V2.
■ Within the superior orbital fi ssure
CN VI passes anteriorly through the superior orbital fi ssure to reach the lateral rectus muscle. Fractures of the orbit or orbital tumors could compromise the nerve. Other CNs (III, IV, and V1) passing through the fi ssure also could also be affected.
■ Within the orbit
CN VI could be injured by an orbital fracture as it passes through the orbit to innervate the lateral rectus muscle.
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CLINICAL TESTING
CN VI is tested in conjunction with CNs III and IV through the assessment of eye movements (see Chapter 13). When testing specifi cally for CN VI, it is important to bring the eye through the full extent of the horizontal plane and to ensure that the eye moves fully away from the midline (Fig. VI–7). The iris should bury itself into the palpebral fi ssure of the corner of the eye and the white of the sclera should not be seen. (See also “Cranial Nerves Examination” on website.)
ADDITIONAL RESOURCES
Brazis P.W. “Isolated palsies of cranial nerves III, IV and VI.” In Seminars in Neurology, vol. 29.
Edited by W.W. Campbell, 14–28. New York: Medical Publishers, 2009.
Brodal, A. 1981. Neurological Anatomy in Relation to Clinical Medicine. 3rd ed., 532–77. New York: Oxford University Press.
Büttner U., and J.A. Büttner-Ennever. 2006. Present concepts of oculomotor organization. Progress in Brain Research 151:1–42.
Büttner-Ennever, J.A. “Anatomy of the oculomotor system.” In Neuro-ophthalmology Developmen-tal Ophthalmology, vol. 40. Edited by A. Straube and U. Büttner,: 1–14. Basel: Karger, 2007.
Glimcher, P.A. “Eye movements.” In Fundamental Neuroscience. Edited by M.J. Zigmond, F.E.
Bloom, and S.C. Landis, 993–1009. San Diego, CA: Academic Press, 1999.
Kiernan, J.A. 2009. Barr’s The Human Nervous System: An Anatomical Viewpoint. 9th ed. Chap. 8.
Baltimore: Lippincott Williams & Wilkins.
Standring, S. Editor in Chief. 2008. Gray’s Anatomy. Chap. 39. London: Churchill Livingstone Elsevier
Wong, A.M. 2008. Eye Movement Disorders, 3–14. Oxford: Oxford University Press.
FIGURE VI–7 Testing the abducens nerve.