ACTA NEUROCHIRURGICA
Acta Neurochirurgica 38, 13--35 (1977)
9 by Springer-Verlag 1977
Department of Neurosurgery, University of Giet~en, Federal Republic of Germany (Head: Prof. Dr. reed. Dr. h. c. H. W. Pia)
Aneurysms of the Posterior Cerebral Artery Locations
and Clinical
Pictures
By
H. W. Pia and H. F o n t a n a * With 3 Figures
Summary A classification of aneurysms of the posterior cerebral artery from a series of 8 personal cases and 34 cases collected from the literature is attempted. The classification is based on the topography and sites of origin of the cortical and central branches of the artery. The artery was divided into six sections which permitted the description of the origin and projection of typical aneurysms. Single cases of atypical aneurysms can be explained by the known vascular anomalies. The predilective site is section B (origins of the posteromedial choroidal artery and quadrigeminal artery), then section D with the main division of the posterior cerebral artery and origins of the anterior temporal artery, the anterior posterolateral choroidal artery, the hippocampal artery and the thalamogeniculate artery, and finally section C--the junction with the posterior communicating artery. Clinical syndromes corresponding to these locations are described. The classification, when considered together with improved angiographic technique and microsurgery, allows exact preoperative and peroperative definition of topography which in turn enables the avoidance of injury to functionally important typical and atypical central branches of the posterior cerebral artery.
Aneurysms of the posterior cerebral artery are infrequent. Systematic analysis of the origin, location, size and form, and consequently of the clinical picture, prognosis, mortality, and morbidity is still lacking. Modern angiography, including angiotomography, as well as angiographic and intraoperative magnification techniques, provided the possibility of attempting a detailed morphological and clinical classification. This paper analyses 34 adequately described cases of posterior cerebral artery aneurysms from the literature. Eight personal cases are added. * Stipendist of the DAD Bonn, Assistant del Servicio di Neurocirurgia (Hospital Guillermo Rawson, Buenos Aires, Argentina).
14
H . W . Pia and H. Fontana:
Analysis of the Cases
Frequency Among the aneurysms of the vertebro-basilar system aneurysms of the posterior cerebral artery constitute l~ to 15.4~ as seen from the review of 8,968 cerebral aneurysms (Table 1). Table 1. Aneurysms of Post. Cerebr. Artery.
Incidence Total N Aneurysms
Vert-Basil. An. Post. Cerebr, An. Post. Cer. Am/Total . . . . . o/o N
%
N
%
Coop. Study 1966, single aneurysms 2,672
144
5.3
22
15.2
0.8
Jap. Neurosurg. Clinics 1971
3,899
156
4
28
17.9
0.7
Helsinki, Neurosurg. Clinic, NystriSm 1973
1,349
45
3.3
14
31
1
33
Gieflen, Neurosurg. Clinic, Pia 1975
410
24
5.8
8
Drake 1974
638
188
29.5
14
7.4
8,968
557
6.2
86
15.4
1.9 2.2 1
Differences in frequency reported from several centres (USA, Japan, Helsinki, Gieflen) could be caused by selection or ethnic reasons, or could be purely coincidental. Of particular interest are the figures from Drake. In his particularly large collection of vertebrobasitar aneurysms the frequency of aneurysms of the posterior cerebral artery is relatively low.
Location Congenital aneurysms of the posterior cerebral artery occur, as expected, at the sites of origin of the branches of the artery, fie., at the origins of its cortical and central branches.
Anatomy (Fig. 1) The basilar artery divides in the interpeduncular fossa into the posterior cerebral arteries each of which runs in the cisterna ambiens round the cerebral peduncle and the midbrain to the
Aneurysms of the Posterior Cerebral Artery
15
posterior edge of the tentorium, and usually divides at that point into two final branches - - the internal occipital artery and the temporal artery. From the main trunks of the posterior cerebral arteries (B) near the basilar bifurcation (A) arise the perforating, posterior thalamic arteries ~ which project dorsally and cranially. Slightly more laterally and close to each other originate the quadrigeminal arteries 2 and the posteromedial choroidal artery 3. These project dorsally.
