Instrumental assessment of velopharyngeal function and resonance: A review

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Journal of Communication Disorders

Instrumental assessment of velopharyngeal function and resonance: A review Kim Bettens a,*, Floris L. Wuyts a,b, Kristiane M. Van Lierde a a b

Department of Speech, Language and Hearing Sciences, Ghent University, Ghent, Belgium Biomedical Physics, University of Antwerp, Antwerp, Belgium

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 January 2014 Received in revised form 14 April 2014 Accepted 16 May 2014 Available online xxx

The purpose of this literature review is to describe and discuss instrumental assessment techniques of the velopharyngeal function in order to diagnose velopharyngeal disorders and resonance characteristics. Both direct and indirect assessment techniques are addressed, in which successively nasopharyngoscopy, videofluoroscopy, magnetic resonance imaging (MRI), cephalometric radiographic analysis, computed tomography (CT), ultrasound, acoustic and aerodynamic measurements are considered. Despite the multiple instrumental assessments available to detect and define velopharyngeal dysfunction, the ideal technique is not yet accessible. Therefore, a combination of different quantitative parameters can possibly form a solution for a more reliable determination of resonance disorders. These multi-dimensional approaches will be described and discussed. The combination of quantitative measurement techniques and perceptual evaluation of nasality will probably remain necessary to provide sufficient information to make appropriate decisions concerning the diagnosis and treatment of resonance disorders. Learning outcomes: The reader will be able to describe and discuss currently available instrumental techniques to assess the velopharyngeal mechanism and its functioning in order to diagnose velopharyngeal disorders. Additionally, he will be able to explain the possible advantages of the combination of several types of complementary measurement techniques. ß 2014 Elsevier Inc. All rights reserved.

Keywords: Velopharyngeal dysfunction Resonance Nasality Instrumental assessment techniques

1. Introduction During speech production, the closure of the velopharyngeal mechanism is necessary to produce oral speech sounds by retraction and elevation of the velum in combination with lateral and posterior pharyngeal wall movements (Perry, 2011). Resonance disorders can arise due to structural abnormalities (i.e. velopharyngeal insufficiency), a neurogenic disorder (i.e. velopharyngeal incompetence) or functional problems (i.e. velopharyngeal mislearning) (Kummer, 2011a). These perturbations can induce resonance disturbance, such as hypernasality, nasal emission and compensatory mechanisms

Abbreviations: MRI, magnetic resonance imaging; CT, computed tomography; SNORS, super nasal oral ratiometry system; MDT, maximum duration time; NSI, nasality severity index. * Corresponding author at: Universitair Ziekenhuis Gent, Vakgroep Spraak-, Taal-en Gehoorwetenschappen, De Pintelaan 185, 2P1, BE-9000 Gent, Belgium. Tel.: +32 9 332 94 26; fax: +32 9 332 54 36. E-mail address: [email protected] (K. Bettens). http://dx.doi.org/10.1016/j.jcomdis.2014.05.004 0021-9924/ß 2014 Elsevier Inc. All rights reserved.

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(Kummer, 2011a). Compensatory mechanisms can consist of compensatory articulation, like glottal stops and pharyngeal fricatives, and compensatory movements, for example the appearance of a nasal or facial grimace (Kummer, 2011a). To select the most effective treatment plan for these problems, several instruments have been developed during the last decades to subjectively or quantitatively investigate velopharyngeal function and resonance. First of all, the speech-language pathologist can perceptually assess resonance and articulation based on a speech sample consisting of, for example, a standardized articulation test, the repetition of syllables or sentences and automatic or spontaneous speech (Kummer, 2011b). In addition, perceptual tests can be applied to clarify this evaluation procedure, such as the Bzoch tests (Bzoch, 1989), used to assess hypernasality, nasal emission or hyponasality, and the Gutzmann a/i test (Gutzmann, 1913), used to evaluate hypernasality. However, articulation errors can influence the perceptual evaluation of resonance (Keuning, Wieneke, van Wijngaarden, & Dejonckere, 2002) and listener judgments between clinical centers are difficult to compare because of different speech samples and various evaluation protocols (Henningsson et al., 2008). Despite these limitations, perceptual assessments still remain the golden standard in the evaluation of resonance (Henningsson et al., 2008), because no existing instrumental assessment technique can yet transcend the possibilities of the human ear. To restrict the limitations of listener evaluations, several instrumental measurements are available to supplement perceptual assessments. These instrumental assessments can be divided in two groups: direct and indirect techniques (Table 1). Direct techniques directly visualize the velopharyngeal closing mechanism, whereas indirect techniques provide information from which the velopharyngeal activity can be inferred. This review discusses instrumental assessment techniques presently available. Because the assessment of resonance disorders is a comprehensive topic with multiple contradictions resulting in different assessment methods, clinical application of these techniques and their advantages and limitations are not always straightforward. Therefore, the current study presents a structured and critical overview for clinicians who make decisions about interventions in patients with resonance disorders. Additionally, possibilities to reduce the mentioned limitations of available techniques, considering a multi-dimensional approach to nasality assessment, will be discussed. 2. Direct assessment techniques Direct assessment techniques directly visualize the velopharyngeal closing mechanism and provide information about velopharyngeal gap size and shape. Based on this information, clinicians will identify patients who could benefit from treatment, quantify the severity of velopharyngeal dysfunction, select the most suitable treatment procedure and quantify changes after treatment (Witt, Marsh, McFarland, & Riski, 2000). The ideal direct assessment technique should be ‘‘noninvasive, easily repeatable, and reproducible; it should not use ionizing radiation, and should allow completely free choice of the image planes in all three dimensions’’ (Beer et al., 2004, p. 791). Successively, the procedure, purpose, advantages and limitations of nasopharyngoscopy, multiview videofluoroscopy, (dynamic) magnetic resonance imaging (MRI), lateral cephalometric radiographic analysis, computed tomography (CT) and ultrasound will be described and discussed. 2.1. Nasoendoscopy During the nasoendoscopic assessment, a fiberoptic scope is passed through a nostril into the nasofarynx and is placed above the velum to obtain a birds’-eye view on the velopharynx in rest and during real-time phonation (Lam et al., 2006; Rudnick & Sie, 2008; Silver et al., 2011) to identify the movement patterns during connected speech (Osberg & Witzel, 1981; Witt et al., 2000). To determine velopharyngeal disorders, a well-defined speech sample (Karnell, 2011) and good cooperation of the patient (Beer et al., 2004; Havstam et al., 2005; Silver et al., 2011; Witt et al., 2000) are necessary. An advantage of nasoendoscopy is that the patient is not exposed to ionizing radiation (Kuehn & Moller, 2000; Witt et al., 2000) which induces the possibility to use this technique during biofeedback training (Berkowitz, 2013; Van Lierde, Claeys, De Bodt, & Van Cauwenberge, 2004). Furthermore, Lam et al. (2006) found a stronger correlation between the grade of velopharyngeal insufficiency and the results of the nasoendoscopic assessment in comparison with multiview videofluoroscopy because nasoendoscopy indicates more precisely the grade of velar closing. However, the insertion of the scope is rather invasive, which can prevent children to cooperate (Beer et al., 2004; Havstam et al., 2005; Silver et al., 2011; Witt et al., 2000). Furthermore, the birds’-eye view represents only one image plane (the axial Table 1 Summary of direct and indirect assessment techniques discussed in this review. Direct assessment techniques

