Embolic posterior cerebral artery occlusion secondary to spondylitic vertebral artery compression Case report
HUMBERT G. SULLIVAN,M.D., JOHN W. HARBISON, M.D., FREDERICK S. VINES, M.D., AND DONALD BECKER, M.D.
Division of Health Sciences, Division of Neurosurgery, Departments of Neurology, Ophthalmology, and Radiology, Virginia Commonwealth University, Richmond, Virginia ms The authors report a case of isolated homonymous hemianopsia secondary to embolic occlusion of the posterior cerebral artery. The cause of embolism was demonstrated to be spondylitic vertebral artery compression. The importance of arteriography is emphasized since the clinical syndrome may be nonspecific and myelographic or plain x-ray changes may be minimal. Surgical therapy is also discussed. KEY WORDS 9 homonymous hemianopsia 9 visual field 9 vertebral artery 9 cervical spondylosis 9 cerebral e m b o l u s
s
PONDYLITIC vertebral artery insufficiency is a well recognized syndrome.1 5,7,8,10-13,15,17 Its symptoms are usually secondary to brain-stem ischemia from mechanical reduction in vertebral artery blood flow. 1-8'~H3,15'17 To our knowledge this is the first reported case of arteriographically demonstrated posterior cerebral artery occlusion secondary to embolus from a site of spondylitic vertebral artery compression. Case Report
This 37-year-old righthanded man experienced the sudden onset of a left frontal headache with obscuration of his left field of vision on March 10, 1973. His symptoms
618
resolved over the next 6 to 8 hours. Two days later he suffered a second attack associated with a left visual field defect. His vision did not improve and he was hospitalized on the following day.
Operation. With the exception of a dense, congruous left homonymous hemianopsia with splitting of fixation (Fig. 1), the physical examination was unremarkable. Optokinetic responses were normal. The patient had no symptoms on head turning. There were no cervical or cephalic bruits. Skull films and brain scan were normal, an electroencephalogram revealed a slightly diminished amplitude of the alpha rhythm over the right occipital lobe. Transfemoral arteriography J. Neurosurg. / Volume 43 / November, 1975
Embolism from spondylitic vertebral artery occlusion showed no evidence of mass lesion. There was a high degree of obstruction of the left vertebral artery at the C5-6 level (Fig. 2 left) and complete occlusion of the right posterior cerebral artery (Fig. 2 right). The right vertebral artery, the carotid bifurcations, and intracranial circulation were otherwise normal. Neither posterior cerebral artery filled from the anterior circulation. Cervical spine films confirmed minimal osteoarthritis at the C5-6 level (Fig. 3 left) and Pantopaque myelography including an oblique crosstable lateral view showed only a small ventral defect at C5-6 (Fig. 4). The patient admitted to some moderate left neck and shoulder pain for the preceding 2 years. He also recalled that this pain had become more intense 4 days before his first episode of visual disturbance. A thorough search for other sources of emboli was unrevealing. Cardiac evaluation and an electrocardiogram were normal. Preoperatively he was treated with heparin for anticoagulation. Operation. On March 29, 1973, the patient underwent decompression of the vertebral artery at C5-6 and anterior cervical fusion at that level. After removal of the osteophyte, the artery remained constricted by a ring of fibrous tissue; this was incised and stripped away from the artery. Postoperatively the visual fields have remained unchanged. There have been no further episodes suggesting posterior circulation emboli. Four months after surgery follow-up arteriography demonstrated normal flow through the left vertebral artery at C5-6 (Fig. 5 left), and the right posterior cerebral artery was patent (Fig. 5 right).
Fro. 1. Preoperative tangent screen visual field examination. There is a dense left homonymous hemianopsia with slitting of fixation.