Fig. 1. Anatomy of posterior cerebral artery with normal and atypical origin and course of cortical and central branches
Further distally the posterior cerebral artery anastomoses with the posterior communicating artery ~ and forms the dorsal part of the circulus arteriosus. The second section of the posterior cerebral artery (C) occupies the distance up to the origin of the first cortical branch--the anterior temporal artery 5. The third section of the trunk of the posterior cerebral artery (D) branches into the following arteries which run very close to each other, but separately: The posterolateral choroidal artery 6, the hippocampal artery 7, and the thalamogeniculate artery 8. The third section of the trunk ends with its division into the posterior temporal artery (a) and the internal occipital artery 9 (E). The latter gives off medially the posterior pericallosal artery l0 which communicates with the anterior pericallosal system. Usually the posterior posterolateral artery arises from the internal occipital artery tl. One
16
H.W. Pia and H. Fontana:
should mention also the cortical calcarine and parieto--occipital branches of the internal occipital artery ,2 (F). Variations are relatively frequent. A common trunk for both temporal arteries is not uncommon 18 The cortical branches run frequently medially and caudally and turn laterally after they have reached the lower surface of the temporal lobe. As far as the central branches are concerned, the posteromedial choroidal artery can originate from the lateral section of the posterior cerebral artery (Schlesinger's caudal variety) 14 Schlesinger described an inconstant cingulothalamic artery which originates from the main trunk or one of the branches 1~ The posterior cerebral arteries supply the cortical and basal parts of the temporal lobe and the medial parts of the occipital lobe and precuneus. Central branches supply the thalamus, lateral and dorsal parts of the midbrain, lamina quadrigemina, choroid plexus, and parts of the fornix and splenium. The thalamo-geniculate arteries supply the sensory nuclei of the thalamus.
Sites of Origin of Aneurysrns of the Posterior Cerebral Artery One has to expect saccular aneurysms at all arterial junctions. 1. a) Posterior cerebral/basilar. } 1. b) Posterior cerebral/posterior perforating thalamic._ Section A 2. Posterior cerebral/posteromedial choroidal. 3. Posterior cerebral/quadrigeminal.
]~ Section B J
4. Posterior cerebral/posterior communicating.
} Section C
5. Posterior cerebral/anterior temporal (or first division site). 6. Posterior cerebral/anterior posterolateral choroidal. Section D 7. Posterior cerebral/hippocampal. 8. Posterior cerebral/thalamogeniculate. 9. Internal occipital/posterior temporal (or bifurcation / of the posterior cerebral). I 10. Internal occipital/posterior pericallosal. } Section E / 11. Internal occipital/posterior posterolateral choroidal. J 12. Peripheral aneurysms: ] a) Calcarine/parieto-occipital, / Section F b) others.
