Chiari 1

General

Technically not a true malformation and rather is more of deformation

AKA

Primary cerebellar ectopia

Adult Chiari 1 malformation

Since it tends to be diagnosed in the 2nd or 3rd decade of life

Classically described as

A rare abnormality

Restricted to caudal displacement of cerebellum with tonsillar herniation below the foramen magnum (below for criteria) and

“peg-like elongation of tonsils”

Definition

A functional obstruction at the foramen magnum by cerebellar tonsils

One that may result in neuraxis compression and/or obstruction to CSF flow across the craniovertebral junction’

Should not use measurements: decent of the cerebellar tonsils > 5 mm below the foramen magnum

Although definitions range from 0 mm (tonsils at the level of the foramen magnum) to 1.5 cm.
Defining Chiari 1 by measurement is not only arbitrary but potentially misleading, risking both over-diagnosis and under-diagnosis
Cerebellar tonsil descent below FM with impaction, while common, is no longer a sine qua non of diagnosis.

Aetiology

Congenital

A small posterior fossa

Underdevelopment of the occipital bone due to a defect in the occipital somites originating from the para-axial mesoderm
Low lying tentorium (the roof of the p-fossa)
Thickened or elevated occipital bone (the floor of the p-fossa)
Space occupying lesion in p-fossa

Arachnoid cyst (Retrocerebellar or Supracerebellar),
Tumor (e.g. FM meningioma or cerebellar astrocytoma),
Hypervascular dura

Arachnoid web or scar or fibrosis around brainstem and tonsils near FM.

Abnormalities of the upper cervical spine

Hypermobility of the craniovertebral junction
Klippel-Feil syndrome
Occipitalization of the atlas
Anterior indentation at foramen magnum: e.g. basilar invagination or retroversion of the odontoid process

Ehlers-Danlos syndrome

Craniosynostosis: especially cases involving all sutures

Retained rhomboid roof: rare

Acquired

Has been described with just about anything that takes up intracranial space

Chronic subdural hematomas
Hydrocephalus

Following LP shunt (p.435) or multiple (traumatic) LPs:

Acquired Chiari 1 malformation (may be asymptomatic)

Primary	Secondary
Tonsilar ectopy	Cranio-spinal pressure dissociation
Small posterior fossa	IIH, Hydrocephalus, Tumour
Isolated abnormality	Spontaneous leak, iatrogenic leak (dural tear, LP shunt overdrainage), CSF-venous fistula
ㅤ	CCJ anomalies (retroflexed odontoid, basilar invagination)

Numbers

Average age at presentation is 41 years (range: 12–73 yrs).

Slight female preponderance (female: male= 1.3:1).

Incidence 1:1000

Average duration of symptoms

If symptoms clearly related to Chiari malformation is 3.1 yrs (range: 1 month-20 yrs)
If nonspecific complaints

e.g. H/A, are included, this becomes 7.3 years
This latency is probably lower in the MRI era.

Pathophysiology of Chiari I malformation

Overt hydrocephalus with ventriculomegaly

Affects 10% of paediatric patients with Chiari 1

Patient should undergo ICP study first

Management

The hydrocephalus should be treated first

Improvement in tonsillar herniation, or indeed complete reversal of the Chiari 1 following hydrocephalus treatment, whether by VPS or ETV, is well documented

Disorder of the craniovertebral junction

Affects 4-20% of Chiari 1 pts

Abnormal biomechanics and anatomy of the craniovertebral junction (CVJ) rather than posterior fossa volume is the cause

Bony malformation commonly found in Chiari 1

Atlanto-occipital assimilation
Basilar invagination
Craniocervical instability
Odontoid retroflexion

Incidence of paediatric Chiari 1 = 80%

Bollo et al. identified radiological parameters that increased the likelihood of occipito-cervical fixation being required.

Descent of the obex as well as the tonsils, below the plane of the foramen magnum (Chiari 1.5),
Basilar invagination
A clivo-axial angle < 125 degrees

Angle determined from the intersection of a line along the slope of the clivus (Wackenheims line) and a line drawn from the posteroinferior point of the C2 vertebral body up along the odontoid (posterior spinal line).

