While many younger children can get 2-3 episodes of viral gastroenteritis each year, having vomiting every month or two is more than you would usually expect a child to have. Still, if a child did have typical viral symptoms, with vomiting and diarrhea and contact with other children with similar symptoms, then it is possible that he is just getting one virus after another.
But if his main symptoms are nause and vomiting, then there is likely something else going on, with cyclic vomiting syndrome being on the top of the list of possibilities.
Children with cyclic vomiting syndrome usually have episodes or cycles of nausea and vomiting which may last a few hours or a few days. During these episodes, children may also have decreased activity and a decreased appetite and they may even become dehydrated. Other symptoms might include abdominal pain, headache, fever, and pallor. Once the episode is over, children are usually symptom free and well until the next time it happens.
Although the cause of cyclic vomiting syndrome is not known, most children with this condition have a family history of migraines and many experts believe the two are related.
The other factor that links migraines with cyclic vomiting syndrome is that they can often both be prevented with the same medications, namely cyproheptadine (Periactin) and amitriptyline (Elavil).
Diagnosis and TestingAs with migraines, there is no specific test that can determine that a child has cyclic vomiting syndrome. Instead, the diagnosis is made by the typical pattern of symptoms, especially the fact that there are long periods without any symptoms at all. However, many tests, usually unnecessarily, are often done to make sure that these children don't have other, more serious, disorders. These tests might include a head CT or MRI, metabolic tests and/or an abdominal ultrasound. One study, The diagnostic evaluation of children with cyclic vomiting: A cost-effective assessment, determined that an Upper GI with small bowel follow through (UGI-SBFT) plus empiric treatment with antimigraine medications (like Periactin or Elavil) was the 'most cost-effective initial strategy to treat cyclic vomiting syndrome.' The UGI-SBFT can help to make sure that your child does not have a malrotation with volvulus, another cause of episodic vomiting in children.
Treatment and PreventionAs already mentioned, antimigraine medications are often effective at preventing episodes of cylic vomiting syndrome. This was confirmed in a study, Effective Prophylactic Therapy for Cyclic Vomiting Sydrome in Children using Amitriptyline or Cyproheptadine, in which 73% of children who were treated with amitriptyline and 66% of those on cyproheptadine had a complete response to the medicines. A complete response was defined as no episoodes of vomiting. An additional 17-18% of children had a partial response, with a 50% or greater decrease in episodes of vomiting.
It may also be helpful to try to figure out the specific things, if any, that trigger your child's episodes of vomiting. Like migraines, these triggers might include stress, excitement, infections, eating certain foods, hot weather, or motion sickness.
References:The diagnostic evaluation of children with cyclic vomiting: a cost-effectiveness assessment. Olson AD - J Pediatr - 01-Nov-2002; 141(5): 724-8
Effective prophylactic therapy for cyclic vomiting syndrome in children using amitriptyline or cyproheptadine. Andersen JM - Pediatrics - 01-Dec-1997; 100(6): 977-81