hemianopsias, such as that seen in our patient, Trobe, et al., ~6 found that 86% have visual fields and histories consistent with unilateral posterior cerebral artery occlusion. In 1960 Sheehan, et al., ~ reported visual field defects in two of 26 patients with angiographically confirmed spondylitic vertebral artery compression. Both of these patients had other manifestations of vertebral basilar insufficiency. Fisher, et al., 4 in 1961 were the first to document embolization from a vertebral artery occlusion. The site of embolization was the posterior cerebral artery. Brain s postulated spondylitic vertebral artery compression to be the cause of a left homonymous hemianopsia noted in a single case. He further suggested that this patient had embolic occlusion of the right posterior cerebral artery; however, his case was not proven arteriographically. McEwan ~~ believed cerebral embolism from a site of spondylitic vertebral artery compression to be the Discussion cause of field defects in two patients. One case Visual field defects associated with vascular may have been traumatic and the other secondisease in the posterior circulation are dary to embolus from a mural thrombosis in homonymous and congruous. Patients the heart. Neither patient underwent generally do not deny the field defect, op- arteriography. In the two largest surgical tokinetic nystagmus is preserved, there is a series (35 cases)7,11 no mention was made of a tendency to split fixation, and the onset of visual field defect. We have been unable to symptoms is apoplectic? 8 The incidence of find another case of isolated visual field defect field defects in patients with vertebral basilar secondary to angiographically proven sponarterial disease is probably quite high. dylitic vertebral artery compression with emSmith ~4 reports 100 cases of unselected bolization. Previous reports 1-8'~-~3'~5'1~ have emphahomonymous hemianopsia seen by him in 21/2 years; 24% of these cases were due to sized mechanical occlusion of the vertebral posterior circulation disease. Of the occipital artery on head turning. Patients may lesions reported by Smith, 64% were vascular. volunteer that rotation of the head produces Analyzing only isolated homonymous symptoms of vertebral basilar artery inJ. Neurosurg. / Volume 43 / November, 1975
619
H. G. Sullivan, J. W. Harbison, F. S. Vines and D. Becker
F~. 2. Left: Left vertebral arteriogram. Note the focal obstruction to blood flow at C5-6 (arrow). Right: Right anteroposterior vertebral arteriogram showing occlusion of the right posterior cerebral artery (arrow).
artery is compressed at the atlantoaxial joint. 3,6,t2,~5 In patients who are symptomatic the uninvolved vertebral artery is frequently hypoplastic. ~-3,e,H-ls Our patient did not have a syndrome consistent with a purely mechanical obstruction of vertebral artery blood flow. He was asymptomatic on head turning and the uninvolved vertebral artery was not hypoplastic. He became symptomatic secondary to an embolus from the area of the vertebral artery compromise. This mechanism of brain injury in spondylitic vertebral artery disease is not generally recognized and certainly is not appreciated as a cause of isolated visual field defect. Does spondylitic compression predispose to arteriosclerosis? Autopsy study has not demonstrated increased arteriosclerotic plaques at the site of spondylitic vertebral compression2 Postoperative angiography has usually shown the vertebral artery to be normal at the site of previous compression without any evidence of arteriosclerotic plaque. 1,7'11,1s,17 In our patient postoperative angiography demonstrated complete relief of the block. The first case of surgical treatment for spondylitic vertebral artery disease was F,o. 3. Lateral cervical spine film showing reported in 1969 by Hardin, et al.; 8 Hardin minimal osteoarthritic changes at C5~6 (arrow). subsequently collected 15 such cases. 7 He persufficiency; this complaint can nearly always be confirmed on physical examination. Symptoms usually occur when the head is turned toward the side of the spondylitic lesion. 1`s,e,lH3,~7 When the head is rotated the ipsilateral vertebral artery is occluded by the spur and the contralateral vertebral
620
J. Neurosurg. / Volume 43 / November, 1975
Embolism from spondylitic vertebral artery occlusion
Ftc. 4. Cervical myelograms. Left." Anteroposterior view. The cervical roots and cord appear normal. Right." Oblique cross-table projection. The ventral bulge is slightly more prominent in this view
(arrow).
formed a wide decompression of the vertebral artery without fusion; eight of his patients showed complete reversal of symptoms, and the other seven were improved. Bakay and Leslie I treated two patients with anterior cervical fusion alone; both were symptomatically and arteriographically improved. VerbiesP 7 treated two patients by removal of the
anterior root of the transverse process as well as the uncovertebral osteophyte; interbody fusion was also performed in one of these cases. Both patients had excellent results. Nagashima 1~ has reported the largest surgical series, 20 cases. He used the Cloward approach with partial excision of the longus coli muscle and removal of the uncovertebral os-
F~6. 5. Left vertebral postoperative arteriograms. Left: The vertebral artery now appears normal at C5-6 (arrow). Right." The right posterior cerebral artery is shown to be patent (arrow).