Management

Posterior craniocervical fixation and decompression

Mainstay of treatment

Is anterior decompression required

If the CVJ abnormality is not reduceable then go for trans-oral decompression
Ventral decompression is not without risk and does not obviate the need for a posterior procedure (fixation with or without decompression).
The degree of odontoid retroflexion and its effect on the neuraxis are most commonly expressed by the Grabb measurement or pBC2 line.

Grabb Oakes Measurement

Midline MRI T2 image used

Line drawn from Basion to the inferior(bottom), posterior aspect of the C2 bone.

Perpendicular line drawn to the dura

Yellow line is the Grab oakes measurement

A normal Grabb Oakes Measurement is 9mm or less

Ventral decompression was not required if this measure was < 9 mm.

Increasingly, it is recognised that traction (prior to or during surgery) will adequately address ventral compression in the majority of cases, obviating the need for trans-oral surgery

Disorder of intracranial venous drainage and craniosynostosis

Reduction of intracranial venous drainage rather than posterior fossa volume is the cause

Interaction between venous hypertension, intracranial pressure and cerebrospinal fluid circulation is complex; disturbances in each of these have been correlated with Chiari 1
Reversibility of the Chiari 1 was only observed when adequate venous drainage from the posterior fossa was established after therapeutic embolization
Reduced jugular venous drainage was among the strongest predictors of poor outcome
Reduce Jugular venous drainage → exacerbation by bulbar palsy causing chronic CO2 retention → progressive extrusion of hind brain
Post fossa volume by itself is a bad predictor for Chiari 1 outcome

Studies have failed to show a consistent correlation between posterior fossa volume and Chiari 1 in syndromic craniosynostosis, indicating that other mechanisms need to be explored
Possibly post fossa venous drainage deficit is a better answer

Jugular foraminal stenosis and collateral venous drainage are much more common among cases of syndromic craniosynostosis compared with controls
18/23 patients with craniosynostosis had either complete or more than 50% occlusion of one or both sigmoid/jugular sinuses

Syndromic craniosynostosis

Chiari 1 is present in

Kleeblatschadel (cloverleaf skull): 100%
Crouzon syndrome (70%)
Pfeiffer syndrome (50%)
Unusual in Apert syndrome (< 2%).

Unusual in Apert because the sutures were seen to persist for longer permitting adequate growth to accommodate the maturing cerebellum

Disorder of growth and metabolism

The Chiari I malformation is commonly a disorder of mesenchyme rather than neural tissue

More a disorder of the ‘container than the contents’

Growth hormone deficiency

In one MRI study of patients with GHD, 20% of cases were found to have Chiari 1
Basiocciput tended to be shorter, and the foramen magnum longer in cases of GHD, factors that might predispose to Chiari 1
GHD and Chiari 1 is further strengthened by observations of improvement, not only in Chiari 1 but also syringomyelia, when GHD patients are treated with growth hormone

Calcium and phosphate metabolism

Due to

Rickets due to

Vitamin D deficiency
Hypophosphataemic rickets: due to excessive urinary loss of phosphate

Rickets can affect skull growth
Chiari 1 was identified in 44% of cases of familial hypophosphataemia
It remains to be seen whether medical management of the underlying disease can modify the natural history of the Chiari 1.
Should Chiari 1 patient be screened for Vit D, Ca and PO4 deficiency

Radiation

Annual growth rate of the clivus was reduced after radiotherapy

Metabolic bone diseases

Pagets disease, fibrous dysplasia, Gorehams disease and osteopetrosis

Although it is often unclear whether the effect is mediated simply through reduced growth, or whether factors such as venous hypertension and secondary craniosynostosis, which commonly coexist these conditions, may also have a role.

Unproven and probably wrong causes of Chiari 1

Occult tethered cord syndrome (OTCS)

Definition: refers to a scenario in which there are clinical features compatible with a tethered spinal cord (sphincter dysfunction, low back pain, lower limb symptoms) yet on MRI, the conus is in a normal position and there are no overt features of spinal dysraphism.
It is still not clear if OTCS is associated with Chiari 1

How can the filum (usually less than 2 mm diameter), in the context of a normally positioned conus, to exert tension sufficient to cause traction of the brainstem
The spinal cord is, after all, not simply anchored by the filum alone but is attached to the spinal canal along its length via the nerve roots and dentate ligaments; therefore, any (likely small) tractional forces emanating from the filum will be exerted over a limited number of segments of the lower spinal cord.
Furthermore, in situations where the spinal cord is unquestionably low lying (for example lumbosacral lipomas), the hindbrain and cerebellum are typically normally positioned.

