Introduction

Cerebral palsy (CP) is the most common physical disability in early childhood. The worldwide prevalence of CP is approximately 2–2.5 per 1,000 live births [27]. It has been clinically defined as a group of motor, cognitive, and perceptive impairments secondary to a non-progressive defect or lesion of the developing brain [4]. Epilepsy is a common problem in patients with CP [27]. Up to 80% of CP cases arise from antenatal factors; birth asphyxia contributes approximately 10% of CP cases [16]. Acquired cases in the postnatal period are usually related to central nervous system infection, trauma, strokes, and severe hypoxic events such as near-drowning. Genetic disorders and acquired insults follow a pattern of selective vulnerability during early brain development. For example, the neonatal neuropathological correlates of hypoxic–ischemic encephalopathy include specific and well-known patterns of brain injury [12, 51] (see Table 1) that interfere with the frontal/insular–basal ganglia–brainstem swallowing pathway [6, 10, 13, 15, 19–21, 23, 24, 26, 39, 52]. We propose that an understanding of paediatric dysphagia might be facilitated by a heightened awareness of the topography pertaining to the neuronal damage. This article focuses on the pathophysiology, clinical features, assessment, and management of swallowing problems in children with CP.

Table 1 Regions with a predilection for hypoxic–ischemic neuronal injury to swallowing
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Members of the Multidisciplinary Outpatient Swallowing/Drooling Clinic at the Radboud University Nijmegen Medical Centre in the Netherlands continually review literature on dysphagia and drooling in neurologically affected patients. References for this review were obtained from personal reprint files, supplemented by PubMed and Scopus searches with varying search periods. The search terms “drooling,” “sialorrhoea,” “swallowing,” “dysphagia,” “cerebral palsy,” “children,” “brain regions,” “fMRI,” “MEG,” and “EMG” were used. Only English-language articles, published from 1970 up to 2011, were included. The final reference list was generated based on originality and relevance to the topics covered in the review.

Neural control of swallowing

Normal swallowing is a goal-directed sequential behaviour that requires the coordinated action and inhibition of the muscles located around the oropharynx and oesophagus. The swallowing process is controlled in a complex manner involving the brainstem as well as cortical and subcortical central pathways. In addition, it requires a higher level of fine-tuning between the central circuits and the enteric nervous systems (ENS) (see Fig. 1).

Fig. 1
figure 1

Overview of the swallowing pathway. DSG dorsal swallowing group, NTS nucleus of tractus solitarius, VSG ventral swallowing group, NA nucleus ambiguus

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Efficient swallowing relies on sensory input from the oropharynx that triggers bilateral afferents in specific regions of the trigeminal sensory nuclei. Subsequently, the inputs reach the brainstem regions responsible for the patterned motor actions of swallowing. Sequential and rhythmic patterns of swallowing are formed and organized by a central pattern generator (CPG) located in the medulla oblongata. The CPG consists of two hemi-CPGs which, under physiological conditions, are tightly synchronized. The swallowing motor sequence is primarily generated in the ipsilateral hemi-CPG which transfers premotor neuron signals to the contralateral CPG [17]. The CPG itself is organized into two groups of neurons: the dorsal swallowing group (DSG) in and around the nucleus of tractus solitarius (NTS) and the ventral swallowing group (VSG) just cranial to the nucleus ambiguus. The DSG contains the generator neurons involved in the triggering, shaping, and timing of the sequential or rhythmic swallowing pattern. The DSG activates all VSG premotor neurons, which in turn distribute the swallowing drive to the various motor neuron pools involved in swallowing. The multifunctional pattern-generating circuits of the brainstem allow rapid modulation of orofacial behaviours (swallowing, respiration, chewing, coughing, and vomiting) [5].

Although our knowledge of the cortical regions involved in swallowing has grown substantially through functional magnetic resonance imaging studies, the exact central control mechanism for swallowing is still not fully understood. The involvement of many functionally and spatially different cortical sites suggests multilevel control for the swallowing pathways. It has been proposed that the control system consists of parallel loops which are able to coordinate and integrate the complex, sequentially based activation for swallowing [26]. The primary motor area and cingulate and insular cortices might all have essential roles in the coordination of the entire swallowing process [21, 24, 52]. Some investigators assume a functional dominance in swallowing [8] or a time-dependent shift of cortical activation from the left to the right sensorimotor cortex during voluntary swallowing [45].

