Suck is a relatively mature ororhythmic motor behavior in a full-term infant and is integral to competent oral feeds. However, preterm infants often demonstrate oromotor discoordination and are unable to suck and feed orally (Comrie & Helm, 1997; Lau, 2006; Barlow, 2009a). This inability represents a serious challenge to both the neonatal intensive care unit (NICU) "graduates" and the physician-provider-parent teams.

Feeding Difficulty Causes and Effects 

Delayed or impaired suck development may result from a variety of causes, including insult to the developing brain following hemorrhage or hypoxic-ischemic event, neurogenetic or craniofacial anomaly, feeding intolerance, surgery, diabetes, or ventilator interventions that interfere with ororhythmic pattern formation. Lengthy oxygen supplementation procedures in the NICU, for example, can have a negative impact on preterm infants with respiratory distress syndrome or bronchopulmonary dysplasia, costing them precious sensory and motor experiences during a critical period of brain development when suck and pre-feeding skills are being refined. Interruption of these vital experiences may impair fragile syntheses of how the brain maps these functions (Bosma, 1970; Hensch, 2004). Even the presence of a nasogastric (NG) feeding tube has negative effects on sucking and breathing (Shiao et al., 1995). Trussing the lower face with tubes and tape restricts the range and type of oral movements and limits cutaneous experiences with the hands and fingers (Figure 1).

For some preterm infants, poor suck and oromotor discoordination persist into early childhood and are correlated with significant delays in the emergence of other oromotor behaviors, including feeding, babbling, and speech-language production (Adams-Chapman, 2006; Ballantyne, Frisk, & Green, 2006). Infants who fail to establish oral feeding skills in the NICU may be sent home on gavage or G-tube feedings, which may hinder the development of coordinated oromotor behavior. The difficulties associated with establishing oral feeding competence—coupled with the additional costs for extended hospitalization—underscore the need for assessment and therapeutic tools that facilitate the development of normal oral motor skills (Lau & Hurst, 1999; Fucile, Gisel, & Lau, 2002, 2005; da Costa, van den Engel-Hoek, & Bos, 2008).

Assessment 

Feeding readiness is often evaluated by an infant's display of non-nutritive sucking and oromotor patterning (Lau, 2006; Bingham et al., 2008). Suck appears in utero during the second trimester and is remarkably stable and well-patterned by 34 weeks post-menstrual age (gestational age plus chronological age) in a healthy preterm infant (Hack, Estabrook, & Robertson, 1985). The non-nutritive suck (NNS)—any repetitive mouthing activity on a blind nipple or pacifier that does not deliver a liquid stimulus (Wolff, 1968)—typically consists of a series of compression bursts and pause intervals. Each burst typically consists of several suck cycles that decelerate in rate over the first five cycles until a steady state of approximately 2 Hz is achieved (Barlow et al., 2010) (see Figure 2A [PDF] online). The maturation and coordination of the NNS precedes the suck-swallow-breathe pattern associated with the slower 1-Hz pattern characteristic of the nutritive suck (Lau & Schanler, 1996; Gewolb et al., 2001; Medoff-Cooper, 2005).

Establishing a patterned NNS for the developing infant has many benefits, including growth, maturation, and gastric motility, and also decreases stress (Abbasi et al., 2008). NNS also improves behavioral state control pre-feed (Pickler et al., 1996) and post-feed (Pickler et al., 1993), decreases the frequency of apnea and cyanosis, and improves breastfeeding scores (Volkmer & Fiori, 2008). In addition, it accelerates swallow frequency and development with differential effects among infants with bronchopulmonary disease (Reynolds et al., 2010a, b, c), promotes development of specific sucking skills (Fucile et al., 2002, 2005), and enhances oral feeds (McCain, 1995). Morever, it accelerates the transition from tube to independent oral feeding (Barlow et al., 2008; Poore et al., 2008) and is presumed to enhance the maturation of neural systems responsible for ororhythmic activity (Bernbaum et al., 1983). Accurate assessment of oromotor discoordination in the preterm infant may also serve as a potent clinical marker for brain development and neurodevelopmental outcomes (Mizuno & Ueda, 2005).