A n e u r y s m s of the Posterior Cerebral A r t e r y
17
The localizations of aneurysms collected from the literature and of our own cases are shown in Fig. 2, Figs. 3 a and 3 b, and Tables 2 and 3. Section A Aneurysms of the final part of the basilar artery are usually classified as bifurcation aneurysms, even if the neck is situated paramedially and if the aneurysm originates rather from one of the posterior cerebral arteries. Table 2 Site Section Section Section Section Section Section Total
Personal cases A B C D E F
Literature
Total
1 2 1 3 -1
2 12 6 11 2 1
2 14 7 14 2 2
8
34
42
In one of our cases (case 1) a large bilobulated aneurysm of the bifurcation was diagnosed on angiography and confirmed during operation. Wrapping with muscle was performed but the patient died because of repeated subarachnoid bleeding. Autopsy revealed two separate aneurysms, one a thrombosed aneurysm of the basilar bifurcation, the second a separate aneurysm of the posterior cerebral artery. This aneurysm was located at the beginning of the posterior cerebral artery and was directed ventrally. The case of Strauss is different in that his patient had a gigantic aneurysm which enlarged in a cranial direction and filled the third ventricle. Menninger in his report does not describe the direction of the aneurysm. Section B
The majority of adequately described aneurysms of the posterior cerebral artery are in this section (14/41). Different origins and directions are possible: a) Ventral (Wichern). b) Cranial-dorsal (Reinhardt, Drake case 8, and probably Jamieson case 4, personal case 2). c) Dorsal and caudal (Jamieson case 4) (the aneurysm perforated the oculomotor nerve). Acta Neurochirurgica, Vol, 88, Fasc. 1--2
2
_J
k~
I
o
I
Aneurysms of the Posterior Cerebral Artery
19
g
h
Fig. 2. Angiographic drawings of eight personal cases with origins and projections of the aneurysms, a) Case 1. L. K. Large multilocular aneurysm (2 X 1 cm) of Section A 1 b. b) Case 2. B. G. Aneurysm of Section B 2 or 3. c) Case 3. W. L. Large multilocular aneurysm (2 X 1.5 cm) of Section B 2 or 3, with encapsulated haematoma. Spasm of internal carotid, middle cerebral and anterior cerebral arteries, d) Case 4. Z. A. Aneurysm of Section C 4. e) Case 5. M. M. Large aneurysm (1.5 X 1.2cm) of Section D 5. f) Case 6. T. T. Aneurysm of Section D. g) Case 7. Z. S. Aneurysm of Section D. h) Case 8. J. H. Aneurysm of Section F b with haematoma of the occipital lobe and the lateral ventricle
2*
1
3
* One of Jamieson's cases ha d 2 aneurysms.
Total
F
E
1
1
6
1
2
4
1
13
1
B
C
1
A
lat. med.
3
1
1
5
5
ventr, dors.
caud.
ventr, dots.
cran.
Projection
Origin
4
4
5
1
cran.
3
3
1
caud.
Table 3
4
1
Iat.
3
3
reed.
10 39
--
2 20
1
6
2
1
--
5
1
11
1
sma l l large
Size
9
--
1
3
3
1
1
giant
36
2
2
13
5
12
2
sacc.
40
Form
4
~-
--
1
2
1
fus.
40
2
2
14
7
13"
2
Total
O
Aneurysms of the Posterior Cerebral Artery
21
Fig. 3. Distribution and extension of posterior cerebral artery aneurysms, a) Own cases, b) Cases of literature
22
H.W. Pia and H. Fontana:
d) Jamieson's case 2 was an aneurysm of a fine branch originating from the first part of the trunk of the cerebral posterior artery, running round the midbrain and supplying a small a.v. malformation in the cisterna ambiens. e) Giant (Squire, Drake) and fusiform (Jamieson) aneurysms. Our case number 3 was interpreted as being an aneurysm of the posterior superior cerebellar artery. At operation it was found that the oculomotor nerve passed below the supplying artery. It now seems probable that we were dealing with an aneurysm of the branch originating from the first part of the posterior cerebral artery, similar to the case described by Jamieson. Section C
Aneurysms of the posterior communicating artery are not so well described in spite of their relative frequency (7,41). They originate centrally and project medially (Drake, Delpech, and perhaps Wilson) as well as distally with lateral projections (Bramwell, Falconer). A giant aneurysm reaching up to the carotid artery was described by Hanot. Jamieson described a case with a fusiform aneurysm originating at this site which spread posteriorly. In our case the aneurysm was connected with the plum-size, encapsulated haematoma situated at the tentorial opening (thrombosed part of a giant aneurysm). (Case 3.) Section D Aneurysms of this section occur relatively frequently (14,41). Classification of the four branches originating from the section was so far not known and was not possible. The aneurysms are situated usually at the junction of the posterior cerebral artery and the anterior temporal artery. Different dispositions of the aneurysms are possible: a) Directly dorsal (Graft, Drake, case 2 of Jain). b) Cranial: These aneurysms were described by Drake. Another example is our case number 5. c) Caudal: Case number 4. d) Medial (our case number 6): Possible origin at the atypical junction of posterior cerebral artery and posteromedial choroidal artery. Section E and F These aneurysms are very infrequent and have not been described precisely in the literature. As far as section E is concerned Bertrams described two aneurysms, both originating from the posterior cerebral arteries. They
Aneurysms of the Posterior Cerebral Artery
23
were between the quadrigeminal plate and the callosal artery and originated from the abnormal anastomosis between both the posterior cerebral arteries. In the case of Obrador an exact description of topography is lacking. We classified these aneurysms as originating from this section after reviewing the pre- and postoperative angiograms. Section F - - t o this group belong our case number 8 and the case described by Ishikawa which are morphologically similar. In both cases there was a haematoma in the occipital lobe.