Indirect assessment techniques

Nasopharyngoscopy Multiview videofluoroscopy (Dynamic) magnetic resonance imaging Lateral cephalometric radiography Computed tomography Ultrasound

Acoustic measurements - Accelerometric techniques - Nasometry - Spectral characteristics Aerodynamic measurements - Nasal and oral airflow - Pressure-flow technique

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or ‘‘en face’’ view) (Kane, Butman, Mullick, Skopec, & Choyke, 2002; Sinclair, Davies, & Bracka, 1982) which does not always give an optimal view to the movements of the pharyngeal walls because of velar or adenoidal blockage (Henningsson & Isberg, 1991; Witt et al., 2000). Additionally, the visualization of underlying structures and muscles is not possible (Bae, Kuehn, Conway, & Sutton, 2011; Kane et al., 2002) and although qualitative evaluations of the velopharyngeal function can be performed, a quantitative analysis is difficult because of the two-dimensional representation of the three-dimensional anatomy (Kane et al., 2002; Lam et al., 2006; Silver et al., 2011). 2.2. Multiview videofluoroscopy Multiview videofluoroscopy is another technique to visualize the structure, movements, closing and timing of the velopharyngeal mechanism (Havstam et al., 2005). After a suspension of colloidal barium sulfate is applied to coat the nasopharynx to increase contrast, fluoroscopic images are acquired in lateral, frontal and base (‘‘en face’’) or Towne views (Kane et al., 2002; Silver et al., 2011; Skolnick, 1970; Stringer & Witzel, 1986) during connected speech. The simultaneous view of all articulators (Shprintzen, 2013) supply unique information about abnormal compensatory movements, such as tongue-backing and possible paradoxical movements of the velopharynx as well as timing abnormalities (Lam et al., 2006). By obtaining the lateral view, abnormalities of the velum, posterior pharyngeal wall and tongue as well as the presence of any Passavant ridge are easy to visualize (Stringer & Witzel, 1986; Witt, Myckatyn, Marsh, Grames, & Pilgram, 1998). Furthermore, the height of velopharyngeal closure can be identified with reference to the vertebrae (Silver et al., 2011) and the size of tonsils can be determined (Witt et al., 2000). The frontal view offers the possibility to demonstrate lateral wall movements (Witt et al., 2000). However, due to overlapping structures, it is sometimes difficult to analyze the images to collect sufficient information about the sphincteric closure (Stringer & Witzel, 1986). Therefore, a third view, such as the basal or Towne projection is necessary. The basal view reveals the relationship between the velum and the lateral-posterior aspects of the pharyngeal wall and is analogous to the nasoendoscopic view (Witt et al., 2000). It also provides information about the closure pattern of the velopharyngeal mechanism which has proved its clinical usefulness for both diagnosis and post-treatment evaluation (Witt et al., 2000), but is difficult to obtain and interpret in patients with large adenoids (Skolnick, McCall, & Barnes, 1973). In this case, the Towne projection, whereby the patient lies supine with the chin on the chest, can form a solution (Stringer & Witzel, 1986). Stringer and Witzel (1986) also found this position to be more comfortable for children to maintain. Although different projections are obtained during multiview videofluoroscopy, the three-dimensional anatomy of the velopharynx is reduced to a two-dimensional image which can create an overestimation of the velopharyngeal closing (Silver et al., 2011). Moreover, it can be challenging to interpret the images because of multiple shadows (Kane et al., 2002; Silver et al., 2011) and visualization of underlying muscles is not feasible (Bae et al., 2011). Even though the time to capture images is restricted, patients are exposed to ionizing radiation (Bae et al., 2011; Beer et al., 2004; Kane et al., 2002; Karnell, 2011; Silver et al., 2011). Finally, injecting the barium sulphate can raise resistance in young children (Beer et al., 2004; Silver et al., 2011), although multiview videofluoroscopy is experienced as less invasive compared to nasoendoscopy (Kuehn & Moller, 2000). 2.3. (Dynamic) magnetic resonance imaging Magnetic resonance imaging (MRI) is a more recent imaging technique that offers the possibility to collect images of different plane views (Beer et al., 2004; Drissi et al., 2011; Silver et al., 2011; Vadodaria, Goodacre, & Anslow, 2000) with high spatial resolution and superior visualization of soft tissues (Atik et al., 2008; Bae et al., 2011; Kane et al., 2002; Ozgu¨r, Tuncbilek, & Cila, 2000). The mid-sagittal view provides information about length, movement and extensibility of the velum, forward movement of the posterior pharyngeal wall during velopharyngeal closure and the presence of a Passavant ridge (Vadodaria et al., 2000). The coronal view shows the width of the pharynx and the contribution of the lateral pharyngeal wall in the velopharyngeal closure (Vadodaria et al., 2000). Finally, the axial view at the level of the hard palate offers information about type and extent of velopharyngeal closure (Vadodaria et al., 2000). One of the main advantages of MRI is that no ionizing radiation is used, so the assessment is repeatable and reproducable (Drissi et al., 2011; Rowe & D’Antonio, 2005). Recently, Maturo et al. (2012) described the possibility to synchronize audio and video samples from connected speech without delay, which creates the opportunity for real-time dynamic visualization in combination with phonatory assessment. Furthermore, MRI can also be used for the early assessment of submucuous cleft palate (Perry, Kuehn, Wachtel, Bailey, & Luginbuhl, 2011). Limitations of MRI are the high costs (Atik et al., 2008; Perry et al., 2012; Silver et al., 2011) together with the difficulty to test young children. Although Tian et al. (2010) reported that children from the age of 5 years old can participate without anesthesia if they receive preparatory instructions, fear, noice and lying quiet for a long time can cause uncooperativeness (Bae et al., 2011; Beer et al., 2004; Kane et al., 2002; Ozgu¨r et al., 2000; Silver et al., 2011; Vadodaria et al., 2000). Several authors (Beer et al., 2004; Ettema, Kuehn, Perlman, & Alperin, 2002; Silver et al., 2011) also indicate the possible consequences of gravity on the velum because of the supine position during the assessment. However, Perry et al. (2012) found only minimal effect of gravity on velar thickness, velar length, velar height, levator muscle length, angles of origin, and