In summary, voluntary and reflexive swallowing are controlled by widely distributed bilateral and multifocal cortical networks which apparently involve overlapping cortical regions. The primary sensory, motor, and cingulate cortices have a major role in these networks. The execution of the sensorimotor aspects related to swallowing relies on functionally connected pathways between (extra) pyramidal cortical motor planning regions, centres controlling the brainstem and cranial nerves, as well as lower motor neurons.

The brain–gut axis

Normal gastrointestinal tract (GI) function results from a balanced interaction between the enteric nervous system (ENS) and the central nervous system (CNS) which is called “the brain–gut axis”. Both neural and hormonal ENS communications have important integrative functions. A detailed discussion of the hormonal pathways is beyond the scope of this article. The ENS neural communications consist of the intrinsic afferent and motor neurons distributed along the gut wall (located in the mesenteric Auerbach and submucosal Meissner plexuses).

Afferent (vagal) sensory fibres terminate in the NTS of the hindbrain. The preganglionic motor innervations to the plexus arise from the dorsal motor nucleus of the vagus in the brainstem. The NTS and the vagal dorsal motor nucleus together comprise the dorsal vagal complex, important in the coordination of the muscular gut activity (by the vago-vagal reflex) [1].

The oesophagus consists of a proximal striated muscle portion (upper oesophageal sphincter, UOS) and a distal smooth muscle portion (lower oesophageal sphincter, LOS). At rest both sphincters are tonically contracted. Relaxation of the UOS (glossopharyngeal and vagal nerves) is initiated in the swallowing centre located in the medulla. Relaxation and contraction of the LOS (vagal and splanchic nerves) are initiated through local peristaltic activity of the oesophagus or distension of the gastric wall.

Thus far, the exact coupling of distinct interneurons (also called local circuit neurons or connector neurons) in the NTS is not known. Also, it is not totally clear which cortical regions are mainly involved in processing information to the GI tract. It has been suggested that the anterior insular cortex (called “visceral cortex”), the prefrontal and sensory/motor regions, the cingulate gyrus, as well as the limbic regions, all participate in the integration of neuronal information to the GI tract [18].

Swallowing problems in cerebral palsy

A recent epidemiological study among 1,357 children recorded by the Northern Ireland Cerebral Palsy Register between 1992 and 2009 showed a dysphagia prevalence of 43% in children with CP in any degree [29]. Results from speech pathology testing and video fluoroscopic swallowing studies in CP children demonstrate the relationship between typically affected brain regions and the associated characteristic patterns of dysfunctional swallowing (see Table 1). Usually, clinical features such as delayed initiation and segmented swallowing during attempted volitional movement might be determined by cortical neuronal networks, while dysfunctional pharyngeal components of swallowing (i.e. automatic components of deglutition, such as throat clearing, laryngeal closure tasks) suggest subcortical brain injury and/or basal ganglia necrosis [23]. In CP, dysphagia is often characterized by problems in both the volitional oral movements and the more reflexive pharyngeal phase of swallowing. Moreover, impaired ability to plan and coordinate swallowing with ventilation (e.g. greater propensity to swallow at abnormal times within the respiratory cycle, such as early inspiration after a thin liquid swallow and variable duration of the deglutition apnoea) are consistent with brainstem involvement [6]. A clinico-pathological correlation between differences in the breath–swallow pattern and the risk for aspiration is likely. Clinically, aspiration or episodic aspiration manifests as frequent coughing and occasional pneumonia. The overall incidence of pulmonary aspiration in CP due to oral motor dysfunctions is not known precisely. Admission to the hospital for presumed aspiration pneumonia in children with CP is common. An earlier study among 238 children with recurrent pneumonia showed that 48% had oropharyngeal incoordination with an aspiration syndrome whereas 50% of these children were diagnosed with CP [28]. Video fluoroscopic study of swallowing (VFSS) has demonstrated pulmonary aspiration in 38% [32] to over 70% of the cases [25], and frequently, the aspiration occurred without coughing, referred to as “silent” [32, 38]. Repeated pulmonary aspiration leads to chronic coughing, sleep-disordered breathing, impaired clearance of airway secretions, colonization of the respiratory tract by pathogenic bacteria, and a high risk of progressive lung parenchymal damage. This process may be lethal [14, 22].