Size and Form of the Aneurysms (Table 3) Aneurysms of the posterior cerebral artery are not different in size and form from other aneurysms. Small and medium size aneurysms (1 cm) are the most frequent (20/40). Large aneurysms (up to 2 cm) were described 10 times. They were in the following sections: B--1/13, C--2/7, D--6/14, E--1/2. Giant aneurysms (over 2 cm) were reported in 9/40 patients and were mostly in sections C and D. Usually saccular aneurysms were described (36/40). Four fusiform aneurysms were in sections B, C (2), and D. Multiple aneurysms were seen on only two occasions.
Clinical Picture Tables 4 and 5 show the correlations between the location and clinical picture. Age: The mean age was 37.4 years. The youngest patient was 1 year old and the oldest 70 years old. Sex: As expected there was a predominance of males (26 patients) over females (16 cases).
General Symptoms Intracranial bleeding: The first and most frequent sign is subarachnoid bleeding (34/41). Subarachnoid bleeding did not occur in 2 out of 13 aneurysms in section B, 3 out of 7 in section C, and in 2 cases of aneurysms in section E. Twenty-five patients had one and eight patients more than one subarachnoid bleed. Haematomas were found in 10 patients. In eight cases ventricular bleeding occurred with the ventricle being partially or totally blocked with a clot. In two cases more detailed data were not available (Fearnsides, Lemmel). In our own series, in case 8 there was an intraventricular haematoma, in case 3 a haematoma and softening in the cerebral peduncle, and in case 4 an encapsulated intra-cisternal haematoma.
36
1872
1878
1887
1912
1913
1916
1923
4. H a n o t
5. Rauchfuss
6. Bramwell
7. Wichern
8. Reinhardt
9. Fearnsides
10. Berger
11.
37
1857
3. Squire
64
70
40
43
12
50
40
1856
2. Van der Byl
20
Age
1842
Year
1. Delpech
Author
f
f
f
m
f
f
m
~
f
m
f
Sex
3 years history of vomiting, 3 weeks history of somnolence, death
coma, grade I V
coma, wide pupils, bilateral Babinski sign
lues, headache, coma
rheumatic fever, endocarditis, sudden coma, neck stiff, 5 m o n t h recurrence, death
dementia, ataxia
rheumatic fever, mitral insuff., I I I r d nerve L, V I I L, la~er fever, coma, and delirium
dementia, blindness, deafness, 10 years history
adynamic fever
headache R, facial pain
transient coma, headache, trigeminal and I I I r d nerve involvement, hemiplegia R
Symptoms
Tab& 4
yes
yes
yes
yes
yes (twice)
--
yes
yes
--
yes
yes multiple
Bleeding
--
--
--
--
--
--
--
--
--
--
--
Angiography
m
--
--
--
--
--
OP
B R flus.) BL
CR
BR
BR
CR
2 aneurysms BL CR bleeding R temp.
C L
B L
D R
C L
Site
o
ba
m
m
f
f
m
m
f
f
f
f
25
34
30
28
21
17
34
29
33
20
1928
1931
1932
1933
1964
1967
1967
12. Graft
13. Lemmel
14. Strauss
15. Menninger, Dixon
16. Jamieson
17.
18.
19.
