In the young child, vomiting is the great imitator:
Gastrointestinal, Neurologic, Metabolic, Respiratory, Renal, Infectious, Endocrine, Toxin-related, even Behavioral.
To help us organize, below is a review of can't-miss diagnoses by age.

The Neonate: Malrotation with Volvulus
In children with malrotation, 50% present within the first month of life, with the majority occurring in the first week after birth. 90% of children with malrotation with volvulus will present by one year of age.   This is a pre-verbal child’s disease – which makes it even more of a challenge to recognize quickly.
The sequence of events usually is fussiness, irritability, and forceful vomiting.  The vomit quickly turns bilious.
Green vomit is a surgical emergency.
Babies may also present unwell, with bloating and abdominal tenderness to palpation.  Be aware that later stages of malrotation may present as shock – they present in hypovolemic shock due to third-spacing from necrotic bowel and/or septic shock from translocation or perforation.   In the undifferentiated sick neonate, always consider a surgical emergency such as malrotation with volvulus.
In the stable patient, get an upper GI contrast study.
Rapid-fire word association for other vomiting emergencies in a neonate:

Fever, irritability and vomiting?  Think meningitis, UTI, or sepsis.
Premature, unwell, and vomiting?  Think necrotizing enterocolitis.  Remember, 10% of cases of NEC can be full-term. Look for pneumatosis intestinalis.
Systemically ill, afebrile, and vomiting for no other reason?  Think inborn error of metabolism.  Screen with a glucose, ammonia, lactate, and urine ketones.
Others include congenital intestinal atresia or webs, meconium ileus, or severe GERD


The Infant: Non-Accidental Trauma
All that vomits is not necessarily from the gut.
Abusive head injury is the most common cause of death from child abuse.   Infants especially present with non-specific complaints like fussiness or vomiting.   Up to 30% of infants with abusive head injury may be misdiagnosed on initial presentation.
Louwers et al. in Child Abuse and Neglect developed and validated a six-question screening tool for use the in ED.  The power of this tool was that it was validated for any chief complaint – it is not an injury evaluation checklist – it is a screen for potential abuse in the undifferentiated child:

Is the history consistent?
Was seeking medical help unnecessarily delayed?
Does the onset of injury fit with the developmental level of the child?
Is the behavior of the child and his interaction with his care-givers appropriate?
Do the findings of the head-to-toe examination match the history?
Are there any other red flags or signals that make you doubt the safety of the child or other family members?

On multivariable analysis, if at least one of the questions was positive, there was an OR of 189 for abuse (CI 97 – 300).  In other words, if any of those six questions are problematic, get your child protective team involved.
Other important diagnoses in the infant: intussusception and pyloric stenosis (rapid review in audio).

The Toddler: Diabetic Ketoacidosis (DKA)
The important diagnosis not to miss in the vomiting toddler or early school age child is the initial presentation of diabetic ketoacidosis.  Children under 5 (especially those under 2) and those from lower socioeconomic groups have a higher risk of DKA as their initial presentation of diabetes.
This is true for any child that isn’t quite acting right – check a finger stick blood sugar as a screen.
The International Society for Pediatric and Adolescent Diabetes (ISPAD) criteria for DKA:

Hyperglycemia, with a blood glucose of >200 mg/dL (11 mmol/L) AND
Evidence of metabolic acidosis, with a venous pH of less than 7.3 or a bicarbonate level of < 15 mEq/L AND
Ketosis, found either in the urine or if directly checked in the blood.

If you have access to checking a serum beta-hydroxybutryrate – the unsung ketone – it can help in diagnosis in un