My scan found something — what does it actually mean?
A precise explanation of what a Chiari Malformation Type I (CM-I) finding on MRI means, what the radiographic threshold does and does not tell you, and what happens next.
Why This Finding Appears
Why this finding appears on scans that were not looking for it
Chiari Malformation Type I (CM-I) is the anomaly you are dealing with unless your report specifies otherwise. The majority of cases discovered today are incidental — found on MRI ordered for headache, head trauma, neck pain, dizziness, tinnitus, or a pre-employment physical. The scan was looking for something else. It found this instead.
This is not accidental. The widespread availability of MRI since the 1990s has created what one literature review aptly described as a generation of patients carrying a radiographic label whose clinical significance is, in the majority of cases, genuinely uncertain and requires specialist evaluation to clarify.
What the scan found is a structural anomaly in which the lower portion of the cerebellum — specifically the cerebellar tonsils — descend through the opening at the base of the skull (the foramen magnum) further than expected. Whether that descent is causing problems, is a bystander finding, or sits somewhere ambiguously between the two, is a clinical question. The MRI alone cannot answer it.
The essential distinction: a radiographic finding of Chiari is not a diagnosis of symptomatic Chiari Malformation. The two require each other — imaging and clinical symptoms — and a specialist to connect them. One without the other is incomplete.
The Radiographic Threshold
The 5mm criterion — and why it is imperfect
The standard definition of CM-I is herniation of the cerebellar tonsils at least 5mm below the foramen magnum, measured on a sagittal T1- or T2-weighted MRI image. This measurement is made from the McRae line — a reference drawn from the basion (front edge of the foramen magnum) to the opisthion (rear edge) — to the lowest tip of the tonsils.
The 5mm threshold has been the dominant criterion since Elster and Chen formalized it in 1992. It is useful, widely applied, and insufficient on its own. Here is what the literature actually shows.
| Tonsillar Descent | Classification | Clinical Significance |
|---|---|---|
| Less than 3mm | Normal variant in most studies | Up to 2.8mm measured in asymptomatic controls (Aboulezz et al.). Not clinically significant in isolation. |
| 3mm to 5mm | Borderline zone | Some institutions use 3mm as the threshold in children. Clinical correlation required. CSF flow study may be informative. |
| 5mm or more | Meets conventional CM-I threshold | Still does not predict symptoms — roughly 14% with this finding are entirely asymptomatic. |
| 13mm or more | Symptoms more likely to correlate | Correlation between descent and symptoms increases at this threshold. Still not deterministic. |
The degree of tonsillar descent does not correlate with symptom severity. A 3mm ectopia can be profoundly symptomatic; a 22mm ectopia can be discovered incidentally on a routine scan. What matters more than millimeters is what the tonsils are doing to the CSF circulation at the foramen magnum.
What radiologists look for beyond the millimeter measurement
- Shape — peg-like or pointed tonsils. Rounded tonsils suggest less crowding. Peg-shaped tonsils indicate compression and are seen in 85% of patients with herniation greater than 5mm. More likely to impede CSF flow.
- CSF spaces — effacement at the foramen magnum. Loss of the normal CSF cushion behind and below the tonsils at the craniocervical junction. This, more than millimeters alone, predicts functional obstruction.
- Brainstem — cervicomedullary kink. A posterior angulation at the junction of the medulla and cervical cord. Present in approximately 71% of symptomatic patients.
- Variant — Chiari 1.5. When the obex (the inferior tip of the fourth ventricle) also descends below the foramen magnum. Present in approximately 17% of those diagnosed with CM-I. Associated with higher revision surgery rates.
A note on borderline reports: if your report describes "low-lying cerebellar tonsils" without measuring them, or uses the phrase "borderline Chiari," you are in the clinically ambiguous zone. This is not reassuring and not alarming — it is a signal that the radiographic finding alone is insufficient and that clinical evaluation by a specialist will be necessary before any conclusions can be drawn.