20. H u n t
21. Obrador
yes, hemiplegia, R III., no hemianopsia
yes
yes, later
no
left sided, headache
blindness, papillary atrophy, paresis B L, focus R
yes
yes, mult. an.
yes
trans, loss of consciousness, mental signs, L III., blindness, atrophy of optic nerve trans, coma, R III., hypertonus
BL
C R (fus.)
AR
AL
B
D R sacc.
yes, hemiplegia, astereognosis
ER
yes, R hemiparesis, B bil. L tremor (N. ruber) R sacc, L fus. B R sacc. yes, hemiplegia, death D R sacc. yes
yes
yes, aneurysm and A-V malformation yes, 2 aneur.
yes
yes, hemianop. hemiplegia, R 3
susp. post._ fossa tumour
yes
trans, loss of consciousness, meningismus
L hemiparesis, hemihypoaesthesia, yes, 3 times L quadrantopsia of the lower quadrant, nystagmus }res~ neck pain, meningismus 1 time 2 years history of headache, yes Back pain, neck stiffness, papilloedema, R VII, vertical nystagmus vomiting, coma, R mydriasis, yes trans. L hemiparesis, bleeding in the L fundus L hemiplegia
b~ kn
>
K
g
>
1969
1974
34. I s h i k a w a
40
3
45
32.
33. U h l m a n
26
31.
12 56 66
49
49 56
1968 1969
24. Jain 25. D r a k e 26.
29. 30.
1968
23. Bertrams
54
59 42
1967
22. Weibel
Age
27. 28.
Year
Author
m
~f
m
m
m f
f f
f m f
m
m
endocarditis, headache, vomiting, R hemianopsia
headache a n d fever, sudden coma
R IH., hemiparesis
headache
headache, coma, respirat, arrest R III., hemiparesis bilateral Babinski, R leg paresis L III. after last bleeding hemiplegia, coma endocarditis R hemianopsia
neck pain, wide left pupil
trans, coma, headache
Sex S y m p t o m s
yes
yes
yes
yes
yes, 4 times yes yes
yes yes yes, 3 times
---
yes
Bleeding
Table 4 (continued)
yes
--
yes, mall., A - V + aneur.
yes
yes yes
yes yes
yes yes yes
yes
yes
Angiography
D R D L sacc.
yes yes, resection of h i p p o c a m p u s yes, aneurysmography, L hypoaesthesia yes, aneur., o e d e m a temp. lobe, R h e m i paresis, R III.
F L
yes, resection of an., r e m o v a l o f haematoma
C L
--
D R multiloc., sacc. B R sacc.
C L sacc. D R fus.
D R B R sacc. D L sacc.
E bil.
D L
Site
-yes, d e a t h
--yes
--
yes, bleeding, dementia, d e a t h 6 years later
OP
~a
o
~a
Personal series
1976
m
m
f
f f f
f
f
38
58
38
48 18 1
43
50
sudden headache, coma, numbness of legs, L hemiparesis headache, R hemiparesis, old paraparesis following infection, aphasia and hemiparesis R 2 times left hemiparesis headaches, vomiting, coma accidental finding, subdural haematoma sudden coma and blindness, R hemiparesis sudden coma, followed by generalized epi. attack yes
yes
? yes --
yes
yes, 3 times yes
Table 4 (continued)
yes
yes
yes yes yes
yes
yes
yes
resection of the occ. lobe clip
clip, removal of the ant. part clip trapping
entrapment with muscle
B? L
FL
DR DL DL
CL
BR
AL
o c3
o
o
25
4I
l 7 1 5 1 1
16
M
33
3 11 4 13 -2
+
SAH
7
-2 3 -2 --
--
General
1 6 ~ 3 1 1
27
2 7 6 10 1 1
--
accidentally.
13
+
Coma
symptoms
* One of our own cases was found
2 6 6 9 1 1
3 13 7 14 2 2
A B C D* E F
Fern.
Total
Section
2
1 ---1 --
-i-
ICP
38
2 13 7 13 1 2
--
4
1 2 -I --
--
+
36
3 12 5 13 I 2
--
Unspec.
