Magnetic Resonance Imaging as an Aid in the Dynamic Assessment of the Velopharyngeal Mechanism in Children

Abstract

Velopharyngeal incompetence is a complication following cleft palate surgery, with an incidence ranging from 5 to 38 percent following palatoplasty.1 It occurs when the velopharyngeal valving mechanism is incapable of complete closure to isolate the oropharynx from the nasopharynx during speech production. The anatomical components of the velopharynx are the velum (soft palate) and the three pharyngeal walls: posterior, right lateral, and left lateral. Velopharyngeal closure depends on three basic factors: (1) superior and posterior movement of the soft palate produced by the contraction of the levator veli palatini muscle, (2) medial movement of the lateral pharyngeal walls, and (3) bulging forward of the posterior pharyngeal wall, creating a Passavant's ridge to facilitate velopharyngeal closure. Both the levator veli palatini muscle and the soft palate play a major role in the pathogenesis of velopharyngeal incompetence. Dysfunction of the levator veli palatini muscle or shortness of the soft palate may cause velopharyngeal incompetence, yet treatment options are quite different. Therefore, accurate diagnosis is crucial. Procedures that rely on restoration of a functional levator sling require the presence of adequate levator musculature for reconstruction of the levator sling. When the diagnosis of velopharyngeal incompetence is confirmed, speech therapy is initiated. Should the patient fail to demonstrate sufficient improvement in nasality despite speech therapy, secondary surgical management to restore a competent velopharyngeal valving mechanism may be indicated. If it is possible to do so by restoration of the levator muscular sling, this may obviate the need for surgical alteration of the posterior or lateral pharyngeal walls as occurs during a pharyngeal flap or sphincter pharyngoplasty. Dynamic reconstruction of the levator sling can be accomplished using either a Furlow palatoplasty in patients with submucous cleft palate or postpalatoplasty velopharyngeal incompetence,2 or by palate re-repair in patients with postpalatoplasty velopharyngeal incompetence as advocated by Sommerlad and colleagues.3 However, should the levator mechanism be negligible or replaced with scar following prior surgery, attempting dynamic levator reconstruction would not result in a competent velopharyngeal valving mechanism, and pharyngeal flap or sphincter pharyngoplasty should be considered. Therefore, the ability to visualize different soft-tissue planes of the velopharynx and to visualize the muscular anatomy preoperatively become crucial when planning secondary surgical management of velopharyngeal incompetence. Current methods for functional visualization of the velopharynx are either invasive (nasendoscopy) or impart ionizing radiation (speech videofluoroscopy), and do not provide anatomical definition of palatal soft-tissue planes. In addition to its invasiveness, nasendoscopy allows only a single viewpoint (from a ventral and cephalad observation point) of the velum. Furthermore, both the passage of the endoscope itself and the local anesthetic agent used during scoping may affect speech. In contrast, although speech videofluoroscopy allows visualization of dynamic interaction (i.e., functional evaluation) of velopharyngeal structures from different angles, interpretation is difficult because of shadows introduced by the overlying structures. Moreover, relative proportion distortion is inevitable in speech videofluoroscopy because three-dimensional structures are converted into two-dimensional images. An ideal test that is noninvasive and does not impart ionizing radiation is not in widespread use for preoperative assessment of the velopharyngeal mechanism in children. Magnetic resonance imaging is noninvasive and poses no ionizing radiation hazard, and allows imaging of the velopharyngeal mechanism during limited speech production. Magnetic resonance imaging also allows anatomical assessment of soft-tissue planes involved in the velopharyngeal valving mechanism, which is not possible with other techniques currently in use. The major hurdle that prevents the widespread application of magnetic resonance imaging for functional evaluation of the velopharyngeal mechanism is poor image quality. To examine the function of the velopharyngeal mechanism during active phonation using magnetic resonance imaging, patients are instructed to sustain phonation of a chosen vowel or consonant during the image-acquisition process. Because most patients, especially children, are only able to sustain phonation for 15 to 20 seconds comfortably, image-acquisition time is limited. This in turn limits the resolution of the acquired image. Fortunately, magnetic resonance image quality is dependent not only on the length of acquisition time but also on the magnetic field strength and head coil design. By using more powerful magnetic resonance imaging scanners and better head coil design, improvement in magnetic resonance images is possible without increasing image-acquisition time. Previous magnetic resonance imaging studies involving functional evaluation of the velopharyngeal valving mechanism in children used magnetic resonance imaging scanners with a strength up to only 1.5 T.4–7 By using a 3.0-T magnetic resonance imaging scanner with a multichannel head coil, we hypothesize that the quality of the images acquired will be sufficient to allow functional evaluation of the velopharyngeal valving mechanism in children. Moreover, resolution of the images will allow delineation of palatal soft-tissue planes for surgical planning.