Associated Findings
What else the scan may have found, and why it matters
CM-I frequently coexists with other structural findings. Some are consequences of the tonsillar herniation; others are independent findings that shape the clinical picture significantly. Your report may mention one or more of the following.
Syringomyelia (syrinx)
A fluid-filled cavity within the spinal cord, developing as a consequence of disrupted CSF circulation at the craniocervical junction. Present in approximately 25% of patients with CM-I across multiple series.
A syrinx is the most consequential associated finding in terms of clinical urgency. Its presence typically shifts management from observation toward more active specialist evaluation, and in many cases toward surgical planning, even in patients with modest symptoms. Syrinx size and extension along the cord are reported in millimeters; a syrinx affecting multiple thoracic levels carries more urgency than one confined to a single cervical segment.
If your report mentions a syrinx, that is a different clinical situation than tonsillar ectopia alone.
Hydrocephalus
Enlargement of the brain's ventricular system due to obstruction or impaired absorption of CSF. Less common in pure CM-I than in Chiari II, but it does occur and requires evaluation for elevated intracranial pressure. Its presence changes the surgical calculus — a shunt may be indicated before or instead of posterior fossa decompression.
Transverse sinus stenosis, which can cause raised intracranial pressure independently, is also increasingly recognized on MRI in some CM-I patients and is clinically relevant when identified.
Basilar Invagination
The odontoid process (the peg-like projection of the C2 vertebra) prolapsing upward into the foramen magnum. Seen in approximately 25% of CM-I patients in surgical series. When present, it constitutes what is now termed "complex Chiari" — a constellation associated with substantially different surgical planning and, often, the need for occipitocervical fusion in addition to or instead of standard posterior fossa decompression.
Ehlers-Danlos Syndrome and connective tissue disorders
Approximately 13% of patients with CM-I in one of the largest prospective cohorts (Milhorat et al., 2,813 patients) met diagnostic criteria for Ehlers-Danlos syndrome or a related hereditary connective tissue disorder. This is not a small subgroup.
EDS and related conditions change the Chiari picture in important ways. The underlying connective tissue fragility that allows the brain to herniate downward also means the ligaments supporting the craniocervical junction are lax. This can produce craniocervical instability (CCI) — a dynamic instability of the skull on the spine that may be invisible on a standard supine MRI but demonstrable on upright or flexion/extension imaging.
In this population, standard posterior fossa decompression alone may fail — or worsen instability — if the CCI component is not identified and addressed. The EDS/Chiari/CCI triad is one of the genuinely complex areas of neurosurgical practice.
If you or your evaluating physician suspects EDS — hypermobility, skin extensibility, joint dislocations, dysautonomia, a family history of connective tissue disorders — that suspicion should be communicated directly to the neurologist or neurosurgeon who reviews your imaging. It changes the differential, the additional imaging recommended, and the surgical conversation if you get there.
Further Imaging
Further imaging that may be recommended, and why
A standard brain MRI showing CM-I is the beginning of the radiographic evaluation, not the end. Depending on what it shows, and what a specialist finds clinically, one or more of the following studies may be ordered.
Indicated in all patients with a confirmed CM-I finding. Surveys the entire spinal cord for syringomyelia, which can occur anywhere from the cervical cord to the conus, and assesses for scoliosis, tethered cord, and spinal dysraphism. Even a small syrinx in the thoracic cord is clinically significant. This is not optional follow-up — it is standard of care in symptomatic patients and those with radiologically significant CM-I.
Uses cardiac-gated phase-contrast MRI to visualize the pulsatile movement of CSF at the foramen magnum — essentially a movie of CSF flow timed to your heartbeat. The most underutilized study in general radiology practice for this condition. When cine MRI shows obstruction at the foramen magnum alongside a syrinx, it is a strong convergent signal for surgical consultation. Best used as one component of a converging clinical picture, not as a standalone decision-making tool.
