Chiari I: Well Known, Not Well Recognized

Seth Brown, MD; Linda Lanier, MD; Chris Sistrom, MD; Robbie Slater, MD; Dhanashree Rajderkar, MD; Ilona Schmalfuss, MD;

Anthony Mancuso, MD

University of Florida : Gainesville, Florida

Background: Call Readiness Simulation

• Computer aided online simulation (SIM) of emergency imaging studies

• Designed to test residents for readiness for call

• Providing proficient & objective assessment of resident competence in the emergency/critical care imaging &affirmation of Milestone achievements

• 8 hour simulation of 65 emergent & critical care cases of varying degrees of difficulty, including normal studies

• Presentation via full DICOM image sets

• Dictation of free responses into text boxes labelled

Critical findings

Incidental findings

Acuity ranking

Background: Call Readiness Simulation

Background: Call Readiness Simulation

• SIM was taken by 127 first (R1) & second (R2) year residents from 16 USA radiology training programs

• Chiari type 1 malformation was presented69% of residents failed to recognize the cerebellar tonsillar ectopia

Only 6% of the residents made the correct diagnosis and gave the appropriate recommendation

Conclusion: An observational gap exists in detecting cerebellar tonsillar ectopia which could delay urgent treatment or place

patients at risk for complications of a lumbar puncture

Teaching Points

• Define cerebellar tonsillar ectopia and Chiari I malformation and discuss CT and MRI imaging findings

• Review CT and MRI imaging findings of critical mimics of Chiari I malformation

• Discuss the importance of making the proper recommendations for patients with cerebellar tonsillar ectopia

Presentation Outline• SIM case presentation

• Chiari I malformation: Clinical presentation and CT & MRI findings

• Mimics of the Chiari I malformation

• Search pattern to prevent observational errors

• Appropriate recommendations

SIM caseHistory: Headache, fever, rash. Evaluate prior to spinal tap.

Non-contrasted CT images show the cerebellar tonsils extending downward through the foramen magnum. There is also anterior displacement of the cervicomedullary junction –Findings that were overlooked by 69% of residents.

The foramen magnum represents a blind spot in

the axial plane

Normal CT/MR for comparison

Axial & sagittal CT and axial T1 images illustrate normal positioning of the cerebellar tonsilswith preservation of the cerebellomedullaris, superior vermian, and interpeduncular &

prepontine cisterns.

Cerebellar Tonsillar Ectopia

• = downward extension of the cerebellar tonsils through the foramen of magnum (low lying tonsils)

• Does not imply the etiology and the radiologist is required to search for additional imaging findings as outlined in this exhibit to distinguish the different causes of cerebellar tonsillar ectopia

Chiari I malformation

Intracranial hypotension

Tonsillar herniation syndrome

Idiopathic intracranial hypertension

Chiari I Malformation • Congenital hindbrain dysgenesis manifesting with cerebellar tonsillar ectopia

• Ectopia distance as defined on MR:Must be >3-5mm (depending on age)

5-10mm variably symptomatic

>10mm almost always symptomatic

• Other imaging findings: Preservation of the majority of CSF spaces

Peg like tonsils

Hydrocephalus & cervicothoracic syrinx

• Up to 25% are associated with a congenital osseous abnormality of the skull base or cervical spine

Chiari I Malformation

Sagittal T1 image illustrates how tonsillar ectopia is measured on MRI. The green line connects the basion & opisthion and represents the plane of the foramen magnum. The red line measures the extent of tonsillar ectopia below the plane of the foramen magnum which is greater than 10 mm in this patient.

The diagnosis of Chiari I malformation was made as the the 4th ventricle and posterior fossa cisterns (interpeduncular, prepontine,& superior vermian) were preserved.

Chiari I Malformation

CT reformatted image in sagittal plane in the same patient reveals that similar information can be obtained on CT as on the sagittal T1 image showing the identical extent of tonsillar ectopia (red line) below the plane of the foramen magnum (green line) with preservation of the posterior fossa CSF spaces.

Tonsillar ectopia is more easily identified on sagittal CT images when compared to the axial plane of the presented SIM case.