Hypermobility

Hypermobility states, in particular hereditary disorders of connective tissue (notably Ehlers-Danlos syndrome, EDS)

Postural orthostatic tachycardia syndrome (POTS)

Clinical features

Asymptomatic

15–30% of patients with adult Chiari malformation are asymptomatic

Pathomechanism of clinical features

Compression of brainstem at the level of the foramen magnum

Hydrocephalus

Hydrocephalus occurs in 7–9% of patients with Chiari type 1 malformation and syringomyelia.

Syringomyelia

Syringomyelia of the spinal cord is present in 30–70% of cases.
True hydromyelia probably doesn’t occur;

CSF flow has not been documented in humans, and it is generally not possible to find communication between the syrinx and the central canal in Chiari 1 patients.

Isolation of the intracranial pressure compartment from the spinal compartment causing transient elevations of ICP intracranial pressure

Symptoms

Pain/Headache (69%)

Especially headache, which is usually felt in the suboccipital region
Worsen by neck extension or Valsalva manoeuvre.
Pain classically should not be present between coughs

Weakness

Is also prominent, especially unilateral grasp.

Lhermitte’s sign

Lower extremity involvement usually consists of bilateral spasticity.

Presenting in 71 cases

Symptom	%
Pain	69
H/A (headache)	34
Neck (suboccipital, cervical)	13
Girdle	11
Arm	8
Leg	3
Weakness (1 or more limbs)	56
Numbness (1 or more limbs)	52
Loss of temperature sensation	40
Painless burns	15
Unsteadiness	40
Diplopia	13
Dysphasia	8
Tinnitus	7
Vomiting	5
Dysarthria	4
Dizziness	3
Deafness	3
Fainting	3
Facial numbness	3
Hiccough	1
Facial hyperhidrosis	1

Signs

Classically

Downbeat nystagmus on vertical movement
Rotatory nystagmus on horizontal movement
Also includes oscillopsia

10% will have a normal neurologic exam with occipital H/A as their only complaint.

Some patients may present primarily with spasticity.

Three main patterns of clustering of signs

Foramen magnum compression syndrome (22%)

Ataxia
Corticospinal and sensory deficits
Cerebellar signs
Lower cranial nerve palsies
Severe H/A (37%)

Central cord syndrome (65%)

Dissociated sensory loss

Loss of pain & temperature sensation with preserved touch & JPS

Occasional segmental weakness
Long tract signs (syringomyelic syndrome)
Lower cranial nerve palsies (11%)

Cerebellar syndrome (11%)

Truncal and limb ataxia
Nystagmus
Dysarthria

Presenting in 127 patients

Sign	%
Hyperactive lower extremity reflexes	52
Nystagmus	47
Gait disturbance	43
Hand atrophy	35
Upper extremity weakness	33
“Cape” sensory loss	31
Cerebellar signs	27
Hyperactive upper extremity reflexes	26
Lower cranial nerve dysfunction	26
Babinski sign	24
Lower extremity weakness	17
Dysesthesia	17
Fasciculation	11
Horner sign	6

Natural history

The natural history is not known with certainty (only 2 reports on “natural history” exist).

A patient may remain stable for years, with intermittent periods of deterioration.

50% remain stable for long periods

Rarely, spontaneous improvement may occur (debated).

Spontaneous resolution is rare

Progression to quadriplegia is rare

? Some deterioration inevitable

Occasional sudden deterioration

Evaluation

Ophthalmology assessment for papilledema

Skull XR

Only 36% were abnormal

26% showed basilar impression
7% platybasia
1 patient each with Paget’s and concave clivus

In 60 C-spine X-rays, 35% were abnormal

Assimilation of atlas
Widened canal
Cervical fusions
Agenesis of posterior arch of atlas

Unenhanced CT

CT is poor for evaluating the foramen magnum region due to bony artifact.

It is very good at demonstrating hydrocephalus (as is MRI).

When combined with intrathecal iodinated contrast (myelogram), reliability improves.

Findings

Tonsillar descent with possible complete blockage of dye at foramen magnum.