MRI is excellent for soft tissue; CT is better for bone. Indicated when basilar invagination, platybasia, atlanto-occipital assimilation, or other bony anomalies are suspected or observed on MRI. These findings — collectively described as complex Chiari — require CT for proper characterization and surgical planning.
Conventional MRI is performed supine. In patients with EDS or suspected craniocervical instability, this may miss dynamic instability that only manifests under gravitational load. Upright MRI (available at a limited number of centers) and dynamic flexion/extension views can reveal translational and rotational instability not visible on standard imaging. A specialist-directed study, not first-line for all CM-I patients.
Intrathecal contrast injected via lumbar puncture, followed by CT imaging of the spine. Now largely supplanted by MRI for most applications, but relevant when MRI is contraindicated, to assess occult CSF leaks (particularly in patients with low-pressure headache or spontaneous intracranial hypotension), and in some complex craniocervical junction assessments. Your neurologist or neurosurgeon will specify if this is indicated.
Who Evaluates This Finding
Who evaluates this finding, and what they are looking for
In most clinical pathways, the first specialty to evaluate an incidental CM-I finding is neurology. The neurologist's role is to establish whether a clinical syndrome consistent with Chiari is present — meaning: are there symptoms that correspond anatomically to what the imaging shows?
The referral to a neurosurgeon follows when the neurologist identifies a clinical case for surgical evaluation. Occasionally, a primary care physician or emergency physician will refer directly to neurosurgery — particularly when there is an associated syrinx, progressive neurological deficit, or known connective tissue disorder. Both pathways are appropriate. What is not appropriate, though common, is managing a confirmed CM-I finding without specialist evaluation of any kind.
The neurological examination in a patient with CM-I is specifically directed at brainstem and upper cervical cord function — not a generic neurological screen. Among the things being assessed: ocular motility (nystagmus), palatal function, gag reflex, tongue strength, shoulder shrug, hand grip and fine motor function, upper and lower extremity reflexes, proprioception, and gait. Many of the most significant Chiari symptoms — swallowing difficulty, sleep apnea, hand weakness, dissociated sensory loss — are missed on cursory examination.
If your GP seems uncertain: the finding of CM-I on an incidental MRI is routinely underestimated by non-specialist physicians, in part because the majority of such findings are clinically benign. "Watch and wait" is a defensible recommendation for a truly asymptomatic patient — but not a substitute for specialist evaluation. If you have symptoms that you believe may relate to the finding, you are entitled to a neurology referral. Ask for it directly.
What the Numbers Tell You
What the numbers actually tell you about your situation
Here is the honest statistical landscape for a patient who has just received an incidental CM-I finding.
Of adults who meet the 5mm radiographic threshold, approximately 14% are entirely asymptomatic and require no intervention beyond specialist confirmation of that status. The remainder carry symptoms of varying degree — from mild, intermittent occipital headache to progressive neurological deficit.
Meadows et al., J Neurosurg 2000Across the full population of radiographically identified CM-I, the proportion who ultimately require surgery is a small minority. The proportion who benefit from specialist evaluation to understand their situation clearly is essentially everyone.
In one large study of over 22,000 brain MRIs, 0.77% had tonsillar ectopia of at least 5mm. Of those, the natural history in conservatively managed patients was described in the authors' own summary as quite benign — meaning few patients who were managed non-surgically later required surgery. This is the statistical backdrop. It should neither minimize your concern nor dismiss it.
What changes the picture: the presence of a syrinx, the morphology of the tonsils, what the cine MRI shows, what the neurological examination finds, and whether you have a concurrent connective tissue disorder. These are the variables that move an individual patient from the statistical average into a specific clinical category.
Next — Room 2: Clinical Diagnosis
The scan is only half the answer.
The other half is what you are experiencing.
A radiographic finding and a clinical diagnosis of Chiari Malformation are not the same thing. Room 2 examines what your symptoms mean, which ones have the strongest anatomical correspondence to your scan, and what a proper neurological evaluation should cover.
Do my symptoms match? Understand the clinical diagnosis
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