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Show Journal of Clinical Neuro- ophlhalmology 12( 4): 263- 267, 1992. Isolated Bilateral Abducens Nerve Palsies Caused by the Rupture of a Vertebral Artery Aneurysm Takato Morioka, M. D., Toshio Matsushima, M. D., Nobuhiko Yokoyama, M. D., Hiroshi Muratani, M. D., Kiyotaka Fujii, M. D., and Masashi Fukui, M. D. ' 91992 Raven Press, Ltd., New York We report two cases with isolated bilateral abducens nerve palsies due to the rupture of a vertebral aneurysm. Surgery revealed that the aneurysm did not directly compress the abducens nerve. Within a year after the subarachnoid hemorrhage, the patients gained full recovery from the bilateral abducens nerve palsies. In view of the clinical and operative findings, it may be regarded as a compression and/ or stretching of the bilateral abducens nerves by a thick clot in the prepontine cistern, and not as a manifestation of the raised intracranial pressure. The mechanisms of the isolated abducens nerve palsy are discussed. Key Words: Abducens nerve palsy- Vertebral artery aneurysm. From the Department of Neurosurgery, Neurological Institute, Faculty of Medicine, Kyushu University, Fukuoka, 812, Japan. Address correspondence and reprint requests to Dr. Takato Morioka, Department of Neurosurgery, Neurological Institute, Faculty of Medicine, Kyushu University 60,3- 1- 1 Maidashi, Higashi- ku, Fukuoka 812, Japan. 263 Involvement of the abducens nerve by direct or indirect effects of intracranial aneurysm is well known. However, an isolated abducens nerve deficit is especially rare, since the majority of cases are due to compression of the 3rd to 6th nerves in the cavernous sinus ( 1- 3). Schrader and Schlezinger ( 4) failed to find aneurysms in their 104 patients with isolated abducens nerve palsy. Dailey et al. ( 5) reported, in their 65 patients with ocular sign due to verified aneurysm, that no patients presented with an isolated abducens nerve palsy. Moreover, little is written about pathophysiology or prognosis with regard to the abducens nerve palsy after rupture of an intracranial aneurysm ( 3,6). We report here two cases with isolated bilateral abducens nerve palsies due to rupture of the aneurysms of the vertebral artery. CASE REPORTS Case 1 A 53- year- old housewife suddenly suffered from severe headache and nausea on December 26, 1990. On the following day, her headache became more severe, and she was transferred to the emergency hospital. Computed tomographic ( CT) scan revealed a subarachnoid hemorrhage ( SAH), especially in the prepontine cistern ( Fig. 1). Intraventricular hemorrhage was seen, but ventricular size was normal. On December 28, she was referred to our hospital. She was alert, however, neurological findings showed bilateral abducens nerve palsies. Left vertebral angiography demonstrated a dissecting aneurysm of the left vertebral artery. The repeated T. MORIOKA ET AL. FIG. 1. Computed tomographic scan shows the massive cisternal hemorrhage, especially in the prepontine cistern ( arrow). angiography on January 31, 1991 revealed an enlarged aneurysm with a small bleb ( Fig. 2). On February 4, a left retromastoid suboccipital craniectomy was done. The aneurysmal shape was fusiform and possessed a large bleb that had no contact with the abducens nerve. There was a clot around the aneurysm, indicating that it had ruptured. The dissecting aneurysm was trapped with-out any abnormality on auditory brainstem response monitoring. After removal of the clots in the subarachnoid space, the left abducens nerve was exposed. Postoperatively, no neurological deterioration was seen. Three months after the onset, the patient's bilateral abducens nerve palsies were completely improved. A R'G. 2~ eN V'~ rMlHal' angl'ogram shows arterial dilatation ( arrows) in the left vertebral artery. J Clin Neuro- ophthalmol, Vol. 12, No. 4, 1992 ABDUCENS NERVE PALSY 265 Case 2 A 69- year- old housewife awoke with a sudden onset of severe neck stiffness and nausea on