Imaging Tests Used to Diagnose Meningitis

Lumbar punctures, CT scans, and MRIs are a few of the imaging tests doctors use to reach an accurate diagnosis. 

By Jean Rothman

Medically reviewed by Pat F. Bass III, MD, MPH

When meningitis strikes, it's vital that the health care team determine as quickly as possible if a virus or bacteria caused the disease. If the problem is bacterial meningitis, it can be fatal or leave the patient with severe health problems, such as deafness, learning disabilities, and brain damage. Meningitis treatment with antibiotics should be started as fast as possible. 

Viral meningitis is less severe than bacterial meningitis and has no specific treatment. 

Diagnosing Meningitis With a Lumbar Puncture

Meningitis is an inflammation of the meninges, the covering around the brain and spinal cord. Most often, the inflammation is caused by an infection of the cerebral spinal fluid that also surrounds the brain and spinal cord. 

If meningitis is suspected, the definitive diagnosis is made by lumbar (low back) puncture, or spinal tap. During the test, fluid is removed and examined under a microscope to determine whether there is an infection, and if so, the type of bacteria causing the infection. Other tests used to diagnose meningitis include CT scans and MRI. 

How CT Scans Aid in Diagnosing Meningitis 

A CT (computed tomography) scan is an imaging test that creates two-dimensional, cross-sectional images. CT can be used to show bones, organs, tissues, and blood abnormalities. When done with a contrast dye that is injected during the test, CT scans can highlight brain tissues to determine whether the meninges are inflamed.

A CT scan can also show whether there is inflammation of the skull or sinuses, which may be helpful in diagnosing meningitis. In addition, CT scans can help doctors determine the presence of:

• Ruptured disks
• Blood clots
• Irregularities of bones or blood vessels
• Some types brain tumors
• Brain damage caused by a head injury
• Other disorders

Most people with meningitis have a CT scan before their lumbar puncture. “We do this to ensure that there isn’t a large, space-occupying lesion, like a tumor, causing increased pressure,” says Jason T. Lerner, MD, assistant clinical professor at the Mattel Children’s Hospital at University of California, Los Angeles. “If there is such a lesion, doing a lumbar puncture can have severe consequences, including herniation [protruding tissue] of part of the brain.” 

Physicians may perform CT scanning to help diagnose meningitis in children and adults in certain categories. These include patients with a history of:

• A suppressed immune system
• Water on the brain (hydrocephalus)
• Shunts
• Trauma to the head
• Increased intracranial pressure
• Focal neurologic signs (signs of impairment of certain neurologic activities, such as perception and behavior)

Using MRI to Diagnose Meningitis

Magnetic resonance imaging (MRI) uses an extremely strong magnet along with computer-generated radio waves to produce pictures of the body. Images from an MRI scan provide even more detail than those from a CT scan. 

MRI can detect whether there is inflammation in the brain and spinal cord, infection, eye disease, or tumors, among many other disorders. 

Why X-Rays and Ultrasounds Aren’t Used for Meningitis Diagnosis

“X-rays don’t help us diagnose or follow-up meningitis because they can only produce images of the skull, because it is bony, but not the brain,” explains Dr. Lerner. 

Conventional X-rays, sometimes called radiographs, use low-dose ionized radiation to produce pictures of the body. Bones show up very well on X-rays, but other kinds of tissues don’t.

Like X-ray, ultrasound cannot penetrate bone, so the only time it could be used to examine the brain is when a baby’s soft spot at the top of the head (fonatelle) hasn’t yet closed. This allows the sound waves to get to the brain without being blocked by the skull. However, even under these circumstances, ultrasound is not used to detect meningitis. 

Observation to Diagnose Meningitis Aftereffects

Lerner prefers to use his own eyes when a meningitis patient returns for a follow-up visit. “If something strikes me as odd during this exam, for instance, if the patient is heavily favoring one side of the body over the other, I refer the patient to a neurocognitive expert for evaluation,” he says. 

If results of the evaluation show that there may be a brain lesion or abscess resulting from the meningitis, an MRI may be ordered, but this is rare. “It is possible to use serial MRI for long-term follow-up, but it is likely the findings will be normal,” he says.