J. Neurosurg. / Volume 43 / November, 1975
621
H. G. Sullivan, J. W. Harbison, F. S. Vines and D. Becker t e o p h y t e and the a n t e r i o r root of the transverse process. H e decompressed two foramina above and below the involved area and did not perform interbody fusion. Excellent results were reported in 13 o f Nagashima's cases and five were noted to be improved. Smith, et al., 1~ achieved good results in two cases with removal of the osteophyte by working laterally from the standard Cloward dowel hole followed by anterior fusion. W e used decompression of the vertebral artery and fusion of the cervical spine to relieve arterial compression, to prevent further osteophyte formation and to insure stability. The presence of fibrous stricture after bone decompression has been commented on by others? lal In our case this fibrous ring was incised and stripped from the artery as recommended by VerbiesP 7 and Nagashima." Cervical spondylosis, vertebral basilar insufficiency, and isolated h o m o n y m o u s hemianopsia are c o m m o n in older patients. Our case points out that the head rotation test, plain cervical spine films, and myelography are not sufficient to select patients for arteriography. An otherwise asymptomatic mechanical compression may act as a source for cerebral emboli. The importance of angiography in what appears to be an uncomplicated stroke syndrome must be emphasized. We feel that arteriography should be considered in any patient who has the apoplectic onset of a visual field defect. Thus, a surgically correctable lesion will not be overlooked.
References 1. Bakay L, Leslie EV: Surgical treatment of vertebral artery insufficiency caused by cervical spondylosis. J Neurosurg 23:596-602, 1965 2. Balla JL, Langford KG: Vertebral artery compression in cervical spondylosis. Med J Aust 1:284-286, 1967 3. Brain L: Some unsolved problems of cervical spondylosis. Br Med J 1:771-777, 1963 4. Fisher CM, Karnes WE, Kubek CS: Lateral medullary infarction: the pattern of vascular
622
5. 6. 7. 8.
9. 10. 11. 12. 13.
14. 15. 16.
17. 18.
occlusion. J Neuropathol Exp Neurol 20: 323-379, 1961 Gorvai D: Insufficiency of the vertebral artery treated by decompression of its cervical part. Br Med J 2:233-234, 1964 Hardesty WH, Whitacre WB, Toole JE, et al: Studies on vertebral artery blood flow in man. Snrg Gynecol Obstet 116:662-664, 1963 Hardin CA: Vertebral artery insufficiency produced by cervical osteoarthritic spurs. Arch Surg 90:626-664, 1963 Hardin CA, Williamson WP, Steegman AT: Vertebral artery insufficiency produced by cervical osteoarthritic spurs. Neurology (Minneap) 10:855-858, 1960 Hutchinson EC, Yates PO: The cervical portion of the vertebral artery: a clinicopathological study. Brain 79:319-331, 1956 McEwan A J: The role of cervical spondylosis in the etiology of cerebral embolism. Br J Clin Pratt 21:465-468, 1967 Nagashima C: Surgical treatment of vertebral artery insufficiency caused by cervical spondylosis. J Neurosurg 32:512-521, 1970 Sheehan S, Bauer RB, Meyer JS: Vertebral artery compression in cervical spondylosis. Neurology (Minneap) 10:968-986, 1960 Smith DR, VanderArk GD, Kempe LG: Cervical spondylosis causing vertebro-basilar insufficiency: a surgical treatment. J Neurol Neurosurg Psychiatry 34:388-392, 1971 Smith JL: Homonymous hemianopsia. A review of one hundred cases. Am J Ophthalmol 54:616-623, 1962 Tatlow WFT, Bammer HG: Syndrome of vertebral artery compression. Neurology (Minneap) 7:331-349, 1957 Trobe JD, Lorber ML, Schlezinger NS: Isolated homonymous hemianopsia. A review of 104 cases. Arch Ophthalmoi 89:377-381, 1973 Verbiest H: A lateral approach to the cervical spine: technique and indications. J Neurosurg 28:191-203, 1968 Westby RK, Dietrichson P: Insufficiency of the vertebral-basilar artery system; with special reference to ocular symptoms and signs. Acta Ophthalmol 41:416-429, 1963
Address reprint requests to: Humbert G. Sullivan, M.D., Division of Health Sciences, Division of Neurosurgery, Virginia Commonwealth University, Richmond, Virginia 23211.
