Colic usually affects babies in the first few weeks of their lives and persists for about 4 months. Crying can be intense and furious and it may last for several hours per day for several weeks. Although crying can occur at any time, it is usually worse in the late afternoon and evening, and occasionally affects the baby's sleep. Infantile colic is not considered a disease, fulfilling instead criteria for a functional disorder. Research shows that babies with colic continue to eat and gain weight appropriately, despite the crying (1). The main risk for these babies relates to the stress and anxiety that the condition creates at home, especially when it affects the first child. The cause of colic is not known. Painful flatus may contribute to colic, but there is little evidence to prove that it is linked to digestive problems (2). Another theory is that while the digestive system is maturing, some babies are more sensitive to substances such as lactose ingested through breast-feeding and formula milk; however, evidence to support this hypothesis also is limited. Other possible causes relate to the baby's behavior and temperament. Among the gastrointestinal (GI) factors, we focus on non-nutritive pathophysiology such as the relation of colic to gastroesophageal reflux (GER), GI motility disorders, the role of gut hormones, and intestinal microflora.
Uncomplicated GER in otherwise healthy infants is common and is not considered a disease. Approximately 50% of all infants 0 to 3 months regurgitate at least once per day. This frequency drops to 5% by age 10 to 12 months. Three percent of parents of 10- to 12-month-old infants view this as a problem (3). Common contributing factors to excessive regurgitation include overfeeding, air swallowed during feeding, crying, or coughing. In uncomplicated GER (termed functional regurgitation in the Rome III criteria (4)), the physical examination is normal and weight gain is adequate. A thorough history and physical examination are sufficient for a confident diagnosis, and conservative, nonpharmacologic therapy usually is recommended. GER disease (GERD) occurs when the consequences of excessive GER cause a true disease, such as esophagitis, aspiration pneumonia, failure to thrive, and hematemesis. Guidelines for the evaluation and treatment of the infant and child with suspected GERD have been published (5). It is tempting to associate GER (and GERD) with excessive crying in some babies. The recent widespread use of medications aimed at increasing acid suppression in newborns and infants (6) may indeed reflect the belief that excessive esophageal exposure to acid is the culprit in many instances of infant colic. GERD symptoms in infants, however, usually include vomiting and feeding difficulty, and these are not common symptoms in babies with colic.
Normal esophageal pH monitoring scores have not been defined adequately in infants of different ages, and many pH studies have indicated that GER is a common event. Reflux index scores in infants often are higher than those considered clinically significant in adults and children. Reflux parameters are altered by changes in arousal state, type and frequency of feeding, and posture (7–9) factors that are rarely standardized in pediatric intraoesophageal pH–monitoring studies. The development of electronic impedance to study reflux events may shed further light on the relation between reflux and crying and irritability in infants. Interestingly, therapeutic studies that have been aimed at reducing gastric acid secretion to treat “reflux symptoms,” including irritability, in infants have failed to demonstrate superiority of the drugs compared with placebo (10,11). Available data seem to suggest that GER and GERD may play a role only in a small subset of infants with colicky symptoms.
Esophageal and GI motility and enteric nervous system abnormalities have been implicated as possibly being involved in the pathophysiology of infant colic. GI motility matures during infancy and early childhood and may be influenced by various factors, including developmental stage, dietary habits, genetics, arousal state, intercurrent illnesses, congenital anomalies, and effects of medical or surgical interventions. Esophageal motility plays an important role in airway protection during episodes of GER. There is evidence that abnormal esophageal reflexes may contribute to difficulty in feeding infants with some of the consequences of prematurity. The evaluation of these protective mechanisms in infants is now feasible but restricted to a few referral third-level centers (12).
