Laboratory Studies
Laboratory studies are important to exclude other etiologies, including electrolyte abnormalities (eg, calcium, magnesium, phosphorus).
Thyroid function tests should also be performed.
Imaging Studies
Abdominal plain films are useful for initial screening and assessment of severity.
After plain films reveal the megacolon, water-soluble contrast enema may be helpful for a number of reasons, as follows [12] :
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Accurately assesses the size of the colon
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Helps to differentiate the presence of megacolon, megarectum, or both
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Helps to define the anatomy
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Can be used therapeutically to evacuate the colon
Distinguishing a colonic inertia etiology from that of a functional outlet obstruction is probably best accomplished by colonic marker transit studies. Numerous ways to perform this test are available. Note the following:
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One method is to instruct the patient to consume 30 g of dietary fiber daily and to stop using laxatives, enemas, and all other nonessential medications for at least 2 days prior to (as well as during) the test.
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The patient swallows the markers, and abdominal plain films are obtained on days 1, 3, and 5.
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Patients with colonic inertia tend to have markers distributed throughout the large bowel from cecum to rectum, while patients with outlet obstruction exhibit markers proceeding normally through the colon but accumulating in the rectum.
Other Tests
Anorectal manometry may help to distinguish congenital from acquired megacolon. The presence of a rectoanal inhibitory response means that there are intact ganglia, and the patient does not have Hirschsprung disease. If the inhibitory response is absent, a rectal biopsy is still needed to confirm the diagnosis of Hirschsprung disease.
Pudendal nerve latency testing may elucidate problems related to peristaltic movement, anatomical and/or mechanical problems with evacuation, and nerve-associated problems with defecation.
Colonoscopy should be used to rule out an obstructive/mechanical cause of colonic dilatation.
Histologic Findings
Histology is helpful for determining the etiology of the condition. Although full-thickness biopsy is the criterion standard to establish a diagnosis of Hirschsprung disease, mucosal suction biopsy is adequate in most instances. The absence of ganglion cells is characteristic of Hirschsprung disease, and specific stains for acetylcholinesterase are used to highlight abnormal morphology. Other than Hirschsprung disease, however, the presence of ganglion cells does not specify one cause over another. For most cases, there is no indication for histology because Hirschsprung disease is not considered or excluded by normal manometric findings.
Ohkubo et al reported that histopathologic abnormalities may precede the clinical manifestations of idiopathic megacolon. [13] They compared histopathologic features of dilated and nondilated loops in 53 full-thickness samples from 31 patients with idiopathic megacolon with 16 samples from 8 controls and defined hypoganglionosis as fewer than 60 ganglion cells/cm. The investigators noted the presence of neuropathy in 61.3% of patients (n=19), myopathy in 35.5% (n=11), and mesenchymopathy in 32.2% (n=10), with some overlap of subtypes. In most cases, there were similar histopathologic abnormalities between the dilated and nondilated loop samples. [13]
