Some Instrumentation for Clinical Use

Last updated: 2025-01-29

Some Instrumentation for Clinical Use

Instrumentation has been developed to study details of tissue and air movement within the glottal cycle, providing quantitative assessment of vibratory patterns beyond what can be observed through laryngoscopy alone. Two principal techniques—photoglottography and electroglottography—offer complementary information about vocal fold oscillation.

Photoglottography (PGG)

Photoglottography measures the amount of light transmitted through the glottis during phonation, providing an indirect measure of glottal area variation over time.

Measurement Principle

A light source is placed at one end of the glottal airway (typically in the oral cavity), and a photosensor is placed at the other end (typically external to the neck). The amount of light transmitted correlates with the minimum glottal area along the light path—essentially the area at the narrowest point from inferior to superior.

Waveform Characteristics

The PGG waveform represents minimum glottal area as a function of time, similar to the displacement waveform shown earlier in this chapter (Figure 4.11a). Key features include:

Opening Phase: Progressive increase in transmitted light as glottis opens

Maximum Opening: Peak of PGG signal corresponds to maximum glottal area

Closing Phase: Progressive decrease in transmitted light as glottis closes

Closed Phase: Minimum signal (ideally zero if complete closure occurs)

Clinical Parameters

Clinicians and researchers extract several parameters from PGG signals:

Open Quotient (OQ): Ratio of open time to total period

OQ = T_open / T_total

Speed Quotient (SQ): Ratio of opening time to closing time, indicating asymmetry

Closing Slope: Abruptness of glottal closure, related to contact velocity

These parameters provide quantitative measures of vibratory pattern characteristics that correlate with voice quality and pathological conditions.

Methodological Considerations

Several factors affect PGG signal quality and interpretation:

Light Source Position: Oral placement is most common but may be affected by vocal tract configuration

Sensor Placement: External neck placement picks up transmitted and scattered light

Signal Amplitude: Depends on light intensity, tissue properties, and sensor sensitivity—absolute calibration is difficult

Vertical Phase Effects: The PGG signal represents minimum area at any vertical level, which may not correspond simply to movement at a single location

Summary Articles

Comprehensive reviews of photoglottography have been provided by:

  • Sonesson (1960): Early comprehensive review
  • Baer, Löfqvist, and McGarr (1983): Comparison with high-speed filming
  • Kitzing and Löfqvist (1979): Clinical applications
  • Hanson, Gerratt, and Berke (1990): Effects of frequency and intensity
  • Gerratt et al. (1991): Clinical synopsis

These resources detail measurement techniques, validation studies, and clinical interpretation.

Electroglottography (EGG)

Electroglottography (also called laryngography or electrolaryngography) measures electrical impedance across the neck at the level of the larynx, providing information about vocal fold contact.

Measurement Principle

A small, high-frequency electrical current (safe and imperceptible to the subject) is passed between two electrodes placed on opposite sides of the neck at the thyroid cartilage level. The electrical impedance between the electrodes varies with vocal fold contact:

Folds Separated: High impedance (air is a poor conductor)

Folds in Contact: Lower impedance (tissue conducts better than air)

The EGG waveform thus reflects the contact area between the vocal folds over time.

Waveform Characteristics

Unlike PGG (which measures opening), EGG emphasizes the closed phase:

Contact Phase: Rising EGG signal as vocal folds come together and contact area increases

Maximum Contact: Peak of EGG signal

Decontact Phase: Falling EGG signal as vocal folds separate and contact area decreases

Open Phase: Minimum signal (though baseline may not reach zero due to parallel current paths)

Clinical Parameters

Key measures derived from EGG include:

Closed Quotient (CQ): Ratio of closed time to total period (complement of OQ when complete closure occurs)

Contact Quotient: More precisely, ratio of contact time to period

Decontact Slope: Steepness of falling edge, indicating speed of separation

Perturbation Measures: Cycle-to-cycle variations in period or amplitude

Advantages of EGG

EGG offers several practical advantages:

Non-invasive: External electrodes, comfortable for extended recording

Continuous: Can record for long periods without fatigue

Synchronized with Audio: Easily recorded simultaneously with acoustic signal

Vertical Information: Sensitive to vertical phase differences (bottom contacts before top in normal vibration)

Limitations and Caveats

Interpretation of EGG requires awareness of several factors:

Not Direct Measurement: EGG measures impedance, which relates complexly to contact area and tissue properties

Baseline Drift: Low-frequency components can drift due to electrode movement or tissue hydration changes