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pharyngeal dimensions. At last, fixed intraoral metallic prostheses can affect the image quality negatively (Drissi et al., 2011; Ozgu¨r et al., 2000; Vadodaria et al., 2000). 2.4. Lateral cephalometric radiographic analysis Lateral cephalograms taken by X-ray and analyzed by specific software programs can be obtained to visualize the anatomy of the velopharyngeal structures at rest or during sustained phonation (Jakhi & Karjodkar, 1990; StellzigEisenhauer, 2001; Witt et al., 2000). With minimal patient compliance, standardized information about the relation of the soft tissues of the nasopharynx to the bony landmarks of the face and the cranium can be obtained (Kuehn & Moller, 2000; Witt et al., 2000). However, the interpretation of this information can be hampered by the presence of multiple shadows and patients are exposed to ionization radiation (Witt et al., 2000). Additionally, only limited and static information about the physiology of the velopharyngeal mechanism in the midsagittal plane can be offered by this technique which also reduces the threedimensional anatomy into a two-dimensional representation (Atik et al., 2008; Berkowitz, 2013; Stellzig-Eisenhauer, 2001; Witt et al., 2000). 2.5. Computed tomography Computed tomograph scans (CT-scans) can provide information about the anatomy of the velopharyngeal system in the axial plane in rest and during sustained phonation (Beer et al., 2004; Honjo, Mitoma, Ushiro, & Kawano, 1984). More specifically, the images can determine the level of velopharyngeal closure in relation to other soft tissues (Honjo et al., 1984) and can quantify surficial and deep craniofacial structures (Suri, Utreja, Khandelwal, & Mago, 2008). However, the limitations of this technique are comparable with those of cephalometric measurements: patients are exposed to ionizing radiation and only limited and static information about the velopharyngeal mechanism is obtained in a two-dimensional way (Atik et al., 2008; Beer et al., 2004; Honjo et al., 1984). Therefore, CT-scans are not often applied in the assessment of velopharyngeal dysfunction. 2.6. Ultrasound To visualize the anatomy and physiology of the lateral pharyngeal walls, ultrasound can be applied (Hawkins & Swisher, 1978). Therefore, a transducer in combination with the use of an acoustic coupling gel has to be placed against the neck, under the ear or behind the ramus of the mandible (Hawkins & Swisher, 1978). Despite the noninvasive character of this technique without ionizing radiation (Ryan & Hawkins, 1976), ultrasound offers too little advantages in the assessment of the velopharyngeal mechanism because of the restricted visibility of the (motion of) the velum (Baken & Orlikoff, 2000; Berkowitz, 2013; Hawkins & Swisher, 1978; Ozgu¨r et al., 2000) and too high interobserver and intersubject variability (Ryan & Hawkins, 1976). 3. Indirect assessment techniques Indirect assessment techniques provide information from which the velopharyngeal activity and possible malfunction can be inferred (Baken & Orlikoff, 2000). In contrast to direct techniques, these assessments aim to provide quantitative information about the degree of resonance or abnormal functioning of the velopharyngeal mechanism. Two categories of evaluation techniques can be distinguished to indirectly assess resonance and nasal airflow respectively: acoustic and aerodynamic measurements. 3.1. Acoustic measurements To determine the presence of resonance disorders, the measurement and analysis of the sounds a patient produces afford useful information. Therefore, several researchers are looking for acoustic parameters that can discriminate between resonance disorders and normal resonance. Acoustic measurements are frequently based on the relation between nasal and oral acoustic energy which can indicate nasality. Within this category, the use of accelerometric techniques, nasometry and spectral analysis will be discussed. 3.1.1. Accelerometry Horii (1980) evolved the Horii’s Oral Nasal Coupling (HONC) Index in which the ratio of the nasal accelerometer signal amplitude to the laryngeal accelerometer signal amplitude is computed to reduce the influence of vocal intensity. Two accelerometers are placed on the external surface of a speaker’s nose and throat. The index is expressed as

