Keywords
terminal end variations - pathological abnormalities - basilar artery
Introduction
The basilar artery formed by two vertebral arteries at the ponto medullary junction
is significantly known to be the major blood source to the hind brain. It terminates
into two major branches, the superior cerebellar artery and posterior cerebral arteries.
It lies ventral to the pons to which it gives a series of pontine branches. Literature
reviewed has shown that the basilar artery is associated with several morphological
variations at its terminal end which in most cases renders it as a susceptible point
for aneurysms, thromboembolisms and presenting neurosurgical complications.[1]
[2]
[3] As a component of the vertebrobasilar circulation it supplies the pons, the cerebellum
and the midbrain thus its occlusion can highly cause infarctions in these areas. Occlusion
of the basilar artery is reported to contribute 8–14% of posterior circulatory stroke.[4] The basilar artery is also an important factor in the pathophysiology of vertebrobasilar
insufficiency that is characterized by transient ischaemia to the vertebrobasilar
circulation.[5] About 20% of cerebrovascular accidents involve the posterior cerebral circulation.[6] There is dearth of literature on stroke from sub-Sahara Africa[7] but recent figures show that it has an annual incidence of 316 per 100,000 population.[8] To improve on the diagnosis of posterior vascular occlusion incidents, it is paramount
that specific anatomical/morphological variations in vertebrobasilar circulation are
explored.
Several morphological studies have documented the terminal end variations of the basilar
artery among various populations including bifurcations, trifurcations, quadrifurcations
and pentafurcations.[9]
[10]
[11]
[12] These multiple branching patterns are believed to be lodging points for thrombo-embolic
phenomena and pathologies like atherosclerosis and aneurysms. Aneurysms with multiple
branching patterns at such points would lead to nerve compressions resulting into
optical abnormalities.[13]
Most studies on pathologies of the basilar artery have used radiological imagery and
just a few have used cadaveric specimens.[11]
[14]
[15] Thus, this cadaveric study set out to describe morphological variations at the terminal
end of the basilar artery and also document the common macroscopic pathologies affecting
this artery.
Materials and Methods
A hundred and fifteen human cadaveric brains with intact basilar arteries and terminal
branches were acquired on autopsy from the Kampala City Council Authority mortuary,
Uganda for this cross-sectional study. Most of the brains were obtained from victims
of road traffic accidents, mob justice and a few abandoned patients from the adjacent
national referral hospital ([Table 1]).
Table 1
Demographic characteristics
|
Study population -115
|
|
Sex
|
Age
|
Height
|
Cause of death
|
|
Range
|
Population
|
Range
|
Population
|
Cause
|
Population
|
|
Males- 77
|
0–24
|
20
|
< 150
|
29
|
Road traffic accidents
|
60
|
|
Females- 33
|
25–29
|
31
|
|
|
Mob justice
|
20
|
|
40–54
|
15
|
151- 175
|
56
|
Abandoned patients
|
30
|
|
≥ 55
|
19
|
|
|
Unknown
|
17
|
|
Unknown
|
30
|
> 175
|
30
|
|
|
|
Total
|
|
115
|
|
115
|
|
115
|
A transverse incision was made across the forehead connecting the mid-ear points bilaterally.
A coronal incision was then made joining the two mid-ear points. The scalp was reflected
from the incised points to expose the skull. The skull was then opened manually using
a metallic saw to remove the calvarium and expose the dura mater. With the supervision
of a pathologist, mortuary attendants carefully removed the brains from the cranium,
after sectioning just inferior to the medulla oblongata to avoid severing the arteries
under study. All brains with damaged arteries were excluded from study ([Fig. 1]). The specimens were then washed under running water and placed on a study tray.
The base of the brains was exposed and the arachnoid mater carefully removed to avoid
rapture. One end of the vertebral artery was tied with a soft string and the other
perfused with clear water using a hypodermic syringe to engorge the arteries and improve
on the visibility of the terminal basilar artery end.
Fig. 1 Selection criteria for the study population.
Terminal end variations and pathological abnormalities of the basilar artery were
observed by the first author, an anatomist and confirmed by the pathologist. Results
were recorded in a laboratory book and representative photographs taken using a 14
megapixels Fuji Film digital camera. Morphologically, the number of terminal branches
of the basilar artery and macroscopic pathological abnormalities were noted. Pathological
abnormalities were defined as, the presence of atheroma plaques plus their locations
along the basilar artery, rigidity, tortuosity and aneurysms. Data was analyzed using
SPSS version 23 and summarized using descriptive and inferential statistics. The relationship
between age pathological abnormalities was analyzed using Pearson's Chis squared test.
A P-value of < 0.05 was considered statistically significant.
The ages of victims of mob justice and traffic accidents who did not have identification
records on them at the time of death could not be documented.
