the experimental works of macklin and macklin provided insights into its pathophysiology [ 4 , 5 ] ; alveolar rupture occurs because of a pressure gradient between the alveolus and the surrounding tissues .
this gradient develops either through overinflation of the alveolus or a reduction of interstitial pressure .
the air that subsequently leaks into the interstitial tissue diffuses toward the peribronchial and perivascular tissue , and then towards the mediastinum , the neck and into the subcutaneous tissue .
however , due to pressure equalisation between the affected and adjacent alveoli in the lungs , the interalveolar walls remain intact and the lungs inflated . the diagnosis of pm is confirmed by frontal chest roentgenogram , including the cervical region .
typical radiological signs of pm include the continuous diaphragm sign ( interposition of air between the pericardium and the diaphragm , which becomes visible in the central mediastinal part ) and linear bands of mediastinal air parallelling the left side of the heart and the descending aorta ( pleura is shown as a fine opaque line ) with extension superiorly along the great vessels into the neck . in infants , the  spinnaker sign 
( an upwards and outwards deviation of thymic lobes ) can be seen when the thymus is raised above the heart by pneumomediastinal air that elevates the thymus and separates it from the cardiac silhouette beneath .
various causes of pm are found in the literature , such as airway obstruction ( e.g. foreign body aspiration ) , iatrogenic ( e.g. mechanical ventilation ) , infections ( e.g. pneumonia ) , obstructive lung disease ( e.g. asthma ) , toxic effects ( e.g. smoking ) , trauma ( e.g. chest trauma ) , valsalva manoeuvres ( e.g. vomiting ) and the weakness of tissue ( e.g. anorexia nervosa ) . in spontaneous pm ,
the underlying lung is healthy and the air leak is thought to be atraumatic . in neonates ,
known predisposing factors are mixed lung diseases , such as pneumonia or meconium aspiration syndrome , with coexisting atelectasis and airway obstruction .
, we retrospectively analysed the incidence , severity and causalities of pm in neonates and children > 4 weeks of life admitted to our intensive care unit , and we investigated the possible differences between the groups .
we retrospectively reviewed all records of children diagnosed with pm who were hospitalised in the interdisciplinary neonatal and paediatric intensive care unit of the university children s hospital in zrich , switzerland , between january 2000 and september 2006 .
the patients were divided into two groups according to their age : neonates ( under 4  weeks of age ) and children ( over 4  weeks of age ) .
we were interested in the causes of pm as documented by the treating physicians , the types and results of radiologic investigations performed , any invasive interventions used to treat pm , the severity of the pm and the length of stay in the intensive care unit .
the incidence of pm in our intensive care unit was 0.08% for children > 4  weeks of age and 0.1% for neonates . in all patients ,
pm was diagnosed by chest x - ray and all had a positive outcome related to the pm .
all five patients with pneumopericardium ( pp ) did not suffer from any complications ( e.g. pericardial tamponade ) .
in addition to pm , two children of this group had subcutaneous emphysema ( se ) , two a pneumothorax ( pt ) and two a pp .
different causes were found for the air trapping . there were two traumatic aetiologies ( rib fracture after a severe car accident , lesion in the hypopharynx after a fall ) .
two children were diagnosed with obstructive bronchitis and in one child , barotrauma occurred intraoperatively due to a clamped expiratory tube during mechanical ventilation ( fig .  1 ) .
one child had exercised vigorously three days before hospitalisation , which may have caused the pm . in one adolescent , pm occurred spontaneously .
all children were hospitalised in the intensive care unit for one to seven days , depending on the severity of their underlying disease .
diagnostics for the pm other than chest x - rays were performed in four patients .
all of these had received a thoracic ct scan . in one child , who also had a huge subcutaneous emphysema and dysphagia , the reason for the air trapping
could only be found by means of a laryngotracheoscopy , which showed a traumatic lesion in the hypopharynx .
the patient s history revealed that she had fallen onto a piece of wood by her neck . 

table  1results for the group of children > 4  weeks of lifeair leakaetiologyage ( years)intubation ( after diagnosis)pleural drainagediagnostic testsdays in icupm , pp , sespontaneous/3 days earlier intensive sport15.8nonochest x - ray 32pm , pttraumatic7.53  daysyesct / chest x - ray 35pm , se traumatic ( lesion in hypopharynx)1.3nonoct / chest x - ray 3 , oesophagogram with contrast medium , laryngotracheoscopy4pm , ppiatrogenic : equipment failure with barotrauma during mechanical ventilation2.3nonochest x - ray 22pmspontaneous15.4nonochest x - ray / ct1pm obstructive bronchitis5.6nonochest x - ray 2/ct2pm , ptobstructive bronchitis1.97  daysyeschest x - ray 77fig . 
