Meconium aspiration syndrome occurs in a small proportion of neonates whose respiratory system is contaminated by the meconium and its content. The exact pathogenesis remains unknown, but conditions that lead to fetal stress and intrauterine hypoxia seem to be the most important risk factors. Tachypnea, grunting, and respiratory insufficiency are the main manifestations of this syndrome. Because of the life-threatening risk, close monitoring for possible respiratory distress in the first few days of life is needed. Together with clinical assessment, the early use of imaging studies (either plain radiography or ultrasonography) is recommended in the workup.
The signs and symptoms of meconium aspiration syndrome stem from the introduction of meconium into the respiratory tree    . It is well known that intrauterine hypoxia (which can occur due to numerous disorders and events, examples being placental insufficiency, preeclampsia, maternal abuse of tobacco and drugs, as well as oligohydramnios) is the main facilitator for the induction of meconium passage . Under physiological circumstances, the meconium (composed of gastrointestinal content, bile acids, pancreatic juice, blood, lanugo, and cellular debris) is a sterile mix of substances before the fetus is expelled from the uterus  . In approximately 1 in 7 births, the amniotic fluid is stained with meconium, but it is still not understood why only 5-9% of neonates exposed to a meconium-stained amniotic fluid (MSAF) develop meconium aspiration syndrome  . As meconium invades the lungs, neonates suffer from decreased lung compliance, eventually leading to hypoxia and tachypnea    . Grunting is a particularly important sign that should immediately raise suspicion toward respiratory distress . In fact, supplemental oxygen is almost always necessary to sustain adequate oxygen saturation in patients with MAS .
The diagnosis of meconium aspiration syndrome can be life-threatening without early treatment , which is why an immediate postnatal clinical assessment is of critical importance. Evaluation of neonates through Apgar scoring is one of the most important steps when it comes to this condition, followed by determination of oxygen saturation, particularly if signs of hypoxia are present. Meconium aspiration syndrome is much more likely to develop if a thick meconium present, if fetal distress is observed, or if the Apgar score is < 7 in the first 5 minutes after birth . For this reason, many authors advise that all neonates who are exposed to MSAF should be monitored for the first 24 hours in case respiratory distress develops . Additional steps that may be useful are imaging studies and arterial blood gas (ABG) analysis . Plain radiography can show overexpansion of the lungs with patchy infiltrates, whereas consolidation, atelectasis, and coalescing B-lines are typical signs observed on ultrasonography  . The convenient real-time imaging provided by the latter technique can be rapidly employed yielding reliable results, with no exposure of the patient to radiation. Ultrasonography is considered superior to X-rays, and should be performed whenever clinical suspicion of meconium aspiration syndrome exists .