MRI of brain and C-spine

Diagnostic test of choice

Findings

Tonsillar herniation

Location of cerebellar tonsils below foramen magnum

Group	Meanᵃ	Range
Normal	1 mm above	8 mm above to 5 mm below
Chiari I	13 mm below	3–29 mm below

ᵃBased on measurements in 200 normals and 25 Chiari I patients taken in relation to the lower part of the foramen magnum

Criteria for Chiari I

Criteria for lowest extent of tonsils accepted as normal	Sensitivity for Chiari I	Specificity for Chiari I
2 mm below FM	100%	98.5%
3 mm below FM	96%	99.5%

> 5mm was defined as clearly pathologic (with 3–5mm being borderline)

Conversely, 14% of patients with tonsillar herniation > 5mm are asymptomatic (average extent of ectopia in this group was 11.4 ± 4.86 mm).

Tonsillar position relative to FM at various ages

Age (years)	Normal (mm)ᵃ	2 S.D.ᵇ(mm)
0–9	–1.5	–6
10–19	–0.4	–5
20–29	–1.1	–5
30–39	0.0	–4
40–49	0.1	–4
50–59	0.2	–4
60–69	0.2	–4
70–79	0.6	–4
80–89	1.3	–3

ᵃnegative number indicates distance below FM
ᵇS.D. = standard deviation. Descent > 2 S.D. beyond normal is suggested as a criteria for tonsillar ectopia
Potentially more significant than the absolute tonsillar descent is the amount of compression of the brainstem at the FM, best appreciated on axial T2WI MRI through the FM.

Complete obliteration of CSF signal and compression of the brainstem at the FM by impacted tonsils is a common significant finding.

Hydrosyringomyelia which occurs in 20–30% of cases.

Ventral brainstem compression when present.
Other

Hydrocephalus
Empty sella

Cine MRI (CSF flow study)

May demonstrate blockage of CSF flow at FM.

Not widely available.

Accuracy is not high → therefore usually does not alter management.

Myelography

Generally used only when MRI cannot be obtained.

Only 6% false negative.

It is critical to run the intrathecal contrast (dye) all the way up to the foramen magnum.

Usually combined with CT scan.

Differential

Vs Chiari type 2

Chiari 1

Medulla is not caudally displaced (some authors disagree on this point)
Brainstem is not involved
Lower cranial nerves are not elongated
Upper cervical nerves do not course cephalad.

Management


---
config:
  layout: dagre
---
flowchart TD
    Chiari1["Chiari 1"] --> Asymptomatic["Asymptomatic<br>"] & Symptomatic["Symptomatic<br>+/- syrinx<br>"]
    Asymptomatic --> NoSyrinx["No syrinx<br>"] & Syrinx["Syrinx<br>"]
    NoSyrinx --> ClinSurv["Clinical surveillance<br>"]
    Syrinx --> ClinSurvMRI["Clinical surveillance<br>+ Interval MRI<br>"]
    Symptomatic --> RaisedICP["Is there evidence<br>of raised ICP?<br>"] & CVJAnomaly["Is there evidence<br>of CVJ anomaly?<br>"] & MetabolicAnomaly["Is there a metabolic<br>anomaly?<br>"]
    RaisedICP --> ICPYes["yes<br>"] & ICPNo["no<br>"]
    ICPYes --> Hydrocephalus["Hydrocephalus<br>"] & Craniosynostosis["Craniosynostosis<br>"]
    Hydrocephalus --> ConsiderShunt["Consider:<br>ETV/Shunt<br>"]
    Craniosynostosis --> ConsiderCVP["Consider:<br>Cranial vault procedure<br>"]
    ICPNo --> FMDecompression["Foramen Magnum<br>decompression<br>"]
    CVJAnomaly --> CVJYes["yes<br>"] & CVJNo["no<br>"]
    CVJYes --> AssessCVJ["Assess reducibility<br>and need for cranio-vertebral fixation<br>"]
    CVJNo --> FMDecompression2["Foramen Magnum<br>decompression<br>"]
    MetabolicAnomaly --> MetYes["yes<br>"]
    MetYes --> EvaluateTherapies["Evaluate role<br>of medical therapies"]

Conservative

Indication

Asymptomatic patients
Symptomatic but stable for years

Indications for surgery

Since patients respond best when operated on within 2 years of the onset of symptoms (see below), early surgery is recommended for symptomatic patients.