J. Neurosurg. / Volume 43 / November, 1975
HUMBERT G. SULLIVAN,M.D., JOHN W. HARBISON, M.D., FREDERICK S. VINES, M.D., AND DONALD BECKER, M.D.
Division of Health Sciences, Division of Neurosurgery, Departments of Neurology, Ophthalmology, and Radiology, Virginia Commonwealth University, Richmond, Virginia ms The authors report a case of isolated homonymous hemianopsia secondary to embolic occlusion of the posterior cerebral artery. The cause of embolism was demonstrated to be spondylitic vertebral artery compression. The importance of arteriography is emphasized since the clinical syndrome may be nonspecific and myelographic or plain x-ray changes may be minimal. Surgical therapy is also discussed. KEY WORDS 9 homonymous hemianopsia 9 visual field 9 vertebral artery 9 cervical spondylosis 9 cerebral e m b o l u s
s
PONDYLITIC vertebral artery insufficiency is a well recognized syndrome.1 5,7,8,10-13,15,17 Its symptoms are usually secondary to brain-stem ischemia from mechanical reduction in vertebral artery blood flow. 1-8'~H3,15'17 To our knowledge this is the first reported case of arteriographically demonstrated posterior cerebral artery occlusion secondary to embolus from a site of spondylitic vertebral artery compression. Case Report
This 37-year-old righthanded man experienced the sudden onset of a left frontal headache with obscuration of his left field of vision on March 10, 1973. His symptoms
618
resolved over the next 6 to 8 hours. Two days later he suffered a second attack associated with a left visual field defect. His vision did not improve and he was hospitalized on the following day.
Operation. With the exception of a dense, congruous left homonymous hemianopsia with splitting of fixation (Fig. 1), the physical examination was unremarkable. Optokinetic responses were normal. The patient had no symptoms on head turning. There were no cervical or cephalic bruits. Skull films and brain scan were normal, an electroencephalogram revealed a slightly diminished amplitude of the alpha rhythm over the right occipital lobe. Transfemoral arteriography J. Neurosurg. / Volume 43 / November, 1975
Embolism from spondylitic vertebral artery occlusion showed no evidence of mass lesion. There was a high degree of obstruction of the left vertebral artery at the C5-6 level (Fig. 2 left) and complete occlusion of the right posterior cerebral artery (Fig. 2 right). The right vertebral artery, the carotid bifurcations, and intracranial circulation were otherwise normal. Neither posterior cerebral artery filled from the anterior circulation. Cervical spine films confirmed minimal osteoarthritis at the C5-6 level (Fig. 3 left) and Pantopaque myelography including an oblique crosstable lateral view showed only a small ventral defect at C5-6 (Fig. 4). The patient admitted to some moderate left neck and shoulder pain for the preceding 2 years. He also recalled that this pain had become more intense 4 days before his first episode of visual disturbance. A thorough search for other sources of emboli was unrevealing. Cardiac evaluation and an electrocardiogram were normal. Preoperatively he was treated with heparin for anticoagulation. Operation. On March 29, 1973, the patient underwent decompression of the vertebral artery at C5-6 and anterior cervical fusion at that level. After removal of the osteophyte, the artery remained constricted by a ring of fibrous tissue; this was incised and stripped away from the artery. Postoperatively the visual fields have remained unchanged. There have been no further episodes suggesting posterior circulation emboli. Four months after surgery follow-up arteriography demonstrated normal flow through the left vertebral artery at C5-6 (Fig. 5 left), and the right posterior cerebral artery was patent (Fig. 5 right).
Fro. 1. Preoperative tangent screen visual field examination. There is a dense left homonymous hemianopsia with slitting of fixation.