1pneumomediastinum ( pm ) , subcutaneous emphysema ( se ) and pneumopericardium ( pp ) in a 2-year - old intubated patient results for the group of children > 4  weeks of life pneumomediastinum ( pm ) , subcutaneous emphysema ( se ) and pneumopericardium ( pp ) in a 2-year - old intubated patient the group of children older than 4  weeks stayed in the intensive care unit for a mean of 3.2  days ( range 17  days ) , depending on the severity of the pm and the underlying disease .
compared to the neonatal group , the length of stay in the intensive care unit was shorter . however , most neonates stayed in the intensive care unit longer , primarily because of comorbid conditions and not because of the pm .
we found nine neonates who were diagnosed with pm ( table  2 ) ; two premature and seven term infants , all of whom presented with signs of respiratory distress .
three neonates were also diagnosed with a pp , one with se and five with a pt .
birth weight ranged from 2,150  g to 4,140  g ( mean 3,340  g ) .
all children were vigorous at birth and none required resuscitation with bag mask ventilation or surfactant . before arriving in the intensive care unit , where the diagnosis of pm was confirmed by chest x - ray , two infants had received ventilatory support by cpap and one of the premature infants had to be intubated for respiratory failure . during hospitalisation in the intensive care unit , two children deteriorated and required mechanical ventilation for three and four days , respectively , and two other children needed cpap for a few hours .
the age at admission to the intensive care unit ranged from a few hours to four days .
one neonate was admitted to the intensive care unit due to convulsions and developed a pm on day six of life .
the treating physicians felt that the pm may have been associated with a valsalva manoeuvre , which occurred during the seizure .
other causes of pm were a pulmonary infection due to maternal infection and a possible barotrauma due to peak inspiratory pressure of 25  cm h2o in a mechanically ventilated premature neonate .
two newborns had pm related to cpap and four neonates were diagnosed with spontaneous pm .
neonates stayed in the intensive care unit for 313  days ( mean 5.6  days ) , depending on the severity of the underlying diseases . 

table  2results for the group of neonatesair leakaetiologybirth weightgestational age ( weeks)mode of deliverymechanical ventilation before diagnosisduration of ventilatory support after diagnosispleural drainagedays in icupmspontaneous4,140  g40 0/7vaginalnonono4pm , pp , sepremature lungs , barotrauma2,150  g34 4/7caesarian section pip max .
25  cm h203  days ( intubation)no4pm , pt , pppremature lungs , spontaneous or cpap2,480  g35 6/7vaginalcpap4  days ( intubation)yes6pm , ptspontaneous3,485  g38 1/7caesarian sectionnonono3pm , ptspontaneous3,440  g37 5/7vaginalno6  hours ( cpap)no3pm , ppspontaneous2,830  g39 1/7caesarian sectionnonono13pm , ptspontaneous3,970  g38 5/7caesarian sectionnonono9pm , ptpulmonary infection due to maternal infection3,440  g38 5/7vaginalcpap1  day ( cpap)no5pmconvulsions or spontaneous4,130  g40 5/7vaginalnonono4 results for the group of neonates
all children with pm had a good outcome without any complications due to air trapping . in the group of children
older than 4  weeks , only two children developed a respiratory insufficiency , leading to mechanical ventilation . in both of them
, respiratory failure was related to their underlying condition ( polytrauma with haematothorax and severe obstructive bronchitis , respectively ) .
all other children were treated with oxygen only and stayed in the intensive care unit until they improved clinically and radiographically .
regarding radiologic diagnostics , four patients of the group of children > 4 weeks of life had ct scans ( three of them had been done in outside clinics from where the patients had been admitted to our intensive care unit ) .
retrospectively , the utility of the ct scans was put into question , as these scans did not change the patient management .
the only patient in whom the ct scan changed management was the child with polytrauma . in this patient , other intrathoracic injuries needed to be ruled out . in the group of neonates , it was much more difficult to find the aetiology of the pm , since all neonates had presented with respiratory disease , and radiologic investigations were partially performed only after the use of cpap or tracheal intubation .
five of the nine neonates had a spontaneous pm without risk factors , such as mechanical respiratory support ( bag mask ventilation after birth , cpap , mechanical ventilation ) or restrictive lung disease .
three of these five babies were delivered by caesarean section . in the remaining four newborns , possible mechanical incidents leading to the air leak
could be revealed : mechanical ventilation with high inspiratory pressure , cpap , pulmonary infection and convulsion .
further investigations are needed to find the aetiology of spontaneous pm in healthy , term neonates . in conclusion ,
it is diagnosed by chest x - ray alone . whereas in older children mechanical events leading to the airway rupture
can be revealed in most cases , about half of the neonates in our series suffered from pm without obvious reason .
 the incidence , aetiology and pathophysiology of pneumomediastinum ( pm ) , an uncommon and potentially serious disease in neonates and children , were evaluated . a retrospective chart review of all patients diagnosed with pm who were hospitalised in the intensive care unit of the university children s hospital zrich , switzerland , from 2000 to 2006 , was preformed . 
 we analysed the incidence , severity and causes of pm and investigated the possible differences between neonatal and non - neonatal cases . 
 seven children and nine neonates were identified with pm . 
 all patients had a good outcome . 
 six cases of pm in the group of children older than 4  weeks were deemed to be caused by trauma , infection and sports , whereas one case was idiopathic . 
 all nine neonatal cases presented with symptoms of respiratory distress . 
 we were able to attribute four cases of neonatal pm to pulmonary infection , immature lungs and ventilatory support . 
 five neonatal cases remained unexplained after careful review of the hospital records . in conclusion , 
 pm in children and neonates has a good prognosis . 
 mostly , it is associated with extrapulmonary air at other sites . 
 it is diagnosed by chest x - ray alone . 
 we identified mechanical events leading to the airway rupture in most children > 4  weeks of life , whereas we were unable to identify a cause in half of the neonates studied ( idiopathic pm ) . 