March 23, 1988. She aspirated her vomitus, which resulted in dyspnea. She lost consciousness en route to emergency hospital. An orotracheal intubation was immediately followed by a tracheostomy. CT scan demonstrated SAH in the prepontine cistern and right ambient cisterns, and dilated lateral ventricles associated with intraventricular hemorrhage ( Fig. 3). With conservative therapy, including glycerol and steroid, she regained full consciousness the following day. She complained of diplopia, and bilateral abducens nerve palsies were diagnosed. Serial CT scans revealed a gradual improvement in the acute hydrocephalus. On May 6, she was referred to our hospital. Neurological examination revealed complete bilateral abducens nerve palsies ( Fig. 4), but no other neurological deficits were observed. She had no diabetes mellitus. Angiography showed a saccular aneurysm of the right vertebral artery ( Fig. 5). On June 6, right lateral suboccipital craniectomy was performed in the left lateral recumbent position. A saccular aneurysm was found on the vertebral artery just below the jugular foramen. The aneurysm was not in contact with the abducens nerve, although the arachnoid membrane around the cranial nerves was thick and yellowish. The neck of the aneurysm was successfully clipped. The abducens nerve was not exposed. The patient's postoperative course was uneventful, and, FIG. 4. Bilateral abducens nerve palsies on the 50th day after aneurysmal rupture. ( A) The patient looking toward her right. ( B) She looking toward the left. upon examination 1 year later, showed a full recovery from the bilateral abducens nerve palsies. DISCUSSION To our knowledge, only seven aneurysmal cases with isolated abducens nerve palsy have been reported. A summary of these cases, including our own, is given in Table 1. Hyland and Barnett ( 7) studied the pathophysiology of cranial nerve involvement by intracranial aneurysms and categorized these into two groups: ( a) those in which the aneurysm sac came in direct contact with the nerve, and ( b) those where the function of the FrG. 3. (,,; omputed tomography scan reveals the hemorrhage in the ( A) prepontine and ambient cisterns and ( B) bilateral posterior horns of the dilated lateral ventricles. I Gill Neuro- ophthalmol. Vol. 12. No. 4. 1992 266 T. MORIOKA ET AL. A FIG. 5. Right vertebral arteriogram shows a saccular aneurysm of the right vertebral artery. ( A) Anteroposterior view. ( B) Lateral view. nerve or nucleus was disturbed by the secondary effects of hemorrhage from aneurysmal rupture. The former mechanism is well known in oculomotor nerve palsy in an internal carotid- posterior communicating aneurysm ( 8). Abrupt aneurysmal dilatation without rupture or incorporation of the nerve within the aneurysmal wall are mechanisms of direct involvement. With regard to an isolated abducens nerve palsy caused by this mechanism, only two cases with vertebral artery aneurysm ( cases 3 and 4 in Table 1) were reported ( 9,10). These aneurysms were large enough to compress the ipsilateral abducens nerve. In our cases, the aneurysm was not in contact with the nerve. One of the latter mechanisms is raised intracranial pressure ( ICP). The abducens nerve is well known for its sensitivity to ICP ( 1,11). Of reported cases with isolated abducens nerve palsy secondary to aneurysmal rupture, all cases, including our own, involved the posterior circulation ( Table 1). This suggests that SAH in the posterior fossa may be important for the development of the isolated abducens nerve palsy. As ICP in the posterior fossa rises, the brainstem is forced downward toward the foramen magnum, resulting in a stretch injury to the nerve at the point there it passes over the sharp apex of the petrous ridge ( 1). However, this explanation seems unlikely, since every case with SAH in the posterior fossa does not demonstrate abducens nerve palsy. Bilateral abducens nerve palsies may be due to vasospasm of the pontine branches of the basilar TABLE 1. Reported