Reading your MRI report

Chiari I malformation

Also written as Chiari malformation type 1, Chiari 1, CM-I

The lowest part of the back of the brain extends below the opening at the base of the skull.

Cerebellar tonsils

The two lowest parts of the cerebellum, at the bottom of the back of the brain (not the tonsils in your throat). The cerebellum helps control balance and coordination.

Tonsillar ectopia

Also written as tonsillar descent, low-lying tonsils, tonsillar herniation

Different ways of saying the tonsils sit lower than usual.

Some reports use "ectopia" or "low-lying" for a small drop that isn't called Chiari; others use all of these words for the same thing. The number in millimeters says more than the word.

On a Chiari report, "herniation" usually describes this same finding: how far the tonsils sit below the opening. Ask your doctor what your report means for you.

Foramen magnum

The opening at the base of the skull where the brainstem becomes the spinal cord.

"7 mm below the foramen magnum"

Also written as below the McRae line, below the basion–opisthion line

How far the tips of the tonsils reach below a line drawn across the skull opening. About 5 mm is the usual cutoff for calling it Chiari I; a few millimeters below can still be normal.

Some people with a clear descent have no symptoms, and some with less than 5 mm do. Doctors also look at the shape of the tonsils, spinal fluid flow, and whether a syrinx is present. The number alone doesn't decide who needs surgery.

Peg-like tonsils

Also written as pointed tonsils

The tonsils are pointed rather than rounded, which is common in Chiari I.

Crowding at the foramen magnum

Also written as effacement of the CSF spaces

The space around the tonsils where spinal fluid flows is squeezed, so there's less room for fluid to pass.

"Effacement" means the space is narrowed or squeezed shut.

CSF

Also written as cerebrospinal fluid

Clear fluid that cushions the brain and spinal cord. With each heartbeat it pulses back and forth between the skull and the spinal canal.

CSF flow study

Also written as cine MRI, phase-contrast MRI

A special MRI that shows how spinal fluid moves back and forth with the heartbeat. Some doctors use it to see whether flow is slowed or blocked at the skull opening.

Syrinx

Also written as syringomyelia, syringohydromyelia, hydromyelia

A syrinx is a fluid-filled cavity inside the spinal cord. It often develops alongside Chiari, most often in the neck part of the spinal cord.

Syringomyelia means having a syrinx. Hydromyelia technically means the spinal cord's thin central fluid channel has widened. The two are often hard to tell apart on MRI, so reports may say "syringohydromyelia". Ask your doctor which one your report means.

Posterior fossa

Also written as small or crowded posterior fossa

The space at the back and bottom of the skull that holds the cerebellum and brainstem. In Chiari I it is often smaller or differently shaped than usual, which crowds the brain.

Hydrocephalus

A buildup of too much spinal fluid in the fluid-filled spaces deep in the brain (the ventricles). It is much less common with Chiari I than with Chiari II.

After surgery

Also written as suboccipital craniectomy, C1 laminectomy, duraplasty, postoperative changes

Suboccipital craniectomy: removing a small piece of bone at the back of the skull, next to the opening. C1 laminectomy: removing the back part of the first neck bone. Duraplasty: opening the brain's covering (the dura) and sewing in a patch to make more room.

"Postoperative changes" means the scan shows changes from the surgery, such as where bone was removed. Your surgeon can explain anything else the report mentions.