Diagnosing Resonance Disorders with Confidence

Resonance disorders can be challenging to diagnose, even for experienced clinicians. However, accurate diagnosis is critical to providing proper recommendations for treatment. Explore what determines resonance for speech, the characteristics of different types of resonance disorders, and methods for diagnosis so you can treat resonance disorders confidently and appropriately.

Normal Resonance

What exactly is “resonance”? If you look up this word, you may discover the following definitions:

• A vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system
• The tendency of a system to vibrate with increasing amplitudes at particular frequencies
• A phenomenon in which an external force or a vibrating system forces another system around it to vibrate with greater amplitude at a specified frequency of operation

Wow! These definitions are hard to understand, particularly as they relate to resonance during speech. Gunnar Fant offered a more accurate explanation in 1960 with his source-filter theory.

Fant explained that sound is generated by vibration of the vocal folds, which is the source. Sound is then altered through selective enhancement of the formant frequencies as it travels through the cavities of the vocal tract, which are the filters.

I would expand on that definition by saying that resonance is the unique quality of the “voice” (as distinct from pitch and intensity from phonation) that occurs when particular frequencies from the phonated sound are selectively enhanced based on the size and shape of the cavities of the vocal tract.

Factors that affect resonance during speech include:

• The function of the velopharyngeal valve, which is consistent with normal speakers:
  • An open velopharyngeal valve sends sound into the nasal cavity
  • A closed velopharyngeal valve sends sound into the oral cavity
• The size and shape of the resonating cavities, which greatly vary among normal speakers, including:
  • the length and volume of the pharynx
  • the size and shape of the oral cavity
  • the configuration of the nasal cavity

Due to the rules of physics, smaller cavities will enhance higher formants, while larger cavities will enhance lower formants. These cavity size differences result in a different perception of pitch between children and adults, men and women, and tall and short people. Statistically, men have larger pharyngeal cavities than women, regardless of height. Overall, resonance is what makes voice quality unique to an individual.

Resonance is particularly important during the production of vowel sounds. Vowels are produced by phonating and then changing the size and shape of the oral cavity with the jaws and tongue to result in a discrete sound that differentiates each vowel.

Resonance Disorders

A resonance disorder occurs when there is an abnormal transmission of sound energy through the oral, nasal, and/or pharyngeal cavities of the vocal tract during speech production. The types of resonance disorders include:

Hypernasality

• Speech characteristics: Hypernasality occurs when there is too much sound resonating in the nasal cavity during the production of oral speech sounds. It is most perceptible on vowels, which are resonance sounds, because vowels are voiced and relatively long in duration. In severe hypernasality, voiced oral consonants become nasalized (m/b, n/d, ŋ/g), which is an obligatory distortion. Additionally, other consonants may be substituted by nasals (n/s) as a compensatory strategy. Hypernasality is accompanied by low volume because the sound becomes muffled as it travels through the nasal cavity. Hypernasality severity depends on the size of the opening, the etiology, and even articulation.
• Causes: Typically, the cause of hypernasality is velopharyngeal insufficiency (VPI), which is a structural abnormality; velopharyngeal incompetence (VPI), which is a neurogenic disorder; or an oronasal fistula secondary to cleft palate. Hypernasality can also be phoneme-specific due to abnormal articulation. Abnormal articulation occurs when a nasal sound is substituted for an oral sound (n/l, ŋ/l, ŋ/r).

Hyponasality

• Speech characteristics: Hyponasality occurs when there is not enough nasal resonance on nasal sounds (m, n, ŋ). As such, nasal phonemes sound similar to their oral cognates (b/m, d/n, g/ŋ). Hyponasality can also affect vowels because some sound is transmitted through the velum during vowel production.
• Causes: The cause of hyponasality is pharyngeal obstruction (i.e., hypertrophic adenoids, narrow pharynx due to midface retrusion, VPI surgery) or nasal congestion or obstruction.

Cul-de-Sac Resonance

• Speech characteristics: Cul-de-sac resonance occurs when there is a blockage at the exit (thus, cul-de-sac) of one of the cavities of the vocal tract. This blockage causes the voice to sound muffled and low in volume. Parents may describe their child’s speech as “mumbling.” Consonants may also be weak and indistinct due to blockage of the airflow.
• Causes: Oral cul-de-sac resonance is caused by a small mouth opening (microstomia). Nasal cul-de-sac resonance is caused by a combination of VPI and stenotic nares (which can occur with cleft lip and palate). Finally, pharyngeal cul-de-sac resonance is caused by large “kissing” tonsils and is the most common form of cul-de-sac resonance.

Mixed Resonance

• Speech characteristics: Mixed resonance occurs when there is hypernasality on oral sounds and hyponasality on nasal sounds.
• Causes: Mixed resonance can occur when both VPI and hypertrophic adenoids are present. It also commonly occurs with apraxia.

Simple Evaluation Methods

You can often determine the type of resonance based solely on the speech characteristics noted above. However, you can easily confirm the diagnosis by using one simple tool—a straw!

The procedure is as follows:

1. Put a bending straw (or tube) in the child’s nostril and the other near your ear.
2. With the straw in place, have the child repeat syllables with pressure-sensitive oral consonants (plosives and fricatives) and then nasal consonants using the vowels /ɑ/ and then /i/ (i.e., pɑ, pɑ, pɑ, pɑ, pɑ; pi, pi, pi, pi, pi; etc.).
   1. If you hear sound through the straw on oral sounds, hypernasality is present.
   2. If you don’t hear sound through the straw, or if the sound is reduced on nasal sounds, hyponasality is present. Note: Be sure to test both nostrils.
   3. If the sound is low in volume and muffled, check the oral cavity opening, nares, and particularly the tonsils for possible blockage.

Referrals for Resonance Disorders

The following is a guideline for appropriate medical and/or surgical referrals based on the type of resonance:

• Hypernasality: Refer to a cleft/craniofacial team. Do not refer to a general ENT. If it is phoneme-specific, recommend speech therapy.
• Hyponasality or cul-de-sac resonance: Refer to an ENT for an evaluation and treatment of the obstruction.

Diagnosing resonance disorders can be made easier for SLPs by understanding the characteristics of each type of resonance disorder. ENTs and surgeons are untrained in speech disorders, making differential diagnosis a task not in their purview. SLPs must make the correct diagnoses for our patients/clients to ensure they receive appropriate and timely care.

To learn more, I offer four MedBridge courses that discuss and demonstrate the latest research and methods for evaluating and treating speech and resonance disorders in pediatric populations.