cases ( all females) with isolated abducens nerve palsy due to aneurysm Case Authors Ruptured or Mechanism of no. ( year): ref. Age Location unruptured Laterality 6th n. palsy Clipping Outcome Hook et al. 30 BA Ruptured Ipsilateral Increased ICP No Death ( 1964): 18 2 Duvoisin & Yahr 62 VA- PICA Ruptured Ipsilateral ? Yes Complete recovery ( 1965): 19 3 Coppeto & Chan 61 VA- PICA Unruptured Ipsilateral Direct Yes Paresis 1 month ( 1982): 9 compression after operation 4 Dumas & Shults 56 VA- PICA Ruptured Ipsilateral Direct Yes Paresis 2 months ( 1982): 10 compression after operation 5 Hirose et al. 25 SCA Ruptured Contralateral Compression by Yes Complete recovery ( 1990): 6 displaced artery 6 Present case 53 VA Ruptured Bilateral Compression by Yes Complete recovery cisternal 7 Present case 69 VA hematoma Ruptured Bilateral Compression by Yes Complete recovery cisternal hematoma Ref, reference; BA, basilar artery; VA, vertebral artery; PICA, posterior inferior cerebellar artery' SCA su . artery; ICP, intracranial pressure. ' , penor cerebellar I Clin Neuro- ophtlullmol, Vol. 12. No. 4. 1992 ABDUCENS NERVE PALSY 267 artery affecting the abducens nuclei ( 12). Abducens nuclear palsies are usually associated with gaze palsy and facial palsies because there are close anatomical relationships between the abducens nuclei, the medial longitudinal fasciculus, and the facial nuclei ( 1,12). In our cases, these accompanying signs were not observed. Another possibility is the effect of a cisternal hematoma. Salcman et al. ( 13) stressed the localizing value of the abducens nerve palsy in SAH patients. Patients who exhibit the sudden onset of the bilateral abducens nerve palsies and SAH should be suspected of harboring an aneurysm of the vertebral artery- basilar artery junction, because the junction is often located at the level of these nerves. One of the mechanisms that has been implicated to explain the vulnerability of the abducens nerve is its long intracranial course ( 14). Thick cisternal hematoma in the prepontine cistern, as seen in our patients, could compress and/ or stretch the bilateral abducens nerves. Cushing ( 15) first called attention to strangulation of the abducens nerve by branches of the basilar arteries, particularly the anterior inferior cerebellar artery and the internal auditory ( or labyrinthine) artery, which have been described as occasionally passing between the roots of the abducens nerve ( 14- 16). In these cases, compression and stretching of the nerve by displaced arteries, as a result of the cisternal hematoma and secondary displacement of the brainstem, may be a possible cause of abducens nerve paralysis, as in case 5 reported by Hirose et al. ( 6). These mechanisms are most likely to be the cause of the bilateral abducens nerve palsies in our patients, although a hydrocephalic factor cannot be excluded in our case 2. Bilateral abducens nerve palsies caused by raised ICP secondary to SAH are often terminal ( 3,7). In Drakes series ( 17), bilateral abducens nerve palsies have been shown to be one of the ominous visual symptoms among 64 patients with unfavorable results from surgical intervention. These symptoms are further complicated by other serious neurological findings, such as unconsciousness or pupillary dilatation. However, our cases are different from Drakes cases in terms of an isolated abducens nerve palsy and good outcome. Almost all reported cases with isolated abducens nerve palsy showed good outcome. Only case 1 in Table 1 showed poor outcome because her ab-ducens nerve palsy was considered to be attributed to the raised ICP. Therefore, isolated abducens nerve palsies do not always reflect a poor outcome. REFERENCES 1. Bajandas FJ. The six syndromes of the sixth nerve. In: Smith JL, ed. Neuro- ophthalmology update. New York: Masson, 1977: 49-- 67. 2. Berlit P. Isolated and combined pareses of cranial nerve III, IV and VI: a retrospective study of 412 patients. 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