Change of

Asymptomatic patients → symptomatic
Symptomatic but stable patients → deteriorating

Surgical techniques

Extent of craniectomy (
Foramen Magnum Decompression and Duraplasty is Superior to Only Foramen Magnum Decompression in Chiari Malformation Type 1 Associated with Syringomyelia in Adults
)

Axial:

3-4cm or when the foramen magnum becomes vertical

Sagittal

3 cm above the foramen magnum

A decompression that is too big can lead to cerebellar slumping

Incision and closure

Try not to manipulate the tonsils as they have poorer outcomes

More and more people do just an extradural operation.

Intraoperature ultrasound is less accurate at predicting those who do not need durotomy in the presence of moderate (below C1 lamina)/severe (below C2 lamina) tonsillar descent.

Literature

Klekamp (Germany)

492 patients
Repeat surgery if:

Large craniectomy
No arachnoid dissection
No tonsillar shrinkage
Autologous duraplasty

Milhorat (Chiari Institute)

2813 patients
Arachnoid intact
No tonsillar resection/shrinkage
Closure of dural defect

Alexander et al 2019

104 (37%) bone-only
177 (63%) PFD with duraplasty
No significant difference in outcomes (p = 0.59)
Analysis of failures revealed a significant difference in tonsillar coagulation versus those whose tonsils were not manipulated (p = 0.02)

Thakar et al 2018

Gait imbalance associated with worse clinical outcome
Motor deficits associated with poor syrinx resolution
Lower position of obex correlates with better outcome from surgery

Sahuquillo et al 1994

Dural graft (10) improved

Xie J et al, Zhonghua 2000, China

Dural Graft (37) 87% had improvement

Munshi I et al, Neurosurgery 2000, Spain

Graft (23) 87% improvement, 100% Syrinx reduction
No graft (11) 73% improvement, 50% Syrinx reduction

Yundt KD, Paediatric Neurosurgery USA

No Graft (7) 100% improvement

Operative findings

Tonsillar herniation is present in all cases (by definition), the most common position being at C1 (62%).

Fibrous adhesions between dura, arachnoid and tonsils with occlusion of foramina of Luschka and Magendie in 41%.

The tonsils separated easily in 40%.

Finding	%
Tonsillar descent	100
Below foramen magnum	4
C1	62
C2	25
C3	3
Unspecified level	6
Adhesions	41
Syringomyelia	32
Dural band (at foramen magnum or C1 arch)	30
Vascular abnormalities¹	20
Skeletal abnormalities	ㅤ
Inverted foramen magnum	10
Keel of bone	3
C1 arch atresia	3
Occipitalization of C1 arch	1
Cervicomedullary "hump"	12

¹vascular abnormalities: PICA dilated or abnormal course in 8 patients (PICA often descends to lower margin of tonsils); Large dural venous lakes in 3

Surgical complications

After suboccipital craniectomy plus C1–3 laminectomy in 71 patients, with dural patch grafting in 69, one death due to sleep apnea occurred 36 hrs post-op.

Respiratory depression was the most common post-op complication (in 10 patients), usually within 5 days, mostly at night.
Close respiratory monitoring is therefore recommended.

Other risks of the procedure include:

CSF leak
Herniation of cerebellar hemispheres
Vascular injuries (PICA)

Operative results

Patients with pre-op complaints of pain generally respond well to surgery.

Sensation may improve when the posterior columns are unaffected and the deficit is due to spinothalamic involvement alone.

Rhoton feels that the main benefit of operation is to arrest progression.

The most favourable results occurred in patients with

Cerebellar syndrome (87% showing improvement, no late deterioration).

Worse outcome are

Presence of atrophy

Weakness is less responsive to surgery, especially when muscle atrophy is present.

Ataxia
Scoliosis
Symptoms lasting longer than 2 years.

Category	Percentage
Early improvement of pre-op symptoms	82%
Percent of above that relapsed¹	21%
Early improvement of pre-op signs	70%
No change from pre-op status	16%
Worse than pre-op	0%

¹these patients deteriorated to pre-op status (none deteriorated further) within 2-3 years of surgery; relapse occurred in 30% with foramen magnum compression syndrome, and in 21% with central cord syndrome.