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Rheoencephalography (REG)

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What is Rheoencephalography (REG)?

Rheoencephalography (REG) is a non-invasive method based on impedance plethysmography, historically developed to study pulsatile blood volume changes in the head. It involves passing a weak, high-frequency alternating electrical current through the scalp via electrodes and measuring the resulting changes in electrical impedance (resistance). The underlying theory was that since blood is more conductive than surrounding tissues, the cyclical increase in blood volume within vessels during each heartbeat would cause a measurable decrease in impedance. These impedance fluctuations, synchronized with the pulse, were recorded graphically as the REG waveform.

The intended purpose of REG was to gain insights into:

  • Cerebral vascular tone (vasoconstriction/vasodilation)
  • Elasticity of blood vessels within the head
  • Overall cerebral blood filling dynamics
  • Regional differences in blood pulsation

Important Historical Context: REG is now considered an obsolete diagnostic technique in contemporary neurology and cerebrovascular medicine. Its clinical utility is severely limited by fundamental flaws, most notably the significant contamination of the signal by blood flow in the extracranial tissues (scalp and muscles). It cannot reliably differentiate between intracranial and extracranial circulatory changes. Modern neurovascular assessment relies on technologies like Transcranial Doppler (TCD) ultrasound, CT Angiography (CTA), MR Angiography (MRA), perfusion imaging (CT/MRI), and Digital Subtraction Angiography (DSA), which provide direct, specific, and reliable information.

Rheoencephalography (REG), a historical method using scalp electrodes to measure impedance changes related to blood volume pulsation, is now considered obsolete due to significant limitations.

Historical Indications for Rheoencephalography

While REG is not recommended for diagnosis by current medical standards, it was historically explored or applied by some physicians for various conditions involving suspected alterations in cerebral circulation. The information obtained was indirect and lacked reliability. These historical uses included evaluation of:

Modern diagnostic standards rely on validated imaging and functional tests for these conditions.

Principles and Technique

REG operates on the principle that blood volume changes within the tissue segment between electrodes alter the segment's electrical impedance. A low-intensity (safe microampere levels), high-frequency (e.g., 30-100 kHz) alternating current is applied. The resulting voltage variations, inversely proportional to impedance changes caused by pulsatile blood flow, are detected, amplified, and displayed as the REG waveform, typically alongside an ECG for timing reference.

The typical procedure involved:

  1. Cleaning the scalp at electrode sites with alcohol.
  2. Applying conductive paste to metal electrodes (often circular plates).
  3. Placing electrodes according to specific montages, such as:
    • Fronto-mastoid: Aiming to capture signals predominantly influenced by the internal carotid circulation territory.
    • Occipito-mastoid: Aiming to capture signals related to the vertebrobasilar circulation territory.
    • Other montages were also used to attempt more localized recordings.
  4. Recording baseline waveforms.
  5. Sometimes performing functional tests like changes in head position, hyperventilation, breath-holding, or administration of vasoactive drugs, to observe waveform changes (interpretation remained problematic).

Interpretation and Limitations

Historical interpretation of REG waveforms involved analyzing various morphological features:

  • Amplitude: Thought to correlate with the amount of pulsatile blood filling.
  • Anacrotic Phase (Upstroke): Speed and shape related to inflow velocity and vessel wall elasticity.
  • Catacrotic Phase (Downstroke): Shape, slope, and presence/prominence of the dicrotic notch related to vascular tone and runoff.

Increased vascular tone was inferred from slower upstrokes, rounded peaks, and reduced dicrotic notches. Decreased tone was inferred from faster upstrokes, sharper peaks, and prominent dicrotic notches. Studies following TBI sometimes showed waveform changes interpreted as evolving vasospasm.

However, the clinical value of this interpretation was severely hampered by major limitations:

  • Extracranial Contamination: The dominant limitation. The measured impedance changes reflect blood flow in *all* tissues between the electrodes, including the highly vascular scalp and muscles supplied by the external carotid artery. It is impossible to reliably isolate the intracranial contribution, rendering conclusions about cerebral circulation highly speculative.
  • Lack of Specificity: Waveform morphology is affected by numerous systemic factors (heart rate, blood pressure, cardiac contractility, blood viscosity) and technical factors (electrode contact, placement, skin resistance) unrelated to intracranial vascular tone or flow.
  • Absence of Anatomical Data: REG provides no information about the structure of blood vessels.
  • Poor Standardization and Reproducibility: Results varied significantly, making reliable comparisons difficult.
  • Weak Physiological Correlation: The exact quantitative relationship between REG parameters and specific physiological variables like cerebral blood flow (CBF) or cerebrovascular resistance (CVR) was never robustly established.

These fundamental flaws led to the abandonment of REG in favor of modern, validated neurovascular diagnostic methods.

References

Note: Due to the obsolescence of REG, references focus on its historical context, principles, or critical evaluation.

  1. Jenker FL. Rheoencephalography: A method for the continuous registration of cerebrovascular changes. Springfield, IL: Charles C Thomas; 1962. (A foundational text describing the method).
  2. Lechner H, Rodler S. Rheoencephalography. In: Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 8B. Elsevier; 1976: 59-78. (Chapter in a historical handbook detailing the technique).
  3. McHenry LC Jr. Rheoencephalography. A clinical appraisal. Neurology. 1965 Feb;15:104-13. DOI: 10.1212/wnl.15.2.104. PMID: 14261397. (Early critical appraisal highlighting limitations).
  4. Perez-Borja C, Meyer JS. A critical evaluation of rheoencephalography in control subjects and in proven cases of cerebrovascular disease. J Neurol Neurosurg Psychiatry. 1964 Feb;27(1):66-72. DOI: 10.1136/jnnp.27.1.66. PMID: 14117682; PMCID: PMC495713. (Another critical evaluation questioning its utility).
  5. Yarullin KhKh. [Clinical Rheoencephalography]. Meditsina; 1983. [Russian]. (Example of a monograph from a period when REG was more widely used, particularly in certain regions).