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CLINICAL RESEARCH: GENERAL CARDIOLOGY

Relationship of neurovascular compression to central sympathetic discharge and essential hypertension

Paul A. Smith, MB, ChB^*,*, James F. M. Meaney, FRCR, Lee N. Graham, MB, ChB^*, John B. Stoker, BSc, MB, ChB^*, Alan F. Mackintosh, MA, MD^*, David A. S. G. Mary, MB, ChB, PhD^* and Stephen G. Ball, MB, BChir, PhD^*

^* Departments of Cardiology and Radiology, Leeds Teaching Hospitals, Leeds, United Kingdom
MRI Department, St. James's Hospital, Dublin, Ireland

Manuscript received July 17, 2003; revised manuscript received November 17, 2003, accepted November 25, 2003.

^* Reprint requests and correspondence: Dr. Paul A. Smith, Cardio-Respiratory Unit, St. James's University Hospital, Beckett Street, Leeds LS9 7TF, United Kingdom.
paulsmith.leeds{at}ukgateway.net

OBJECTIVES: We planned to examine the relationship between neurovascular compression (NVC) of the rostral ventrolateral medulla (RVLM) and the magnitude of central sympathetic hyperactivity in normal subjects and in patients with untreated and uncomplicated essential hypertension (EHT).

BACKGROUND: Previously it has not been possible to establish a definitive relationship between EHT and NVC of the RVLM, a location containing efferent sympathetic vasoconstrictor neurons. Furthermore, the relationship between NVC and magnitude of sympathetic nerve hyperactivity has not been adequately examined, despite the knowledge that hyperactivity varies according to EHT severity.

METHODS: In 83 subjects, we used magnetic resonance imaging to detect NVC and, independently, peroneal microneurography to quantify muscle sympathetic nerve activity (MSNA), expressed as the mean frequency of multi-unit discharge (m-MSNA) and of single units (s-MSNA). Subjects were classified according to arterial pressure values into groups with normal (NT) (n = 24) or high-normal (HN) (n = 14) arterial pressure and mild (EHT-1) (n = 26) or severe (EHT-2/3) (n = 19) EHT.

RESULTS: A significantly greater sympathetic activity was found in 23 subjects with NVC, compared with 60 subjects without NVC. The prevalence of NVC and the magnitude of sympathetic hyperactivity were greater in the EHT-1 group (p < 0.05) than in the other three groups. There was no significant difference in confounding variables between the groups. Although increased sympathetic activity was strongly predictive of NVC, this was not significantly related to baroreceptor sensitivity controlling the pulse interval (cardiac baroreceptor reflex sensitivity).

CONCLUSIONS: Neurovascular compression of the RVLM may cause central sympathetic activation in normal and hypertensive populations and therefore has significant implications regarding the pathogenesis of EHT.

Abbreviations and Acronyms   BRS = baroreceptor reflex sensitivity   EHT = essential hypertension   EHT-1 = group classification for mild essential hypertension   EHT-2/3 = group classification for severe essential hypertension   HN = group classification for high-normal arterial pressure   JNC = Joint National Committee   m-MSNA = multi-unit muscle sympathetic nerve activity   NT = group classification for normal arterial pressure   NVC = neurovascular compression   RVLM = rostral ventrolateral medulla   s-MSNA = single-unit muscle sympathetic nerve activity

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