Table 5
8
1 5 2 ----
~
III
15
2 8 5 13 2 2
--
2 3 3 4 1 1 14
+
26
1 10 4 9 1 1
--
Hemipar.
Focal symptoras
2
---2 ---
+
38
3 13 7 11 2 2
--
Hemihyp.
3
---2 -1
+
37
3 13 7 11 2 1
--
Hemianops.
2
1 1 ---
---
+
38
3 13 6 12 2 2
--
Others
22
2
7
4
6
2
o
:~
-~
ga
g
Aneurysms of the Posterior Cerebral Artery
29
Coma and disturbance of consciousness which is not entirely explicable by subarachnoid bleeding (i.e., not a transient loss of consciousness, but a deep coma lasting often until death) was a particularly frequent sign (13/41). These deep disturbances of consciousness occuring with bleeding from aneurysms located in central sections of the posterior cerebral artery (A 1/3, B 6/13), contrasted with cases of peripherally located aneurysms (C 1/7, D 3/13), could be considered as indicating mesencephalic involvement. Accompaning decerebration signs are not analyzed here. The clinical picture of increased intracranial pressure was noted on two occasions--once with the clinical picture of a tumour of the posterior fossa (Strauss) with papilloedema and nystagmus, and on another occasion with the picture of chronic intracranial hypertension with blindness and optic nerve atrophy. There was no direct compression of the chiasma or of the optic nerves (Obrador). Nonspecific signs such as dementia and pseudoencephalitis were described on four occasions. Local Signs: Twenty-two patients had local signs such as oculomotor nerve paresis, hemiparesis, pain limited to half of the body or hypaesthesia, hemianopsia, and aphasia. Paresis of the oculornotor nerve occurred in 8/41 cases and was noticed only in cases with aneurysms located in sections A (1/3), B (5/13), and C (2/7). Patients with aneurysms in section B had weakness of the oculomotor nerve more frequently than hemiparesis (5/3); in cases with aneurysms in section C this relation is inverted (2/3). Should carotid angiography be negative, these findings can be of diagnostic importance in cases of suspected aneurysm of the internal carotid artery at its junction with the posterior communicating artery. Herniparesis or herniplegia occurred most frequently in cases of aneurysms in sections D (7/13) and F (2/2). Sensory disturbances are infrequent. Two instances--one of hemihypaesthesia and one of thalamic pain--occurring postoperatively with aneurysms in section D indicated direct thalamic lesions or blood supply disturbances within the territory of the posterolateral choroidal or thalamogeniculate arteries. Hemianopsia was observed in 3 out of 41 cases in patients with aneurysms in sections D (2) or F (1). Other Local Symptoms The cause of trigeminal pain or weakness which was observed in a few cases 8, 47 is compression of the nerve trunk through the tentorium 26
30
H.W. Pia and H. Fontana:
Although one would expect signs o] trochlear nerve involvement v because of the topography, these are so far lacking. Speech disturbances were observed in case 4 in our series. They were interpreted as a result of blood supply disturbances within the carotid artery territory.
Angiography Aneurysms of the posterior cerebral artery can be demonstrated by carotid angiography 4~ and by vertebral angiography 57. Total angiography in cases of subarachnoid bleeding is nowadays a routine procedure and these aneurysms are as demonstrable as any others. Angiography in two projections enables one to classify the site of aneurysms into the sections A-B-C, D-E, or peripheral (F). Because of superimposition of the posterior cerebral arteries and the superior cerebellar arteries in a lateral projection, difficulties in exact localization can arise (case 3 of our series). An exact topographical diagnosis with determination of the site of origin and direction of the aneurysm is only exceptionally possible, if at all, and is limited to the large branches. An analysis of our own series and literature shows that the usual angiography in two projections proves to be inadequate. It must be supplemented by oblique and tangential projections and, if possible, by angiotomography and magnification angiography. Importance of exact preoperative diagnosis based on topographic classification is obvious and the preoperative knowledge of point of origin of the aneurysrns facilitates intraoperative isolation of the neck, sparing the artery and lowering the morbidity and mortality. This is particularly true for the sections A, B, and D of the posterior cerebral artery 1% Morphological and angiographic studies by the junior author (H. Fontana) 14 showed that the posterolateral choroidal artery originating in section D can be seen not only on lateral projection ~s. 54 but also on AP pictures. Thus, the aneurysms situated at the point of origin of this artery 24 can be separately identified.