hemianopsias, such as that seen in our patient, Trobe, et al., ~6 found that 86% have visual fields and histories consistent with unilateral posterior cerebral artery occlusion. In 1960 Sheehan, et al., ~ reported visual field defects in two of 26 patients with angiographically confirmed spondylitic vertebral artery compression. Both of these patients had other manifestations of vertebral basilar insufficiency. Fisher, et al., 4 in 1961 were the first to document embolization from a vertebral artery occlusion. The site of embolization was the posterior cerebral artery. Brain s postulated spondylitic vertebral artery compression to be the cause of a left homonymous hemianopsia noted in a single case. He further suggested that this patient had embolic occlusion of the right posterior cerebral artery; however, his case was not proven arteriographically. McEwan ~~ believed cerebral embolism from a site of spondylitic vertebral artery compression to be the Discussion cause of field defects in two patients. One case Visual field defects associated with vascular may have been traumatic and the other secondisease in the posterior circulation are dary to embolus from a mural thrombosis in homonymous and congruous. Patients the heart. Neither patient underwent generally do not deny the field defect, op- arteriography. In the two largest surgical tokinetic nystagmus is preserved, there is a series (35 cases)7,11 no mention was made of a tendency to split fixation, and the onset of visual field defect. We have been unable to symptoms is apoplectic? 8 The incidence of find another case of isolated visual field defect field defects in patients with vertebral basilar secondary to angiographically proven sponarterial disease is probably quite high. dylitic vertebral artery compression with emSmith ~4 reports 100 cases of unselected bolization. Previous reports 1-8'~-~3'~5'1~ have emphahomonymous hemianopsia seen by him in 21/2 years; 24% of these cases were due to sized mechanical occlusion of the vertebral posterior circulation disease. Of the occipital artery on head turning. Patients may lesions reported by Smith, 64% were vascular. volunteer that rotation of the head produces Analyzing only isolated homonymous symptoms of vertebral basilar artery inJ. Neurosurg. / Volume 43 / November, 1975
619
H. G. Sullivan, J. W. Harbison, F. S. Vines and D. Becker
F~. 2. Left: Left vertebral arteriogram. Note the focal obstruction to blood flow at C5-6 (arrow). Right: Right anteroposterior vertebral arteriogram showing occlusion of the right posterior cerebral artery (arrow).
artery is compressed at the atlantoaxial joint. 3,6,t2,~5 In patients who are symptomatic the uninvolved vertebral artery is frequently hypoplastic. ~-3,e,H-ls Our patient did not have a syndrome consistent with a purely mechanical obstruction of vertebral artery blood flow. He was asymptomatic on head turning and the uninvolved vertebral artery was not hypoplastic. He became symptomatic secondary to an embolus from the area of the vertebral artery compromise. This mechanism of brain injury in spondylitic vertebral artery disease is not generally recognized and certainly is not appreciated as a cause of isolated visual field defect. Does spondylitic compression predispose to arteriosclerosis? Autopsy study has not demonstrated increased arteriosclerotic plaques at the site of spondylitic vertebral compression2 Postoperative angiography has usually shown the vertebral artery to be normal at the site of previous compression without any evidence of arteriosclerotic plaque. 1,7'11,1s,17 In our patient postoperative angiography demonstrated complete relief of the block. The first case of surgical treatment for spondylitic vertebral artery disease was F,o. 3. Lateral cervical spine film showing reported in 1969 by Hardin, et al.; 8 Hardin minimal osteoarthritic changes at C5~6 (arrow). subsequently collected 15 such cases. 7 He persufficiency; this complaint can nearly always be confirmed on physical examination. Symptoms usually occur when the head is turned toward the side of the spondylitic lesion. 1`s,e,lH3,~7 When the head is rotated the ipsilateral vertebral artery is occluded by the spur and the contralateral vertebral
620
J. Neurosurg. / Volume 43 / November, 1975
Embolism from spondylitic vertebral artery occlusion
Ftc. 4. Cervical myelograms. Left." Anteroposterior view. The cervical roots and cord appear normal. Right." Oblique cross-table projection. The ventral bulge is slightly more prominent in this view
(arrow).
formed a wide decompression of the vertebral artery without fusion; eight of his patients showed complete reversal of symptoms, and the other seven were improved. Bakay and Leslie I treated two patients with anterior cervical fusion alone; both were symptomatically and arteriographically improved. VerbiesP 7 treated two patients by removal of the
anterior root of the transverse process as well as the uncovertebral osteophyte; interbody fusion was also performed in one of these cases. Both patients had excellent results. Nagashima 1~ has reported the largest surgical series, 20 cases. He used the Cloward approach with partial excision of the longus coli muscle and removal of the uncovertebral os-
F~6. 5. Left vertebral postoperative arteriograms. Left: The vertebral artery now appears normal at C5-6 (arrow). Right." The right posterior cerebral artery is shown to be patent (arrow).