HONC ¼

ArmsðnÞ ðk  ArmsðvÞÞ

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where Arms(n) is the root-mean-square amplitude of the nasal accelerometer signal, Arms(v) is the root-mean-square amplitude of the vocal accelerometer signal and k is a constant which corresponds with a HONC value of one during sustained phonation of the sound/m/ (Horii, 1980). Due to the implementation of this constant, which compensates for variations of the accelerometer signals and interindividual differences, comparisons between and in speakers are possible. The HONC index ranges from zero to one, in which zero represents a complete oral signal and one represents the signal of a sustained sound/m/, or can be expressed in dB (Horii, 1980). The index can differentiate between normal and hypernasal speech, as well as between oral and nasal texts (Horii & Lang, 1981; Mra, Sussman, & Fenwick, 1998; Sussman, 1995), it has a moderate to strong correlation with perceptual evaluation (Horii, 1980; Horii & Lang, 1981; Laczi, Sussman, Stathopoulos, & Huber, 2005) and good interobserver reliability (Mra et al., 1998; Sussman, 1995). Furthermore, the assessment can be done based on sustained sounds or running speech and is noninvasive (Horii, 1980). However, the HONC index is rarely used in clinical or research settings (Kuehn & Moller, 2000) because it is not commercially available as a preassembled package (Laczi et al., 2005). The Nasality Oral Ratio Meter (NORAM) (Karling, Lohmander, De Serpa-Leita˜o, Galyas, & Larson, 1985) also utilizes nasal and laryngeal accelerometers by which the duration of the nasal (tN) and oral (tL) signal is measured and the ratio is calculated with the formula

n¼

tN tL  100

where n is the percentage of nasality. Although NORAM can be used to examine nasality before and after therapy (Karling, Larson, Leanderson, Galyas, & De Serpa-Leita˚o, 1993; Lohmander-Agerskov, Dotevall, Lith, & So¨derpalm, 1996), the low interand intraobserver reliability and the impossibility to distinguish normal resonance from hypernasality (Karling et al., 1993) cause a limited clinical and research application of this technique (Baken & Orlikoff, 2000). 3.1.2. Nasometry Another acoustic measurement technique that is based on the relation between nasal and oral acoustic energy is The Oral Nasal Acoustic Ratio (TONAR), originally developed by Fletcher and Bishop (1970) and marketed by Kay Elemetrics as the Nasometer. To determine the percentage of ‘nasalance’, a plate on a headset is fixed between the upper lip and the nose of the patient. Two microphones, one on the upper side and one on the underside of the plate, record the nasal and oral signal by a connected computer. The signals are then passed through a filter with a frequency of 500 Hz and a bandwidth of 300 Hz. The nasal and oral signals are compared to each other and a nasalance percentage is calculated with the formula

Nasalance % ¼

Nasal signal  100 Nasal þ oral signal

The results range from 0 to 100%. To detect hypernasality, a speech sample without nasal consonants can be used. To detect hyponasality, a speech sample loaded with nasal consonants is necessary. The Nasometer is applied in several clinical centers and constituted the subject of sundry studies (Shprintzen & Marrinan, 2009; Vijayalakshmi, Reddy, & O’Shaughnessy, 2007). As a quantitative measurement technique (Karnell, 2011), it has a good test-retest reliability (Watterson & Lewis, 2006), is noninvasive, convenient and easy to interpret. Therefore, the device can also be used during biofeedback training (Van Lierde, De Bodt, Van Borsel, & Van Cauwenberge, 1999). Subtle differences for language (Nichols, 1999; Okalidou, Karathanasi, & Grigoraki, 2011; Seaver, Dalston, Leeper, & Adams, 1991; Van Lierde, Wuyts, De Bodt, & Van Cauwenberge, 2001), gender (Rochet, Rochet, Sovis, & Mielke, 1998; Seaver et al., 1991; Van Lierde, Wuyts, Bodt, & Van Cauwenberge, 2003), age (Brunnegard & van Doorn, 2009; Haapanen, 1991; Rochet et al., 1998; Van Lierde, De Bodt, Baetens, Schrauwen, & Van Cauwenberghe, 2003; Van Lierde, Wuyts, et al., 2003) and race (Mayo, Floyd, Warren, Dalston, & Mayo, 1996) were found. Although, loudness (Watterson, York, & McFarlane, 1994), speech rate (Gauster, Yunusova, & Zajac, 2010) and type of oral consonants used in the speech sample (Watterson, Lewis, & Deutsch, 1998) have no significant effect on nasalance scores, the type of vowels can influence the results (Lewis, Watterson, & Quint, 2000). The authors (Lewis et al., 2000) mention that high vowels can lead to higher nasalance results. Several authors state good correlations with perceptual judgment (Hardin, Demark, Morris, & Payne, 1992; Sweeney & Sell, 2008; Watterson et al., 1998), however, different cutoff scores are used to determine sensitivity and specificity of the Nasometer (Brancamp, Lewis, & Watterson, 2010, 22%; Dalston, Warren, & Dalston, 1991, 32%; Hardin et al., 1992, 26%; Sweeney & Sell, 2008, 35%; Watterson et al., 1998, 26%). Moreover, it is difficult to compare these different cutoff scores because of the application of different methods across studies (Watterson et al., 1998). Some authors, on the other hand, report low correlations with perceptual assessment (Karnell, 2011; Keuning et al., 2002; Nellis, Neiman, & Lehman, 1992), which leads to a disagreement about the validity of the Nasometer. 3.1.3. Spectral analysis New techniques to determine nasality are introduced thanks to the current trend of digitalization approaching resonance by using digital processing techniques which are based on specific spectral characteristics of hypernasal speech. In literature,