Besides dysphagia, chronic pulmonary aspiration may also occur as a result of the gastro-oesophageal reflux (GOR) [53]. The incidence of GOR has been estimated at approximately 50% [40] and might be explained by lesions in the neuronal-anatomic swallowing centre located in the medulla oblongata leading to dysfunction of the vago-vagal reflex. In addition, GOR in children with CP may also result from direct lesions in cortical areas that modulate brainstem activity [1, 3].

Constipation, a common dysmotility disorder of the gut in children with CP, is often overlooked. More than half of the children with severe generalized CP are constipated [50]. The high incidence of the dysmotility disorders emphasizes the defective integration and modulation of information in the brain–gut axis in CP [30, 34, 35, 41, 43, 49], for which some investigators had proposed the term “Dysphagia–GOR complex” with a central role for the vagal nerve [33, 34]. It is reasonable to assume that vagal disruption is responsible for defective feedback to the distinct cortical regions and to the brainstem, those features being associated with swallowing disorders, defective ventilation, as well as dysmotility problems. At this time, more studies are needed to investigate the clinical relevance of integrated breathing, GI and swallowing function on the health and nutritional outcomes of children with CP.

Drooling is caused by the swallowing disorder and occurs in 10–58% of children with CP [11, 44, 46]. From a clinical point of view, it makes sense to distinguish between “anterior” and “posterior” drooling. Anterior drooling is the unintentional loss of saliva from the mouth; it can impose a significant disability on children with CP, leading to psycho-social, physical, and educational consequences. The most severely affected children may be rejected by their peers and even by their caregivers. Excessive anterior drooling damages books, computer, and keyboards and as such threatens essential tools for education and communication in neurologically disabled patients. In addition to cosmetic effects, drooling can produce peri-oral infections and can impair dentition. In contrast to anterior drooling, the so-called “posterior drooling” refers to the spill of saliva over the tongue through the faucial isthmus [18]. In particular, the children with most severe pharyngeal dysphagia are at medical risk due to saliva aspiration to the lungs. As mentioned above, aspiration in a child with CP often occurs without obvious coughing or choking (i.e. silent), and therefore, chronic aspiration of saliva might not be diagnosed prior to development of significant lung injury.

In case of chemical irritation such as that caused by GOR, salivary secretion is increased to protect the oral, pharyngeal, and oesophageal mucosa mediated by the vago-vagal complex in the brainstem. Unfortunately, in children with oral motor dysfunction, this protective increased saliva volume may accumulate in the pharynx and/or oesophagus, leading to an increased risk for aspiration. It is still a matter of debate whether GOR can cause severe drooling and whether or not treatment of pathological GOR diminishes drooling in children with CP.

Assessment and management of swallowing problems in CP

The investigation and treatment of swallowing problems in children with CP are challenging. Individualized care plans should be formulated accounting for the degree of oral motor impairment, feeding ability, aspiration, epilepsy, and ambulation. Generally, swallowing is more problematic in non-ambulatory children with CP. Furthermore, marked disturbed consciousness such as drug overdose and seizures interfere with the voluntary swallowing act and is a common cause of aspiration. Some children develop aspiration in association with GOR. Some anti-epileptics, such as clobazam and clonazepam, and neuroleptic drugs will induce the drooling risk.

In short, there is a growing awareness among clinicians that at early stage, particular note should be given to the importance of dysphagia [7] and excessive drooling contributing substantially to the burden of a child with CP and his or her family [48]. Ideally, the management of patients with swallowing problems requires the coordinated expertise of a number of health care professionals. Regular reassessment is necessary to gauge the response to oral motor training, nutrition, and drooling interventions. Tables 2 and 3 summarize our recommendations for evaluation and treatment of dysfunctional swallowing and drooling in children with CP.

Table 2 Recommendations for evaluation of dysfunctional swallowing (expert opinion)
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Table 3 Drooling treatment (expert opinion)
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