Operative Treatment Aneurysms of the proximal posterior segment are approached, similarly to the basilar bifurcation aneurysms, through the frontotemporal route. Aneurysms in sections D and E are dealt with by the temporal approach. Peripheral aneurysms, according to site, are approached by occipital or temporo-occipital craniotomy. Thanks to optic magnification one can avoid injuries to small central branches and damage to the midbrain ~7 or thalamus i0. The
Aneurysmsof the Posterior Cerebral Artery
31
same is true for the oculomotor nerve ev. The posterior communicating artery should be preserved if possible 27 Drake 10 points out the danger of postoperative temporal lobe oedema due to the necessary elevation of the temporal lobe and possible injury to the basal temporal veins. He suggests reduction of intracranial pressure with diuretics and CSF drainage while approaching the aneurysm. Aneurysms located in the cisterna chorioidea (section D) sometimes require subpial resection of parts of the hipp0campal gyrus 10, 11 The disadvantage of this is that one approaches the fundus of the aneurysm first. Transcisternal approach with isolation of the neck of the aneurysm and the supplying artery is preferable. Giant aneurysms and those of peripheral branches constitute a particular problem 7, iv. ~8
Prognosis The mortality of aneurysms of the posterior cerebral artery is low in series reported by experienced surgeons 10, 11, 27 as well as in our own series. The morbidity seems to be higher. Direct lesions of the third nerve (case 2) occur frequently 11, ~5, but the oculomotor paresis is usually reversible. More serious are the direct and indirect lesions of the central branches, with midbrain, brain peduncle, and thalamic damage which remains permanent in spite of a good collateral supply. Discussion
Aneurysms of the posterior cerebral artery can be divided into six different topographic groups. Although the relations of the aneurysms to the branches of the posterior cerebral artery were only rarely and inadequately described in the literature, we attempted to analyse and to classify the topography of these aneurysms from the clinical and radiological material. It seems that further anatomical and topographical studies are necessary. In sections A and B the aneurysms directed cranio-dorsally are the most frequent. Their projections correspond to the courses of the only branches in that region, which are the perforating thalamic arteries, and they are situated at the origins of these arteries. The relations of the posterior or mesencephalic parts of these vessels to the third nerve in the interpeduncular cistern are known. Sometimes one of these arteries perforates the nerve (Schlesinger). In this respect the case of Jamieson is particularly interesting because the neck of the aneurysm he described perforated the oculomotor nerve, and in
32
H . W . Pia and H. Fontana:
order to clip the aneurysm some of the fibres of the nerve had to be sacrificed. The single ventrally projecting aneurysm of these sections (Wichern) cannot be clearly classified. It is possible that its relation to the posterior communicating artery has not been recognized. Aneurysms of section C originate centrally or distally to the junction between the posterior cerebral artery and the posterior communicating artery. Large aneurysms which point toward the tentorium displace the posterior communicating artery and the anterior choroidal arteries and are indistinguishable clinically from aneurysms at the origin of the communicating posterior artery. The first part of the posterior cerebral artery (section D) at the site of origin of the anterior temporal artery or of the common trunk of the anterior and posterior temporal arteries, and at the sites of origin of the posterolateral choroidal and hippocampal arteries constitute the principal locations of aneurysms. These project along the courses of the vessels dorsally, caudally, or, less frequently, cranially. It is not certain whether more detailed classification of aneurysms in this region is possible. In the clinical picture the signs of cerebral peduncle and midbrain involvement predominate. The rarity of aneurysms of sections D and F does not permit detailed description of them. It is interesting to note that half the number of aneurysms of the posterior cerebral artery exhibit local signs and symptoms which can be attributed to the three main sites of the aneurysms. References
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