(Hypertension. 2001;37:176.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
Neurovascular Contact of Cranial Nerve IX and X Root-Entry Zone in Hypertensive Patients
From the Department of Internal Medicine (H.H., R.O., J.O., H.Z., A.D., A.M.S.), Division of Endocrinology and Nephrology, the Department of Radiology (S.A.S., K.-J.W.), and the Department of Neurology (H.-C.K.), Universitätsklinikum Benjamin Franklin, Berlin, Germany.
Correspondence to Prof Arya M. Sharma, Abteilung für Nephrologie und Hypertensiologie, Franz-Volhard Klinik–Charité, Wiltbergstrasse 50, 13122 Berlin-Buch, Germany. E-mail sharma{at}fvk-berlin.de
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Abstract |
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Abstract—Neurovascular contact of the left rostral ventrolateral medulla has been implicated in the pathogenesis of "essential" hypertension, and recent intervention studies suggest that surgical decompression of the ventrolateral medulla lowers blood pressure in these patients. We assessed the prevalence of this vascular anomaly in patients with essential hypertension by using an advanced MRI technique. We performed MRI of the brain stem in 125 hypertensive patients and in 105 age-matched, sex-matched, and body mass index–matched normotensive control subjects. Imaging of the root-entry zone of cranial nerves IX and X was performed by combining a high-resolution 3D constructive interference in steady-state sequence with a flow-sensitive time-of-flight technique, and images were independently assessed by 4 readers using predefined criteria. Left-sided neurovascular contact was found in 23% of the hypertensive patients and in 16% of the normotensive individuals (P=0.12). Blood pressure level, heart rate, and number of antihypertensive medications in treated hypertensive patients were similar among patients with positive, borderline, and negative brain stem findings. Our findings cast doubt on the importance of left-sided neurovascular contact as a frequent cause of essential hypertension or as a major factor determining the severity of hypertension in patients with this anomaly.
Key Words: nervous system • hypertension, essential • magnetic resonance imaging • hypertension, neurogenic • brain
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Introduction |
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Central neurogenic mechanisms have long been suspected to play a fundamental role in the development of "essential" hypertension.1 Particular attention has been focused on the left rostral ventrolateral medulla (RVLM), which contains major centers of cardiovascular control.2 3 After the initial observation by Jannetta and Gendell,4 5 ie, that surgical decompression of neurovascular contact in the left rostral root-entry zone of the glossopharyngeal (IX) and vagus (X) nerves can cure hypertension, several experimental6 7 8 9 10 and clinical5 11 12 13 14 15 16 17 studies have suggested an association between compression of the left ventrolateral medulla and essential hypertension. More recently, compression of the left ventrolateral medulla was found to cosegregate to 100% with hypertension in a Turkish family with monogenic hypertension and brachydactyly.17 Preliminary data from interventional studies have also reported enthusiastic results of vascular decompression in patients with essential hypertension.14 18 19
Therefore, the aim of the present study was to examine the prevalence of left-sided vascular contact of the cranial nerve IX/X root-entry zone in hypertensive patients and in age- and sex-matched normotensive control subjects. Because the MRI technology used by previous investigators has been criticized for having low spatial resolution,20 21 possibly resulting in an overestimation of the prevalence of neurovascular contact in the rostral medulla, we used a novel imaging protocol that combines 2 high spatial resolution 3D sequences. A constructive interference steady-state sequence (CISS) with optimized contrast between solid structures and cerebrospinal fluid was anatomically matched with a magnetic resonance angiographic sequence optimized for arterial flow, resulting in a more precise definition of the morphological features of the brain stem.
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Methods |
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Subjects
We studied 125 hypertensive patients and 105 age- and sex-matched normotensive control subjects. In patients, diagnosis of hypertension was based on the prescription of antihypertensive medication and/or an average 24-hour ambulatory blood pressure level
140/90 mm Hg (SpaceLabs 90207). Secondary causes of
hypertension were ruled out by appropriate physical and laboratory
evaluation. Diabetes was defined as a history and treatment of diabetes
or elevated fasting blood glucose levels; dyslipidemia was
defined as hypercholesterolemia and/or
hypertriglyceridemia or treatment with
lipid-lowering drugs. The presence of left ventricular
hypertrophy was based on echocardiographic
findings, and coronary artery disease was based on a history of
myocardial infarction, coronary bypass operation, or
angioplasty as well as a positive coronary angiogram. In
control subjects, hypertension was ruled out by repeated blood pressure
measurements documenting resting blood pressure levels <140/90
mm Hg. Patients were recruited from our hypertension clinic.