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several characteristics are described, more specifically the introduction of pole-zero pairs in the region of the first formant (Maeda, 1982; Pruthi, Espy-Wilson, & Story, 2007; Schwartz, 1968; Vijayalakshmi et al., 2007), reduction of the amplitude of the first formant (Fant, 1970; Pruthi et al., 2007; Schwartz, 1968), increase of bandwidth of the first and second formant (Fant, 1970; Pruthi et al., 2007), shifts in formant frequencies (Fant, 1970; Hawkins & Stevens, 1985; Pruthi et al., 2007; Schwartz, 1968), rise in the amplitude between the first and second formant (Chen, 1997; Kataoka, Michi, Okabe, Miura, & Yoshida, 1996; Kataoka, Warren, Zajac, Mayo, & Lutz, 2001; Lee, Ciocca, & Whitehill, 2003; Vijayalakshmi et al., 2007; Yoshida et al., 2000) and decrease in the amplitude at or above the second formant (Kataoka et al., 1996, 2001; Lee, Ciocca, et al., 2003; Lee, Yang, & Kuo, 2003; Yoshida et al., 2000). Based on these spectral characteristics, computer logarithms are composed to discriminate between normal and hypernasal speech. Examples of such digital processing techniques are a onethird octave spectra analysis (Kataoka et al., 1996, 2001), the linear predictive model developed by Rah, Ko, Lee, and Kim (2001), a group delay-based formant extraction method (Vijayalakshmi et al., 2007) and the Voice Low Tone to High Tone Ratio (VLHR), developed by Lee, Yang, et al. (2003). For further information about the technical detail reference is made to the listed articles. The collection of spectral characteristics is objective, noninvasive and cost-effective (Vijayalakshmi et al., 2007). The equipment only consists of a good quality microphone, a computer with an analog-to-digital converter (Vijayalakshmi et al., 2007) and free software (Maier, Reuß, Hacker, Schuster, & No¨th, 2008). Nevertheless, the features are sometimes difficult to interpret (Baken & Orlikoff, 2000), are influenced by loudness or individual differences (Kataoka et al., 1996; Yoshida et al., 2000) and can only be extracted from vowels. Moreover, the effect of therapy cannot be verified because no degree of hypernasality is determined (Cairns, Hansen, & Kaiser, 1996) and the described correlations with nasalance scores are rather low (Rah et al., 2001). 3.2. Aerodynamic measurements Aerodynamic measurements can also be performed to evaluate the function of the velopharyngeal closure mechanism. Based on the principle that insufficient closure of the velopharyngeal mechanism causes increased nasal air escape (Warren, 1967), several tests measuring airflow and air pressure have been evolved to provide more information about the velopharyngeal functioning. Successively, techniques based on nasal and oral airflow and the pressure-flow technique (Warren & DuBois, 1964) will be described en discussed. 3.2.1. Nasal and oral airflow techniques To visualize the amount of nasal air escape, the mirror-fogging test by Glatzel (Foy, 1910) can be used. During this test, a cold mirror is held 0.5 cm under the nose of the subject during phonation of vowels or consonants. According to Glatzel, the degree of condensation is represented by four concentric circles (0–4) by which 0 corresponds to no condensation and 4 to severe condensation. The modified Glatzel mirror includes more than 4 concentric circles with 1 cm distance between the lines by which the degree of condensation can be calculated by the mathematical formula for an ellipse: S = a.b.p (Brescovici & Roithmann, 2008; Gertner, Podoshin, & Fradis, 1984). Because the application of this technique is simple, noninvasive and inexpensive, it has been applied in some clinical studies to detect nasal air emission (Van Lierde, De Bodt, et al., 2003; Van Lierde et al., 2004). Nevertheless, some studies report a questionable reliability and validity of the mirror-fogging test (Brescovici & Roithmann, 2008; Pochat et al., 2012). This can be due to influencing variables such as temperature, air humidity, resistance of the nasal airways and tilting errors (Brescovici & Roithmann, 2008). However, these studies only assessed nasal breathing without speech production. A more complex, but still user-friendly device to determine nasal emission is the aerophonoscope (Devani, Watts, & Markus, 1999; Rineau, 1993). Three airflow sensors, one for each nostril and one for the oral airflow, enable the simultaneous visualization of both, nasal and oral airflow, as well as voice levels. Additionally, an understandable graphic form displays these data qualitatively which offers the possibility to use the devise during biofeedback training (Devani et al., 1999). A limitation of this device, however, is that the handpiece is held under the nose in front of the mouth which can influence speech movements. Dotevall, Ejnell, and Bake (2001) examined nasal airflow dynamics during the velopharyngeal closure phase in speech by using a pneumotachograph attached to a nasal continuous positive airway pressure mask. They stated that the detection of nasal airflow patterns during velopharyngeal closure is a sensitive method to determine velopharyngeal functioning quantitatively so that it can accurately distinguish between perceptually abnormal and normal resonance in children with and without cleft palate (Dotevall et al., 2001; Dotevall, Lohmander-Agerskov, Ejnell, & Bake, 2002). Despite the good correlation with perceptual evaluation of hypernasality, this technique cannot determine a degree of hypernasality (Dotevall et al., 2001). Additionally, the nasal mask may have an influence on the sensory feedback for speech activity (Dotevall et al., 2002). Another possibility to measure nasal and oral airflow is the use of a warm-wire anemometer attached to a facial mask. Airflow causes a cooling of the electrical heated wire filament by which higher velocities of airflow create a major cooling effect. To maintain the fixed temperature ratio of the heated filament more current is needed. This current is shown on a display to make interpretations possible (Quigley, Shiere, Webster, & Cobb, 1964). This system, however, has a limited response speed, so it is unable to detect rapid movements of the velum (Main, Kelly, & Manley, 1999). Therefore, the Super Nasal Oral Ratiometry System (SNORS), by using high speed sensors, can overcome this deficit (Main et al., 1999). A modified

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oxygen mask, including airflow sensors and microphones, registers the amount of nasal airflow as a percentage of total airflow which can be displayed together with a speech envelope on a computer screen (Main et al., 1999; McLean, Kelly, & Manley, 1997). Due to its noninvasive and inexpensive character, this device can be applied during biofeedback training (Main et al., 1999; McLean et al., 1997). However, it is important to realize that nasal airflow during speech can be influenced by many factors such as nasal pathway resistance, velopharyngeal airway resistance, oral air pressure, amount of air release from the lungs and respiratory effort (Warren, 1967). Furthermore, the amount of nasal emission is strongly influenced by articulatory movements and tongue position (Machida, 1967; Selley, Zananiri, Ellis, & Flack, 1987). 3.2.2. Pressure flow technique To expand the measurements of nasal airflow, Warren and DuBois (1964) developed the pressure-flow technique to objectively evaluate velopharyngeal function during speech production. Based on a modification of the Theoretical Hydraulic Principle, the rate of nasal airflow in combination with the differential pressure across the velopharyngeal orifice can determine the area of that orifice. The equation is