This study was approved by the Makerere University School of Biomedical Sciences Higher
degrees and Research Ethics Committee (SBSHDREC) (Ref. No: SBSHREC-443). Administrative
clearance to access cadaveric specimens at the Kampala City Council Authority (KCCA)
mortuary was obtained from the mortuary In-charge and the Head of Forensic Pathology
at Uganda Police Headquarters. A waiver of consent was obtained from SBSHDREC. All
cadavers and specimens were treated with utmost respect and no personal identifiers
were recorded to maintain confidentiality.
Results
One hundred fifteen cadaveric brains were included in this study, of which 38 (33%)
were female and 77 (67%) male. The mean age was 38 years (SD ± 19, range 18–85 years).
The majority of specimens (56/115, 48.7%) had normal bifurcation while trifurcations,
quadrifurcation, pentafurcation and hexafurcation were also observed as summarized
in [Table 2]. One unique case of hexafurcation was observed with triplicated right and duplicated
left Superior superior cerebellar arteries arising bilaterally from the terminal end
([Fig. 2]). There were no differences in morphology between both sexes (p = 0.55).
Fig. 2 Hexafurcation at the terminal end of the Basilar artery. Arrows point to the terminal
branches at the distal end of the basilar artery. Note the right triplication and
the left duplication of the superior cerebellar arteries.
Table 2
Terminal end variations of the basilar artery among the Ugandan population*
|
Terminal end variations
|
Frequency
|
Percent
|
|
Bifurcation
|
56
|
48.7
|
|
Hexafurcation
|
1
|
0.9
|
|
Pentafurcation
|
7
|
6.1
|
|
Quadrifurcation
|
25
|
21.7
|
|
Trifurcation
|
26
|
22.6
|
|
Total
|
115
|
100.0
|
*Termination variations.
Bifurcation - termination into 2 Posterior cerebral arteries branches; Triplication
- termination into 3 branches 2PCAs and 1 Superior cerebellar artery; Quadrifurcation
- termination into 4 branches 2SCAs and 2 PCAs at the same point; Pentafurcation -
termination into 5 branches, 2 duplicated SCA, 1 single SCA, and 2 PCA; Hexafurcation
- termination into 6 branches, 3 triplicated SCAs, 2 duplicated SCAs, 2 PCAs.
Sixteen specimens (14%) had pathological lesions (atheroma plaques) of the basilar
artery; with 14/16 being 50 years of age and above. The most common pathological abnormality
was rigidity (22.6%); other abnormalities included tortuosity ([Fig. 3]) and there were no aneurysms seen ([Table 3]). We also investigated the location of the pathological lesions where 10/16 (62.5%)
of atheroma plaques were located along the entire length of the basilar artery ([Fig. 4]). Other plaques were located either at the formation point or termination or both
points of the basilar artery (6/16, 37.5%) ([Table 4]). There was strong correlation between age and pathological abnormalities (p = 0.001).
Table 3
Pathological vascular abnormalities
|
Pathological abnormality
|
|
Atheroma
|
Rigidity
|
Tortuosity
|
Aneurysm
|
|
Cases
|
(%)
|
Cases
|
(%)
|
Cases
|
(%)
|
Cases
|
(%)
|
|
Presence
|
16
|
13.9
|
26
|
22.6
|
21
|
18.3
|
0
|
0
|
|
Absence
|
99
|
86.1
|
89
|
77.4
|
94
|
81.7
|
115
|
100
|
|
Total
|
115
|
100
|
115
|
100
|
115
|
100
|
115
|
100
|
Table 4
Location of atheroma plaques along the basilar artery*
|
Location point
|
Frequency
|
Percent
|
|
Formation point
|
2
|
1.7
|
|
Termination point
|
3
|
2.6
|
|
Entire artery
|
10
|
8.7
|
|
Formation and termination points
|
1
|
0.9
|
|
No atheroma plaques
|
99
|
86.1
|
|
Total
|
115
|
100.0
|
*Atheroma plaques along the entire artery were observed among elderly individuals
above 50 years.
Fig. 3 Tortuosity of the basilar artery. Arrows point to the tortuous proximal end of the
basilar artery. Blood supply is cut off from the two vertebral arteries. Note: Quadrifurcation
at the terminal end.
Fig. 4 Atherosclerosis of the basilar artery. Arrows point to parts with atheromatous plaques.
Note: the terminal end trifurcates and the right superior cerebellar artery shares
a common trunk with the posterior cerebral artery (longer arrow).