Chiari I Malformation • May impede CSF flow causing

Obstructive hydrocephalus (up to 30%)

Cervicothoracic syrinx (in 30-40%)

• Symptoms are nonspecific • Headaches

• Nausea

• Cerebellar symptoms

• Paralysis and dysautonomia

• Symptoms are usually slowly progressive

Typical progression with degree of tonsillar descent

Two different patients with Chiari I malformation with cerebellar tonsil ectopia. In one patient, the impaired CSF flow causes obstructive hydrocephalus while in the other patient

it results in a cervical syrinx. A small syrinx can be easily overlooked on a head MRI.

Chiari I Malformation - Complications

** *

Symptomatic Chiari I malformation is treated by a shunting procedure and / or suboccipitaldecompression in which the inferior portion of the occipital bone is resected to widen the

foramen magnum and improve the CSF flow.

Chiari I Malformation - Treatment

Intracranial Hypotension• Defined as CSF opening pressure < 6-7cm H2O during an LP (or

fundoscopic examination)

• Symptoms: postural headache (most common), nausea/emesis, neck pain, hearing/visual disturbances, vertigo and occasionally coma

• Most cases result from a CSF leak Primary or spontaneous: dural dehiscence of perineural cysts, degenerative

dural tears

Secondary: trauma, iatrogenic (lumbar puncture or spinal anesthesia), overshunting

Intracranial Hypotension• Hypothesized that an increase in venous intracranial blood volume

compensates for the decrease in CSF volume

• Imaging findings in addition to tonsillar ectopiaBilateral subdural fluid collections

Engorgement of the dural venous sinuses and pituitary gland

Pachymeningeal enhancement

Decreased mamillopontine (MP) distance <5.5mm

Pontomesencephalic (PM) angle <50 degrees

• Findings are best seen on MRI and rarely all are present in a single patient

Resutling in obliteration of the interpeduncular cistern

Sagittal CT images show cerebellar tonsillar ectopia in two different patients. The patient on the left also shows loss of the interpenduncular cistern, dural venous sinus engorgement & extra-axial effusion indicating intracranial hypotension. None of the additional findings are seen in the patient on the right with Chiari I malformation.

Intracranial Hypotension versus Chiari I

Sagittal T1 image shows cerebellar tonsillar ectopia with severely decreased PM angle when

compared to normal

Intracranial Hypotension versus Normal

and markedly decreased MP distance when compared to normal. These findings are consistent with intracranial hypotension.

MRIs of 3 different patients with cerebellar tonsillar ectopia illustrate additional findings of intracranial hypotension: dural venous sinus engorgement, severely decreased MP distance &

PM angle, small subdural effusions and diffuse pachymeningial enhancement.

Intracranial Hypotension

Tonsillar Herniation Syndrome • Herniation of the cerebellar tonsils secondary to significant

mass effect in the posterior fossaTumorHemorrhageEdemaInfectionHydrocephalus

• Symptoms are rapidly progressive and can quickly lead to respiratory depression and death

Tonsillar Herniation Syndrome

• Often associated with upward transtentorial herniation

• May be related to severe supratentorial pathology causing downward transtentorial and subsequent tonsillar herniation

• Manifests on imaging as obliteration of the following CSF spaces:Superior vermian & tectal plate cistern

4th ventricle

Prepontine cistern

At the foramen magnum

Suprasellar cistern in case of supratentorial pathology

Axial & sagittal CT images illustrate imaging findings of cerebellar herniation syndrome with obliteration of the CSF spaces at the foramen magnum due to cerebellar tonsillar ectopia, of

the superior vermian cistern due to upward transtentorial herniation, and compression of the prepontine cistern and of the 4th ventricle caused by diffuse cerebellar edema bilaterally

related to metastatic disease. Notice the preservation of the MP distance & PM angle.

Tonsillar Herniation Syndrome

Axial CT images reveal unilateral cerebellar tonsillar ectopia and complete obliteration of the 4th ventricle & prepontine cistern as well as of the superior vermian & tectal plate cisterns. These findings are related to tonsillar and upward transtentorial herniation caused by a left

cerebellar lesion with subtle surrounding edema.