Orifice area ¼

0:65

Volume rate of airflow through the orifice pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðð2  ðintraoral air pressure  nasal air pressureÞÞ=density of airÞ

with 0.65 a correction factor to account for ‘‘unsteady, non-uniform, and rotational’’ characteristics of airflow during speech production (Warren & DuBois, 1964). However, Yates, McWilliams, and Vallino (1990) assume that this correction factor may be significantly higher than 0.65 based on the influence of the inlet shape of the orifice and depending on the orifice geometry. To collect the requisite data simultaneously, two flexible catheters, one within the mouth and another in the nostril, collect intraoral and nasal air pressure (mm H2O) respectively and transmitting these pressures to pressure transducers. Furthermore, airflow is measured (ml/s) by a pneumotachograph connected by plastic tubing to the patient’s other nostril (Warren & DuBois, 1964). To make this procedure more comfortable, a nasal mask can be used to collect airflow and nasal air pressure (Gauster et al., 2010). Besides information about nasal airflow rate, oral and nasal air pressure levels and velopharyngeal orifice size, the pressure-flow technique also provides information about timing characteristics associated with speech (Warren, Dalston, Trier, & Holder, 1985). However, the needed, specialized equipment is often not available and the procedures are technically complex and require substantial cooperation (Karnell, 2011; Kuehn & Moller, 2000). Therefore, the described technique has probably more significance for research than for daily clinical practice (Karnell, 2011). 4. Discussion Current diagnosis of resonance disorders is based on a combination of perceptual assessments, information about the anatomy and functioning of the velopharyngeal mechanism obtained by direct assessment techniques and data from indirect techniques providing additional information regarding acoustic and aerodynamic features in speech. Appendix A provides a summary of the described assessment techniques, including their advantages and limitations as discussed above. Regarding the direct assessment techniques, no ideal imaging method is yet available (Beer et al., 2004; Silver et al., 2011). At the moment, nasopharyngoscopy and multiview videofluoroscopy are the most convenient instruments to assess velopharyngeal dysfunction in daily clinical practice (Lam et al., 2006). Both techniques are complementary (Lam et al., 2006) and it depends on the cause and the severity of the speech disorder whether nasopharyngoscopy or multiview videofluoroscopy is preferred (Havstam et al., 2005; Lam et al., 2006; Rowe & D’Antonio, 2005). Because these assessments rely on subjective interpretation of qualitative visual information, reliability is not always acceptable. Therefore, in 1990, a multidisciplinary international workgroup introduced a standardized rating scale to report the outcomes of instrumental assessments for velopharyngeal disorders (Golding-Kushner, 1990). However, the interrater reliability is still too low for the scale to be used for intercenter comparisons (Sie et al., 2008). Furthermore, neither nasopharyngoscopy nor multiview videofluoroscopy matches the ideal imaging method, so innovative application of existing technologies, such as magnetic resonance imaging (MRI), creates new opportunities. In the future, a combination of dynamic MRI and audio recordings will probably provide the possibility to discriminate between normal and abnormal resonance based on both neuromuscular and anatomic analysis (Silver et al., 2011). However, refinement and analysis of these techniques as well as normative imaging characteristics are necessary before MRI can accurately diagnose velopharyngeal disorders with high sensitivity and specificity (Maturo et al., 2012). As seen in the overview of the indirect measurement techniques, elaborate information about the velopharyngeal functioning can be derived from these methods. An extensive choice of techniques is available ranging from easily applicable and available assessments to expensive equipment in combination with complex implementation. A widespread acoustic technique to assess nasalance is the Nasometer (Fletcher & Bishop, 1970; Shprintzen & Marrinan, 2009), although no clear consensus exists about the correlation with perceptual assessment (Keuning et al., 2002; Sweeney & Sell, 2008). Possible explanations are that hypernasality may be rated more severely when articulation errors are present (Keuning et al., 2002) or