Normotensive control subjects were recruited from genetically
nonrelated individuals identified through the patient (eg, spouse or
friends) or through newspaper announcements. The present study was
approved by the ethics committee of our hospital, and written informed
consent was obtained from all subjects.
Magnetic Resonance Measurements
The MRI protocol developed and used for the present study
has been previously reported.22 In brief, the imaging was
carried out with the use of a standard head coil on a 1.5-T magnetic
resonance imager (Vision, Siemens). Two magnetic resonance pulse
sequences were used to generate 1-mm-thick slices at identical anatomic
levels. To visualize the morphology of both cranial nerves and vessels,
a CISS sequence, which offers a high contrast between solid structures
like the brain stem, nerves, and vessels (black) and surrounding
cerebrospinal fluid (white), was used. For magnetic resonance
angiography, a 3D fast low-angle shot sequence was used.
Arterial flow was selectively highlighted by the
time-of-flight technique.
Image Analysis
Hard copies of axial continuous CISS and source magnetic
resonance angiographic images as well as 3D maximum intensity
projection (MIPs) of the arterial tree in 12 rotational
views were reviewed independently by 4 readers. Two readers had
neuroimaging experience for >5 years, and 2 readers were trained
novices. All 4 readers were unaware of the medical history of the
probands. The presence of neurovascular contact on either side of the
brain stem was evaluated by using the following criteria: Upper and
lower borders of the root-entry zone were determined by uppermost and
lowest fibers of the IX/X nerve bundle entering the medulla. The
anterior border of the root-entry zone was defined as the transition of
the olivary convexity to the concavity of the retro-olivary sulcus, and
the posterolateral border was located at the junction of parenchymal
brain tissue to individual nerve fibers (Figure 1). Vascular structures were
characterized as arterial by identification of continuity
to the vertebral or basilar feeding artery and by the presence of an
arterial flow signal.
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A positive finding was defined as an arterial vessel or vascular loop with its convexity touching the surface of the left brain stem within the region defined above. A negative finding was defined as no contact between arterial vessels and either brain stem or nerve fibers on the left. Cases in which the arterial vessel touched either the left cranial nerve bundle or the left medulla within 2 mm above or below the root-entry zone were defined as borderline. Cases in which the criteria for nerve contact were present on the right side only were also noted but were considered negative. Cases in which no consensus decision could be achieved were defined as undetermined and were excluded from the analysis.
Data Analysis
Interreader agreement was characterized by using 
statistics, with 0 indicating no agreement and 1 indicating complete
agreement between readers. In cases of disagreement between readers,
studies were subjected to a review meeting to seek a consensus
decision. The data were analyzed by descriptive and comparative
statistics (t test and 
2 test). The
level of significance was considered to be P<0.05.
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Results |
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In the hypertensive and control groups, 3 and 4 subjects were excluded, respectively, because of technical failure, and 7 and 5 studies were excluded because of lack of consensus agreement. There were no significant differences between the 2 groups with regard to age, sex, and body mass index between the hypertensive and normotensive groups (Table 1).
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Out of 115 hypertensive patients, 26 (23%) were identified as positive, 55 (48%) were identified as negative, and 34 (29%) were identified as borderline for neurovascular contact by consensus decision. Within the borderline group, contact with the medulla just above or below the root-entry zone was present in 4 patients, contact with cranial nerves IX/X was present in 21 patients, and contact with both structures outside the defined area was present in 8 patients. Vascular contact to the medulla on the right root-entry zone was noted in 25 (22%) patients, 6 of whom also had a positive contact on the left medulla.