J. Neurosurg. / Volume 43 / November, 1975
621
H. G. Sullivan, J. W. Harbison, F. S. Vines and D. Becker t e o p h y t e and the a n t e r i o r root of the transverse process. H e decompressed two foramina above and below the involved area and did not perform interbody fusion. Excellent results were reported in 13 o f Nagashima's cases and five were noted to be improved. Smith, et al., 1~ achieved good results in two cases with removal of the osteophyte by working laterally from the standard Cloward dowel hole followed by anterior fusion. W e used decompression of the vertebral artery and fusion of the cervical spine to relieve arterial compression, to prevent further osteophyte formation and to insure stability. The presence of fibrous stricture after bone decompression has been commented on by others? lal In our case this fibrous ring was incised and stripped from the artery as recommended by VerbiesP 7 and Nagashima." Cervical spondylosis, vertebral basilar insufficiency, and isolated h o m o n y m o u s hemianopsia are c o m m o n in older patients. Our case points out that the head rotation test, plain cervical spine films, and myelography are not sufficient to select patients for arteriography. An otherwise asymptomatic mechanical compression may act as a source for cerebral emboli. The importance of angiography in what appears to be an uncomplicated stroke syndrome must be emphasized. We feel that arteriography should be considered in any patient who has the apoplectic onset of a visual field defect. Thus, a surgically correctable lesion will not be overlooked.
References 1. Bakay L, Leslie EV: Surgical treatment of vertebral artery insufficiency caused by cervical spondylosis. J Neurosurg 23:596-602, 1965 2. Balla JL, Langford KG: Vertebral artery compression in cervical spondylosis. Med J Aust 1:284-286, 1967 3. Brain L: Some unsolved problems of cervical spondylosis. Br Med J 1:771-777, 1963 4. Fisher CM, Karnes WE, Kubek CS: Lateral medullary infarction: the pattern of vascular
622
5. 6. 7. 8.
9. 10. 11. 12. 13.
14. 15. 16.
17. 18.
occlusion. J Neuropathol Exp Neurol 20: 323-379, 1961 Gorvai D: Insufficiency of the vertebral artery treated by decompression of its cervical part. Br Med J 2:233-234, 1964 Hardesty WH, Whitacre WB, Toole JE, et al: Studies on vertebral artery blood flow in man. Snrg Gynecol Obstet 116:662-664, 1963 Hardin CA: Vertebral artery insufficiency produced by cervical osteoarthritic spurs. Arch Surg 90:626-664, 1963 Hardin CA, Williamson WP, Steegman AT: Vertebral artery insufficiency produced by cervical osteoarthritic spurs. Neurology (Minneap) 10:855-858, 1960 Hutchinson EC, Yates PO: The cervical portion of the vertebral artery: a clinicopathological study. Brain 79:319-331, 1956 McEwan A J: The role of cervical spondylosis in the etiology of cerebral embolism. Br J Clin Pratt 21:465-468, 1967 Nagashima C: Surgical treatment of vertebral artery insufficiency caused by cervical spondylosis. J Neurosurg 32:512-521, 1970 Sheehan S, Bauer RB, Meyer JS: Vertebral artery compression in cervical spondylosis. Neurology (Minneap) 10:968-986, 1960 Smith DR, VanderArk GD, Kempe LG: Cervical spondylosis causing vertebro-basilar insufficiency: a surgical treatment. J Neurol Neurosurg Psychiatry 34:388-392, 1971 Smith JL: Homonymous hemianopsia. A review of one hundred cases. Am J Ophthalmol 54:616-623, 1962 Tatlow WFT, Bammer HG: Syndrome of vertebral artery compression. Neurology (Minneap) 7:331-349, 1957 Trobe JD, Lorber ML, Schlezinger NS: Isolated homonymous hemianopsia. A review of 104 cases. Arch Ophthalmoi 89:377-381, 1973 Verbiest H: A lateral approach to the cervical spine: technique and indications. J Neurosurg 28:191-203, 1968 Westby RK, Dietrichson P: Insufficiency of the vertebral-basilar artery system; with special reference to ocular symptoms and signs. Acta Ophthalmol 41:416-429, 1963
Address reprint requests to: Humbert G. Sullivan, M.D., Division of Health Sciences, Division of Neurosurgery, Virginia Commonwealth University, Richmond, Virginia 23211.
J. Neurosurg. / Volume 43 / November, 1975