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that the human ear assesses a larger speech sample and a wider range of speech characteristics compared to the Nasometer (Karnell, 2011; Keuning et al., 2002). This disagreement about the correlation between nasalance and perceptual judgments confirms the persistent need to combine quantitative, indirect measurement with perceptual assessment (Bressmann et al., 2000; Hardin et al., 1992; Keuning et al., 2002). Additionally, different personal variables, such as nasal pathway resistance, oral air pressure, respiratory effort (Warren, 1967) and the amount of nasal emission (Machida, 1967; Selley et al., 1987), can influence the outcome of acoustic and especially aerodynamic measurements so that the usefulness of some techniques is questionable (Warren, 1967). Therefore, it is important to be aware of these influencing factors when interpreting the test results of for example the Horii’s Oral Nasal Coupling (HONC) index, the Nasality Oral Ratio Meter (NORAM) or the pressure-flow technique (Baken & Orlikoff, 2000; Karnell, 2011; Kuehn & Moller, 2000). According to several authors (Shprintzen & Bardach, 1995; Van Lierde, Wuyts, Bonte, & Van Cauwenberge, 2007), these subjective and quantitative measurements have to be interpreted with care and can lead to contradictory results when examining the nasality of an individual. The limitations of single assessment methods can possibly be overcome by combining several types of complementary measures. A possible solution is the simultaneous combination of direct and indirect assessment techniques during a specific speech task, for example dynamic MRI in combination with audio recordings or SNORS+ (Sharp, Kelly, Main, & Manley, 1999). Dynamic MRI with simultaneous audio recordings can investigate the acoustic-physiologic relation between the velopharyngeal mechanism and specific speech samples (Bae et al., 2011) or can evaluate velopharyngeal closure (Maturo et al., 2012; Silver et al., 2011). The goal of this procedure is to individualize speech therapy or surgery based on the detected functional anatomic defect (Maturo et al., 2012). However, as mentioned above, this is an expensive and time-consuming procedure and the influence of gravity on the velum is not yet clear. The SNORS+-system (Sharp et al., 1999) includes the images of videofluoroscopy, nasoendoscopy, waveforms of electrolaryngography, data of nasal anemometry (SNORS) and tongue-palate contact based on electropalatography. All these techniques cover the functioning and coordination of the key articulators: velum, tongue, pharynx and larynx (Sharp et al., 1999). The data are simultaneously provided on a computer screen, allowing to interpret a combination of the assessment results. This real-time display with direct visual feedback can be used in therapy or in a diagnostic setting. Following the authors (Sharp et al., 1999), it is a clinical, user-friendly device. However, the afore-mentioned limitations of the included direct and aerodynamic assessment techniques remain: ionization radiation, two-dimensional representation of the velopharyngeal mechanism, influencing variables such as nasal pathway resistance and velopharyngeal airway resistance, discomfort and influence of sensory feedback due to the nasal mask (anemometry) and the artificial palate (electropalatography). Furthermore, the artificial palate can have a negative impact on speech and saliva production (McLeod & Searl, 2006). Another multi-dimensional approach to assess resonance by the combination of only indirect techniques was presented by our research group, more specifically by Van Lierde et al. (2007). They comprised acoustic and aerodynamic characteristics obtained from an individual in one outcome measure to provide a more comprehensive descriptive of the resonance disorder, more specifically the Nasality Severity Index (NSI). The NSI was based on the optimal statistical discrimination between healthy children and those with a cleft palate, and constructed in a stepwise statistical approach, with sensitivity and specificity as the serving criteria. Based on the data set, the NSI has a sensitivity of 88% and a specificity of 95%. The five parameters used in the NSI calculation are the nasalance scores of respectively the phoneme/a/, an oral and an oronasal passage. Next, the Maximum Duration Time (MDT) of the phoneme/s/was used as well as the mirror-fogging test by Glatzel (Foy, 1910) during the production of/a/. All digital values were determined by the Nasometer (Fletcher & Bishop, 1970). The equation yields NSI = 60.69  (3.24  nasalance oral text (%))  (13.39  Glatzel value/a/) + (0.244  MDT (s))  (0.558  nasalance/a/(%)) + (3.38  nasalance oronasal text (%)). The more negative the NSI value, the higher the degree of hypernasality. Such an index allows to implement measurements complementary to nasalance values that should be considered in the judgment of nasality as recommended by several authors (Dalston et al., 1991; Seaver et al., 1991; van Doorn & Purcell, 1998). Although the clinical usefulness of the NSI has been shown by some case reports by Van Lierde et al. (2007), further research is required to confirm the possibility of the NSI to discriminate between hypernasal and normal resonance in a larger group of participants with a wider age range. To advance the validity and reliability of the index, adaptation of the included parameters may be required following the aforementioned limitations considering the mirror-fogging test by Glatzel (Foy, 1910) and possible influence of articulation errors and vital capacity on the MDT of the phoneme/s/ (Bettens, Wuyts, De Graef, Verhegge, & Van Lierde, 2013). Furthermore, it will be necessary to determine normative values before the NSI can be implemented in daily clinical practise. Because this index is only based on indirect assessment techniques, direct assessment techniques will be indispensable in treatment planning to provide information about the anatomy and functioning of the velopharyngeal mechanism. 5. Conclusion For decades, researchers have been searching for the most ideal measurement technique, which provides accurate information to diagnose the amount of velopharyngeal dysfunction based on direct techniques, and hypernasal resonance, provided by indirect techniques and perceptual assessments, in order to decide on the most apposite treatment.

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9

Successively, different direct and indirect measurement techniques were discussed, revealing different aspects of the velopharyngeal mechanism or resonance characteristics. As the ideal technique, as described by Horii (1980), has not been found, treatment decisions should not be based on a single source of information on patient performance. Therefore, multiparameter approaches such as dynamic MRI with audio recordings (Maturo et al., 2012; Silver et al., 2011), SNORS+ (Sharp et al., 1999) and the Nasality Severity Index (Van Lierde et al., 2007) can be an added value to evaluate resonance and the underlying anatomical defect. However, the combination of quantitative assessment techniques and perceptual measurement will probably remain necessary to provide sufficient information to make appropriate decisions concerning the treatment of resonance disorders.

Disclosure statement All authors have approved the final article. None of the authors has any commercial associations, supporting funds, or financial disclosures that might pose or create a conflict of interest with information presented in this article.

Appendix A. Summary of the described instrumental assessment techniques including their advantages and limitations.

Technique

Procedure

Direct assessment techniques Nasopharyngoscopy A fiberoptic scope is placed through a nostril into the nasopharynx and placed above the velum

Multiview videofluoroscopy

After high-density barium is applied to coat the nasopharynx, fluoroscopic images are taken in lateral, frontal and base or Towne’s views

(Dynamic) magnetic resonance imaging

Magnetic resonance images in different plane views using an MRI scanner (in combination with a microphone)

Lateral cephalometric radiographic analysis

Lateral cephalograms by X-ray and tracing of the radiographs by using specific software programs

Computed tomography

Computerized tomograph scan (CTscan) on the axial plane

Advantages

Limitations

- No ionizing radiation - Identification of velopharyngeal function during speech, especially the grade of velar closure - Can be used during biofeedback training - Identification of velopharyngeal function during speech - Different views providing unique information about abnormal compensatory movements, timing abnormalities, height of velopharyngeal closure (lateral view), lateral wall movements (frontal view), closure pattern (base or Towne view) - Simultaneous view of all articulators - Different views providing unique information about length, movement and extensibility of the velum, forward movement of the posterior pharyngeal wall (mid-sagittal view), lateral wall movements (coronal view), type and extent of velopharyngeal closure (axial view, level hard palate) - High spatial resolution and superior visualization of soft tissues - No ionizing radiation, repeatable - Early assessment of submucuous cleft palate - Standardized information - Minimal patient compliance - Allocation of the soft tissues of the nasopharynx to the bony landmarks of the face and the cranium

- Only axial view - Invasive for children - Two-dimensional representation of the threedimensional anatomy - Ionization radiation - Interpretation can be difficult because of multiple shadows - No visualization of underlying muscles - Two-dimensional representation of the threedimensional anatomy

- Determination of velopharyngeal closure level referring to organs - Quantifying surfacial and deep craniofacial structures

- Time-consuming procedure - Possible consequences of gravity on the velum - Expensive - Reduction of image quality because of fixed intraoral metallic prostheses