Out of the 96 normotensive subjects, 15 (16%) were identified as positive, 55 (57%) were identified as negative, and 26 (28%) were identified as borderline by consensus decision. Within the borderline group, contact with the medulla just above or below the root-entry zone was present in 6 subjects, contact with cranial nerves IX/X was present in 17 subjects, and contact with both structures outside the defined area was present in 3 subjects. Vascular contact to the medulla on the right root-entry zone was noted in 10 (9%) subjects, of whom 1 also had a positive contact on the left medulla.
In positive hypertensive and normotensive subjects, the offending vessel was identified as the posterior inferior cerebellar artery in 16 and 10, the anterior inferior cerebellar artery in 4 and 3, and the left vertebral artery in 6 and 2, respectively. Typical cases of neurovascular contact are shown in Figures 1 and 2.
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Blood pressure levels, heart rate, and other clinical characteristics were comparable between subjects with positive, borderline, and negative neurovascular contact, both in the normotensive (Table 2) and hypertensive groups (Table 3). In treated hypertensive patients, the distribution of medication was likewise similar between the patients with different brain stem findings. In untreated hypertensive patients, resting and ambulatory blood pressure, heart rate, and blood pressure and heart rate variability (as assessed by standard deviation of ambulatory measurements) were not different between the groups (Table 4). In separate post hoc analyses for patients with right-sided medullary contact, there were likewise no significant differences between patients with and without neurovascular contact (data not shown).
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Discussion |
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In the present study, left-sided neurovascular contact of the IX/X cranial nerve root-entry zone was more frequent in patients with essential hypertension (23%) than in age- and sex-matched normotensive control subjects (16%), but this difference was not statistically significant (P=0.12). Furthermore, blood pressure levels, heart rate, and number of antihypertensive medications in treated hypertensive patients were similar among patients with positive, borderline, or negative brain stem findings. Thus, the present study does not entirely rule out an association of left-sided neurovascular brain stem contact with hypertension, but it does cast doubt on the importance of this feature as a sufficient cause of essential hypertension or as a major factor determining the severity of hypertension in affected patients.
Previous reports, estimating the frequency of left-sided neurovascular brain stem contact at 60% to 80% of patients with essential hypertension,11 12 16 clearly represent a gross overestimation of the prevalence of this vascular anomaly. Rather, our findings are more in line with recent reports,21 23 24 which have suggested a frequency of neurovascular contact of 30% to 50% in patients with essential hypertension. However, the present study also demonstrates that left-sided vascular contact to the brain stem within the area believed to be relevant for blood pressure control is by no means a rare finding in age- and sex-matched normotensive control subjects. Evidently, left-sided neurovascular contact similar to that found in some patients with hypertension by itself is not enough to produce hypertension in these individuals.
According to microanatomic studies,9 25 the determination of neurovascular contact in the assessment of hypertension requires visualization of the most cranial and caudal fibers of the IX/X nerve complex, because these structures define the cranial and caudal borders of the root-entry zone. However, as pointed out by previous investigators,20 21 the limited spatial and contrast resolution of imaging techniques used in previous studies11 12 16 17 21 23 may have resulted in some of the discrepant results reported in the literature, inasmuch as the exact boundaries of the area of contact were probably not always evident in these studies.
Increased resolution, even allowing the visualization of nerve fibers in the 1-mm range, although essential for appropriate characterization of anatomic structures, also augments the issue of where exactly neurovascular contact should be considered positive and where not. This is because experimental or interventional data that precisely define the rostral and posterior border of the brain stem region where vascular contact within the retro-olivary sulcus may cause hypertension in humans are currently lacking. It can also not be ruled out that as in other neurovascular compression syndromes,26 vascular contact to individual nerve fibers or the nerve bundle rather than direct contact to the medullary parenchyma may be of pathophysiological significance. Another relevant point to be determined is whether looping or some kind of curvilinear appearance or pulsatility of the offending artery must be present, as suggested by Jannetta and Gendell.4
In the present study, we focused on the presence of left-sided contact, because this was the lesion originally described by Jannetta and Gendell5 and subsequently by Naraghi et al.11 Furthermore, experimental evidence also demonstrated that left-sided pulsatile compression of the medulla raises blood pressure in primates.10 These authors11 also speculated on the importance of the left medulla, because this region receives important afferent fibers from the myocardial receptors of the left ventricle and atrium to the nucleus tractus solitarii via low-myelinated cardiac C fibers of the left vagus nerve.27 However, it may be of interest to note that exploratory analysis of the data revealed a higher frequency of right-sided brain stem contact in the hypertensive subjects (P<0.05). In fact, when positive brain stem contact was defined as a vascular contact present on the left, right, or both sides, this finding was more common in hypertensive than in normotensive subjects (39% versus 25%, respectively; P<0.05). However, because this was not the "primary end point" of the present study, one must be cautious in interpreting this finding. Nevertheless, future studies may seek to address the presence and relevance of neurovascular contact on either side of the brain stem in hypertensive patients.