Amplitude Calibration: Absolute impedance values vary with electrode placement, tissue composition, and hydration—relative changes within a recording are more reliable

Open Phase Ambiguity: EGG provides little information about vocal fold position when they are separated

Individual Variation: Optimal electrode placement and signal interpretation may vary among individuals

Methodological Reviews

Comprehensive reviews of electroglottography include:

  • Lecluse (1977): Detailed technical description (in Dutch)
  • Childers and Krishnamurthy (1985): Critical review of methodology
  • Baken (1987): Clinical measurement context
  • Colton and Conture (1990): Problems and pitfalls
  • Titze (1990): Interpretation guidelines
  • Kitzing (1990): Clinical applications with extensive bibliography

These sources provide technical background, validation studies, and guidance for clinical use.

Combined Techniques

Optimal assessment often combines multiple approaches:

EGG + Audio: Simultaneous recording allows correlation of vibratory patterns with acoustic output

EGG + Videostroboscopy: Karnell (1989) demonstrated synchronized recording to identify specific movement events within the cycle

PGG + High-Speed Imaging: Direct validation of PGG interpretation against observed tissue motion

Multiple Modalities: Comprehensive assessment uses laryngoscopy (for structure), stroboscopy (for vibratory pattern visualization), EGG or PGG (for quantitative measures), and acoustic analysis (for output characteristics)

Clinical Applications

These instrumental techniques serve several clinical purposes:

Diagnosis

Quantitative measures may reveal:

  • Asymmetry in left-right fold vibration
  • Incomplete closure (high OQ, low CQ)
  • Aperiodicity suggesting mass lesions or neurological disorder
  • Abnormal contact patterns in muscle tension dysphonia

Treatment Monitoring

Repeated measurements track:

  • Response to voice therapy
  • Recovery after surgery
  • Effects of medical treatment
  • Progression of degenerative conditions

Biofeedback

Visual display of EGG or PGG can provide:

  • Real-time feedback during therapy exercises
  • Targets for achieving specific vibratory patterns
  • Immediate reinforcement of correct production

Research

Instrumentation enables:

  • Systematic study of normal vocal fold function
  • Validation of theoretical models
  • Investigation of voice disorder mechanisms
  • Development of new diagnostic criteria

Future Directions

Ongoing development aims to:

Improve Accuracy: Better understanding of how signals relate to underlying tissue motion

Enhance Accessibility: Lower-cost, more portable systems for wider clinical use

Expand Parameters: New measures that capture clinically relevant vibratory characteristics

Integrate Technologies: Combining multiple modalities for comprehensive assessment

Standardize Protocols: Consensus on measurement procedures and interpretation guidelines

The continued evolution of these tools promises enhanced capability for assessing vocal fold oscillation in both clinical and research contexts.

Summary

Photoglottography and electroglottography provide complementary information about vocal fold vibratory patterns. PGG measures transmitted light as an index of glottal area variation, emphasizing opening characteristics. EGG measures electrical impedance as an index of contact area, emphasizing closing and contact characteristics.

Both techniques yield quantitative parameters including quotients (proportions of the cycle) and slopes (speeds of opening or closing) that correlate with voice quality and pathology. Methodological considerations include signal interpretation, calibration challenges, and individual variation. Combined with laryngoscopy and acoustic analysis, these instrumental techniques enable comprehensive assessment of vocal fold oscillation for diagnosis, treatment monitoring, and research.

Key Takeaways

  • ✅ Photoglottography measures glottal area variation through transmitted light, emphasizing opening phase
  • ✅ Electroglottography measures vocal fold contact through electrical impedance, emphasizing closed phase
  • ✅ Both techniques provide quantitative measures of vibratory patterns complementing visual observation
  • ✅ Parameters like open quotient, closed quotient, and various slopes correlate with voice quality and pathology
  • ✅ Combined instrumental assessment with multiple modalities yields most comprehensive evaluation

Further Reading

  1. Titze, I. R. (1990). Interpretation of the electroglottographic signals. Journal of Voice, 4, 1-9.
  2. Baken, R. (1987). Clinical Measurement of Speech and Voice. Boston: College Hill Press.
  3. Gerratt, B. R., Hanson, D. G., Berke, G. S., & Precoda, K. (1991). Photoglottography: A clinical synopsis. Journal of Voice, 5(2), 98-105.
  4. Kitzing, P. (1990). Clinical applications of electroglottography. Journal of Voice, 4(3), 238-249.
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