In regard to alteration in gastric motility and its interplay with regurgitation in infants, gastric distension and impaired fundic relaxation as a result of disturbed gastric motility may play a role. Transient lower esophageal sphincter relaxation seems to be triggered by gastric distension via activation of the stretch receptors in the stomach (13,14) and there is evidence that gastric accommodation may be impaired in the first few days of life (15). The enlarged antral area, as a consequence of distal displacement of gastric contents when fundic function is impaired, could trigger nausea and discomfort and consequently provoke regurgitation. Other studies suggest that acid exposure may be reduced in the case of delayed gastric emptying (16). Recent data on infants with cow's-milk allergy show a close link between GI symptoms, GER, and gastric-emptying time (17). Although the available evidence fails to provide insight into the exact triggers of infant colic, the data about the relation among gastric and oesophageal motor function, allergy, and dyspeptic symptoms allow for the hypothesis that certain infants may be predisposed to dietary protein intolerance and disturbed gut motility in the first few weeks of life. These processes also may lead to altered gut perception and normal stimuli (eg, intestinal distension) may be misinterpreted as painful events (18), much as happens in other pain-predominant functional disorders affecting older children, such as functional abdominal pain and irritable bowel syndrome (19,20). In support of the hypothesis that disturbed gut sensory-motor function may play a role in infant colic, there is the finding that ghrelin and motilin (2 hormones that affect gastric emptying) concentrations in blood are higher in infants with colic than in controls. Elucidation of the role of ghrelin in GI motility may open new doors to better understand the etiology of infant colic (21). Treatments of colicky symptoms have included medications with anticholinergic effects with the goal of reducing “spasms” or lessen the amplitude of potentially painful GI contractions. Evidence supporting their use is controversial, suggesting that motility disturbances may be involved only in a subgroup of children with infant colic (22,23).
ROLE OF MICROBIOTA
Reduction in crying time in colicky newborns fed with breast milk with the addition of probiotics has been reported by Savino and coworkers (24) and confirmed by other studies in preterm and term infants (25,26). The action of microbiota and its modulation by probiotics on upper GI motility can be explained in several ways (27). Volume and chemical characteristics of meals in the gut may modulate vagal signaling and affect gastric emptying (28). Fiber content increases gastric antrum motility compared with other diets (29). Intestinal bacteria metabolites such as short-chain fatty acids (SCFA) may stimulate smooth muscle (30). In the colon, these compounds inhibit peristaltic activity and may stimulate tonic activity. SCFA modify upper motility, inducing relaxation of the proximal stomach, lower esophageal sphincter and reducing gastric emptying via the mediation of GI hormones such as polypeptide YY (31). Cross-talk among the digestive nervous and motor activities, immune-related mechanisms, and probiotics has been thoroughly investigated. There is evidence that postinfectious enteric muscle dysfunction represents a state of persistent dysfunction of the neuromuscular tissues maintained by the production of mediators such as transforming growth factor-β and prostaglandin E2 by the intestinal muscle layers themselves (32). The interstitial cells of the Cajal network, the pacemaker of GI electrical activity, also may be damaged by inflammation and such alteration may lead to motor abnormality (33). The administration of probiotics could restore muscle function after a GI infection through action on multiple proteins and other components of excitation-contraction coupling (34). Probiotics also interact with the gut-associated lymphoid tissue (35,36). The coordinated interplay among these components is fundamental for the proper functioning of the gut. De Weerth and coworkers showed a microbiota diversity in children affected by colic in the first week of life compared with healthy controls (37).
Little evidence supports a substantial role of GER or GERD in the majority of infants with colic. It is possible that delayed gastric emptying, associated with abnormal antral contractions, leads to prolonged gastric stasis and antrofundic incoordination, resulting in increased wall tension in the gastric body and the fundus. This, in turn, may activate tension and pain receptors in the stomach to generate the characteristic distress present in children with colic. Heightened visceral sensitivity and altered gut hormones in these children also may contribute to enhanced pain perception. Such hypotheses have not been confirmed yet by in vivo studies and have not translated into viable therapeutic options. The role of microbiota in modifying intestinal environment and host responses in both the peripheral and central nervous systems represents a promising but not yet proven target for therapeutic interventions in dealing with distressed behavior in infants.
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