- Ionization radiation - Interpretation can be difficult because of multiple shadows - Limited, static information about the physiology of the velopharynx, only in the midsagittal plane - Two-dimensional representation of the threedimensional anatomy - Ionization radiation - Limited, static information about the physiology of the velopharynx, only in the axial plane - Two-dimensional representation of the threedimensional anatomy

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10 Appendix A (Continued ) Technique

Procedure

Advantages

Limitations

Ultrasound

Transducer placed against the neck, under the ear and behind the ramus of the mandible in combination with an acoustic coupling gel

- No ionizing radiation - Noninvasive, repeatable

- Limited visualization because of movements of the transducer and signal reflection - Intersubject and interobserver variability

- Ratio reduces influence of vocal intensity - Inter- and intra-speaker comparisons possible - Moderate to strong correlation with perceptual evaluation - Simple, noninvasive - Good interobserver reliability

- Limited application - Not commercially available as preassembled package

- Examination of nasality before and after surgery

- Low inter- and intraobserver reliability - Impossible to distinguish normal resonance from hypernasality - Limited application - Low correlation with perceptual assessment - Use of different cut-off scores to determine sensitivity and specificity of the Nasometer

Indirect assessment techniques Acoustic techniques Accelerometric Horii’s Oral Nasal Coupling (HONC) techniques index - Nasal and laryngeal accelerometer on nose and throat resp. or nasal accelerometer and microphone - Determination of constant k - Ratio of nasal to vocal accelerometer signal amplitude Nasality Oral Ratio Meter (NORAM) - Nasal and laryngeal accelerometer on nose and throat resp. - Ratio of nasal to vocal accelerometer signal amplitude Nasometry

Spectral characteristics

Aerodynamic techniques Nasal and oral airflow

- Fixation of a plate on a headset between upper lip and nose - Two microphones collect the nasal and oral signal - Filtering of the signals through 500 Hz filter with 300 Hz bandwidth - Calculation of nasalance score, range 0–100% Digital processing techniques based on specific spectral characteristics of hypernasal speech: pole-zero pairs in the region of F1, reduction of amplitude of F1 and F2, increase of bandwidth of F1 and F2, shifts in formant frequencies, rise in amplitude between F1 and F2

- Good correlation with perceptual judgment - Noninvasive, convenient, easy to interpret - Good test-retest reliability - Extensive clinical and research application - Quantitative - Can be used during biofeedback training - Techniques can be used as screening instrument - Quantitative, noninvasive, cost-effective - Limited need for equipment: microphone, computer and free software

- Features can only be extracted from vowels - No degree of hypernasality is determined - Difficult to verify the effect of therapy - Influencing variables - Low correlations with nasalance scores

Mirror-fogging test by Glatzel - Holding a cold mirror 0.5 cm under the nose during phonation of vowels or consonants - The degree of condensation is represented by 4 concentric circles by which 0 corresponds with no condensation and 4 to severe condensation Aerophonoscope Three airflow sensors, one for each nostril and one for oral airflow, enable the simultaneous visualization of both, nasal and oral airflow as well as voice levels Pneumotachograph Measuring the amount of nasal airflow by a pneumotachograph attached to a nasal continuous positive airway pressure mask

- Easy to use, simple training - Inexpensive, rapid, noninvasive

- Low reliability and validity - Tilting errors - Influencing variables

-

- Impairment of speech movements possible because of positioning of the handpiece in front of the mouth

Super Nasal Oral Ratiometry System (SNORS) - Modified oxygen mask including airflow sensors and microphones - High-speed sensors detect sudden changes in airflow caused by rapid movement of the velum (heated thermistor techniques)

- Quantitative, inexpensive and noninvasive - Information regarding timing of the velopharyngeal closure - Can be used during biofeedback training

Continuous speech possible Determination of nasal emission User-friendly, portable Can be used during biofeedback training

- Safe and easy to perform - Noninvasive - High sensitivity and specificity - Good correlation with perceptual evaluation

- No direct quantification of the degree of hypernasality and movements of the velopharyngeal mechanism - Possible influence on the sensory feedback of speech because of the nasal mask - Influencing variables - Possible influence on the sensory feedback of speech because of the nasal mask

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Appendix A (Continued ) Technique

Procedure

Advantages

Limitations

Pressure-flow technique

- Two flexible catheters (within the mouth and nostril) collect intraoral and nasal air pressure, transmitting it to pressure transducers - A pneumotachograph, connected by plastic tubing to the other nostril, measures nasal airflow

- Information about timing characteristics associated with speech - Verifying treatment outcome - Significant for research

- No clarity about the correction factor - Equipment often not available - Technically complex procedures - Limited significance for daily clinical practice

Appendix B. Continuing education questions CEU Questions 1. Why is the reliability of direct assessment techniques not always acceptable? a. There are no standardized rating scales available. b. The two-dimensional representation of the three-dimensional anatomy provides too limited information. c. The assessments rely on a subjective interpretation of qualitative visual information. d. The inclusion of a speech sample during direct assessment techniques is not possible. 2. What is the difference between acoustic and aerodynamic assessment techniques in the diagnosis of velopharyngeal disorders? a. Acoustic techniques provide indirect information about velopharyngeal functioning and resonance; aerodynamic techniques provide indirect information about velopharyngeal functioning and airflow. b. Acoustic techniques provide direct information about velopharyngeal functioning and resonance; aerodynamic techniques provide direct information about velopharyngeal functioning. c. Acoustic techniques provide indirect information about velopharyngeal functioning and airflow; aerodynamic techniques provide indirect information about resonance and airflow. d. Acoustic techniques provide direct information about resonance and airflow; aerodynamic techniques provide direct information about velopharyngeal functioning and airflow. 3. Which condition is not related to the ideal, objective assessment technique used to diagnose velopharyngeal disorders? a. The assessment has to be noninvasive. b. The assessment needs the possibility to assess the velopharyngeal mechanism during speech. c. The assessment may not interfere with the sensory feedback during speech. d. The assessment needs to include a perceptual analysis of the speech. 4. True or false - During the examination of a patient with a velopharyngeal disorder, the acquisition of an extensive examination including different objective measurements is sufficient. - Influencing variables are the cause of low correlations between the results of acoustic measurements and perceptual assessments.

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