Tonsillar Herniation Syndrome

The subsequently performed MRI of the same patient confirms the cystic lesion in the left cerebellum with marked surrounding edema causing obliteration of the 4th ventricle and prepontine cistern. The rim enhancement in combination with the restricted diffusion is

consistent with an abscess.

Tonsillar Herniation Syndrome T1+Gd T2 DWI

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Axial CT image illustrating a case of obstructive hydrocephalus causing cerebellar tonsillar ectopia. Note these images were taken shortly after a VP shunt was placed.

Tonsillar Herniation Syndrome

Follow up (FU) axial & sagittal CT images reveal resolution of the obstructive hydrocephalus and of the cerebellar tonsillar ectopia.

Prior FU FU

Axial & sagittal CT images show cerebellar tonsillar ectopia in association with complete obliteration of the basilar, prepontine & superior vermian cisterns as well as complete

compression of the 4th ventricle. These findings could be mistaken for central hypotension syndrome, however the loss of gray-white matter differentiation and the hyperdense

appearance of the tentorium & vasculature should lead to the correct diagnosis of diffuse hypoxic brain injury.

Tonsillar Herniation Syndrome

Idiopathic Intracranial Hypertension • Historically referred to as pseudotumor cerebri

• Most commonly affects middle aged, overweight females

• Defined as increased intracranial pressure ( >20-25 cm H2O) without an identifiable cause (e.g. mass lesion, hydrocephalus, or duralvenous sinus thrombosis) and normal CSF analysis

• Symptoms: headache (most common), tinnitus, visual disturbances that can lead to blindness due to papilledema, and eye pain

Idiopathic Intracranial Hypertension

• Poorly understood pathogenesis, postulated to be related to reduced CSF resorption, elevated CSF production, or increased cerebrovascular volume

• Imaging findingsFlattening of the posterior sclera & bulging optic discs (due to papilledema)

Tortuosity of the optic nerve with widening of the perioptic nerve CSF spaces

Enlarged arachnoid spaces (empty sella turcica, enlarged trigeminal cave)

Venous sinus stenosis (especially affecting the lateral transverse sinuses)

Cerebellar tonsillar ectopia in some cases

• Findings are best seen on MRI and rarely all are present in a single patient

Axial & sagittal CT images show cerebellar tonsillar ectopia in a young, slightly obese female that might be mistaken for Chiari I malformation. Notice the partially empty sella that is usually better appreciated on MR imaging and provides an important clue to the correct diagnosis of idiopathic intracranial hypertension which was confirmed on subsequent LP.

Idiopathic Intracranial Hypertension

MR images illustrating the ocular findings in idiopathic intracranial hypertension including bulging optic discs, flattening of the posterior sclera, and tortuous optic nerves with dilation

of the CSF space surrounding the optic nerves.

Idiopathic Intracranial Hypertension

Axial T1+Gd Coronal T2 Sagittal T2

Conclusion• Our SIM results revealed an observational gap in detection of cerebellar

tonsillar ectopia on axial CT images & an interpretational gap in regard to the correct diagnosis when tonsillar ectopia was detected

• Head CT remains the study of choice for acute intracranial findings and is commonly preformed prior to LP

• Therefore it is critical to recognize tonsillar ectopia on initial CT & look for diagnostic clues that can help to make the correct diagnosis and prevent potentially life threatening LP

• Consider reformation of the axial images in sagittal plane in every patient as cerebellar tonsillar ectopia is easier appreciated in the sagittal plane

ConclusionChecklist for imaging evaluation• Foramen magnum to detect cerebellar tonsillar ectopia

• CSF spaces = closure of the interpretational gapPosterior fossa cisterns & 4th ventricle that are obliterated in

intracranial hypotension & posterior fossa edema/mass

Interpeduncular cistern that is obliterated in intracranial hypotension & downward transtentorial herniation

• Additional clues = closure of the interpretational gapEmpty sella in idiopathic intracranial hypertension

Venous and pituitary gland engorgement in intracranial hypotension

Subdural hematoma / effusion in intracranial hypotension

Ocular findings in idiopathic intracranial hypertension

Closure of theobservational gap

Closure of theinterpretational

gap

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