If one assumes, as suggested by previous investigators,6 7 8 10 that neurovascular contact compromises autonomic blood pressure control, the use of advanced imaging technique in combination with extensive hemodynamic studies on baroreceptor reflexes and vagal function will be required before the relevance of morphological abnormalities and their potential involvement in the pathophysiology of hypertension can be determined. This question has recently been addressed by Makino et al,28 who performed extensive autonomic function tests in 14 hypertensive patients with left- or right-sided neurovascular contact versus 11 hypertensive control subjects. There was no difference in average 24-hour ambulatory systolic blood pressure between the groups, although diastolic blood pressure and heart rate were significantly higher in the positive group. However, responses of blood pressure and heart rate to mental stress, cold pressor, hand grip, Valsalva maneuver, phenylephrine infusion, and clonidine tests were not different between the groups. Thus, the functional relevance of the "positive" finding remains doubtful.
Although we found no clear association between hypertension and neurovascular contact in the present study, it should be noted that given the heterogeneity of hypertension, a single factor is unlikely to account for a large proportion of essential hypertension in the population. Our findings in white individuals may not be applicable to other populations in which different genetic factors may be operable. Furthermore, the presence of neurovascular contact in normotensive subjects by no means rules out the possible role of this mechanism in the development of hypertension, because normotensive individuals have a great compensatory capacity that may counteract the development of hypertension in these individuals.
Despite several reports on cure or amelioration of hypertension in patients with essential hypertension after surgical decompression of the ventrolateral medulla,14 18 19 29 these reports have been received with due skepticism from experts in the field of hypertension.30 31 32 Thus, as pointed out by Kaplan30 and DeQuattro and Giannotta,31 these reports are best described as uncontrolled anecdotal observations in which a nonspecific effect of the surgical intervention cannot be ruled out. The criteria presented to define "uncontrolled" hypertension, which was regarded as an indication for surgery in these studies, were inadequate or did not apply to the patients presented in these studies. In fact, truly uncontrollable or resistant hypertension is a rare finding in clinical practice and is present in <1% of all patients referred as "uncontrollable" to the hypertension specialist.30 In light of our present observation that left-sided ventromedullary contact is not associated with higher blood pressure or heart rate in patients with hypertension and is also present in a considerable proportion of age- and sex-matched normotensive control subjects, we suggest that additional criteria, such as functional data demonstrating impairment of autonomic blood pressure control in patients with positive morphological features, should be present before surgery (even in the setting of experimental protocols) can be considered. Clearly, future morphological imaging should make use of advanced magnetic resonance protocols, as in the present study, which provide a higher resolution of the anatomic features of the brain stem.
In summary, the present study shows that left-sided neurovascular contact can be found in a considerable proportion of both normotensive and hypertensive individuals and appears to be unrelated to the severity of hypertension or to blood pressure levels in these individuals. Therefore, these data do not support the hypothesis that neurovascular contact of the IX/X root-entry zone is, per se, an important cause of essential hypertension. However, our findings do not rule out the possibility that neurovascular contact may affect autonomic control and perhaps contributes to the development of hypertension in some individuals. Nevertheless, further studies are required to prove this assumption.
Received March 20, 2000; first decision April 12, 2000; accepted July 10, 2000.
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