Discussion
This study investigated the morphological variations of the terminal end of the basilar
artery and also documented the common macroscopic pathologies affecting this artery
in a selected Ugandan population. Almost half of the study population had normal bifurcation
but there were several other variations in the branching patterns. We also found several
age-related pathological abnormalities of the basilar artery. Knowledge on the terminal
end variations of the basilar artery is essential when investigating arterial occlusion
in the posterior circulatory system. This is essential because the vertebrobasillar
circulation supplies the entire hind brain and a portion of the midbrain; therefore,
lesions of the basilar artery may culminate in fatal complications considering the
extent of its supply.
We report a higher percentage of basilar artery terminal end variants from the normal
bifurcation with more than half of the study population (51.3%) being either trifurcations,
quadrifurcations, pentafurcations or hexafurcations. Similar studies have reported
the same variations though with a varying prevalence rates.[10]
[11] Ogeng'o et al.[11] reported 82.1% normal bifurcation in a black Kenyan population and Gunnal et al.[10] 82.5% in a turkish population. These two studies reported a significantly higher
prevalence of bifurcations as compared with our study. The most common variant terminations
were trifurcations (22%) and quadrifurcations (23%). Similar terminal variants were
reported by Ogeng'o et al.[11] but with a lower prevalence. Uganda is bordered by Kenya to the East and both countries
have black populations however there are notable differences in the occurrence of
morphological variants. One limitation of this and Ogeng'o et al.[11] studies is the small sample size. This therefore calls for a wider in-depth investigation
with larger numbers.
The difference in frequencies of these variations among different populations is unknown
however the similarity in morphological nature can only be explained embryologically.
For example, Bala et al.[9] theorizes that these variations particularly trifurcation result from lack of normal
fusion of the basilar artery at the origin of the superior cerebellar artery during
embryogenesis. Aydin et al.[16] proposed the mal-fusion of the basilar artery at the origin of the superior cerebellar
artery during embryogenesis of the basilar artery from the primitive neural arteries.
They further theorize that the posterior cerebral artery anastomoses with the basilar
artery caudally at a point lower than the normal site. This abnormal fusion, leads
to multiple branches bifurcating close to the trunk of the basilar artery instead
of the normal lateral bifurcation, with the number of terminal branches depending
on the extent of fusion.[17] These multiple branching points interrupt the haemodynamics resulting into aneurysms
and thrombo-embolism.
In one unique case of hexarfurcation, the basilar artery terminated into 5 SCAs and
2 PCAs ([Fig. 1]). Ogeng'o et al.,[18] reported a similar case of hexafurcation though with prominent therlamoperforator
arteries. The increase in number of terminal branches is most likely to decrease the
angles of bifurcation of the basilar artery which in turn will increase the rate of
arterial occlusion.[19] The high prevalence of arterial variants could predispose to age-related pathological
abnormalities such as aneurysms and thromboembolic phenomena in cerebral circulation.[20]
The most frequent pathological abnormalities observed macroscopically were rigidity.
These were observed in 22.6% most of which were associated with atheroma plaques.
The cause of cerebral arterial rigidity is associated with cardiovascular and renal
malfunction, since these are the predominant blood pressure regulators. Multiple branching
patterns of the terminal end of the basilar artery tend to alter the hemodynamics
resulting in arterial remodeling secondary to hypertension; this in turn increases
arterial rigidity, plaque depositions and atherosclerosis limiting brain perfusion.[21] Atheroma plaques were observed in 16 cases (14%) within the age range of 38–85 years
from the current study. Those formed at the formation and termination ends were frequent
among age groups of 38–50 and those that spread entirely across the arteries were
among individuals with 50 and above years This could imply that atheroma plaques are
most likely to commence at the basilar termination and formation points but spread
as the person ages. In comparison to another study by Songur et al.[22] atheromatous plaques are most likely to be frequent among individual of 50 years
and above. Terminal end variations of the basilar artery, with increase in age are
relatable factors to formation of atheromatous plaques.
Tortuosity cases are usually detected on MRI though these are missed out this being
a non-common procedure among the third world countries. We noticed 1 severe case of
tortuosity (with the rest being mild cases) which could have compromised blood supply
or caused accumulation of blood in the basilar artery ([Fig. 3]). It was noted that the basilar artery in this case was extremely longer and highly
folded at the ponto medullary junction. Severe cases of tortuosity are usually associated
with fatal effects of ischemic attacks.[23]
[24] The obvious cause of tortuosity is unknown but, atherosclerosis, and aging are culminating
the risk factors.[25]
Tortuosity like atheroma and rigidity were common cases among individuals above 40
years from our study hence age can be considered as a determining risk factor for
occurrence of pathological vascular abnormalities of the basilar artery.
Conclusion
There are several morphological terminal end variants of the basilar artery among
the Ugandan population. Arterial pathological abnormalities are also not uncommon
and tend to increase with age. Thus, the need for routine imaging investigations in
patients with cerebrovascular disorders and individuals of 40 years and above. Generalization
of our findings may not be practical due to the limited scope of the study and small
sample size.