Suck Central Pattern Generator 

The mammalian suck is one the earliest-appearing somatic motor rhythms. In the fetus and late-gestation human infant, it is regulated primarily by a neural network known as the suck central pattern generator (sCPG), which consists of bilateral neural networks within the brainstem pontine and medullary reticular formation (Tanaka et al., 1999; Barlow et al., 2010). Based on animal models, the minimal circuits for ororhythmic activity reside between the trigeminal motor nucleus and the facial nucleus in the brainstem (Tanaka et al., 1999) and are situated to function as premotor inputs to lower motor neurons. Among many fascinating features, the sCPG can be modulated by the sensorimotor cortex and cerebellum. This connection underscores the importance of careful posturing and orientation during clinical testing to help the infant regulate his or her behavioral state because the nature of descending and peripheral inputs to the sCPG will be affected by this regulation. The sCPG is highly responsive to oral stimulation (i.e., breast, pacifier, or bottle nipple; tactile and thermal touch; olfactory cues) and orocutaneous entrainment (Finan & Barlow, 1998).

During entrainment, the baby synchronizes his or her oromotor activity to an externally delivered, patterned orocutaneous stimulus. This process represents a powerful method of achieving neural synchrony among sensorimotor pathways of the orofacial system to drive suck development. Therefore, it is not surprising that stimulation of the lips and tongue is a common method used to evoke sucking (Poore et al., 2008; de Costa et al., 2008; Barlow, Poore, & Zimmerman, 2010).

Future Applications  

A new randomized controlled trial (RCT) funded by the National Institute on Deafness and Other Communication Disorders is underway to assess the effects of a new synthetic patterned orocutaneous stimulation technique designed to entrain the sCPG in human infants. This technique of oral entrainment stimulation mimics the temporal features of the NNS burst, including age-appropriate NNS burst length and the frequency-modulated component of suck typically observed during burst initiation (Barlow et al., 2010). Using this technique, the infant's pacifier is transformed into a "pulsating nipple" that resembles the temporal pattern of well-formed NNS bursts. Application of the oromotor entrainment therapy is correlated to the rapid organization of suck in preterm infants who exhibit poor feeding skills (Barlow et al., 2008; Poore et al., 2008; Barlow, Poore, & Zimmerman, 2010), including improved NNS burst structure, lower NNS spatiotemporal index measures (sCPG activity pattern invariance), and a shorter transition to oral feeds.

In the RCT underway, oromotor entrainment stimulation will be delivered as early as 30 weeks post-menstrual age among several groups of babies, including infants with respiratory distress syndrome, bronchopulmonary dysplasia, abdominal wall defects, or Down syndrome; infants who are small for gestational age or born to mothers with diabetes; and healthy preterm infants. Infants receive alternating three-minute blocks of low-amplitude (0.15 millimeter) patterned orocutaneous stimulation during a 20-minute period, typically paired with gavage feeds, up to three times per day for 10 days in the NICU until the infant attains 90% to 100% oral feeds for two consecutive days (depends on individual NICU feed criteria). This schedule helps the infant develop an association between pleasurable oral stimulation and satiation from nutrients entering the stomach. Infants are held in a developmentally supportive posture (Figure 2) with dimmed background/overhead lighting to promote eye contact with the developmental specialist. Stimulation and sampling of NNS behavior begins when the infant achieves a drowsy-to-active alert behavioral state (NIDCAP) (Als, 1995). Infants remain connected to their pulse-oximetry monitors at all times to ensure that respiration, pulse rate, and oxygen saturation levels are adequate to support oromotor activity.

Primary outcome variables include daily measures of non-nutritive suck dynamics, spatiotemporal index of the non-nutritive suck, ororhythmic motor behavior, NNS cycle period analysis, early feeding skills assessment, transition time from tube-to-oral feed, feeding efficiency, and length of hospital stay. A second major objective is to conduct a comprehensive assessment of neurodevelopmental outcomes at 3 years of age, using the Bayley III and standardized speech-language test instruments. More than 350 preterm infants will be included in the study.

Prematurity is a major factor leading to feeding and swallowing issues and may be further complicated by insults to the developing nervous system or the challenging environment of the NICU. A patterned oral somatosensory experience through entrainment of neural pathways supporting the orofacial system presents a new neurotherapeutic application for the habilitation of suck in premature infants. Repeated exposure to patterned orocutaneous concurrent with NG tube feeds facilitates the development and presumably strengthens neural networks that regulate suck. This intervention is non-invasive, safe, and pleasurable for the neonate, as well as quantitative and easily administered in the NICU. Entrainment therapy provides the infant with additional benefits, including improved behavioral state control pre- and post-feed, growth, maturation, and gastric motility, while decreasing stress and enhancing the transition to oral feeds. 

Other contributors include Mimi Urish, MA, CCC-SLP, of Children's Hospital, Aurora, Colo., and University of Kansas faculty members Diane Loeb, PhD, CCC-SLP; Meredith E. Estep, PhD; Susan Stumm, ABD, CCC-SLP; Lalit Venkatesan, MSEE; and Joan Wang, MSEE.