The Chiari Malformations: A Review With Emphasis on Anatomical Traits

THE CHIARI MALFORMATIONS:
A REVIEW WITH EMPHASIS ON
ANATOMICAL TRAITS
Alper Cesmebasi,1 Marios Loukas,1 Elizabeth Hogan,1
Sara Kralovic,1 R. Shane Tubbs,1,2* And Aaron A. Cohen-gadol3
1Department Of Anatomical Sciences, St. George’s University, St. George’s, Grenada, West Indies
2Section Of Pediatric Neurosurgery, Children’s Hospital, Birmingham, Alabama
3Department Of Neurosurgery, Goodman Campbell Brain And Spine & Indiana University, Indianapolis, Indiana
Clin Anat. 2015 Mar;28(2):184-94. doi: 10.1002/ca.22442. Epub 2014 Jul 25
Presenter: Dr Kaushal Deep Singh
MCh SR, Department of Neurosurgery, SKIMS, Srinagar

INTRODUCTION
• Hindbrain herniations may come in many forms.
• For cerebellar tonsillar ectopia, they can be divided into acquired and congenital forms.
• Acquired hindbrain herniations are due to increased intracranial pressure.
• Congenital hindbrain herniation makes up the majority of these congenital malformations.
• Congenital malformations are often found to harbor additional anatomical derailments whether
it be the cerebellar tonsils (Chiari I malformation) or the cerebellar vermis (Chiari II
malformation).
• This article reviews these forms of cerebellar ectopia and describes the recent advances in
understanding the details of their anomalous anatomy.
• This article also compares and contrasts all the recent embryological theories and advances
in understanding the pathogenesis of this disease.

PRESENT CONCEPT
• ACM classification is based on the morphology of the malformations:
• Chiari I: >5mm descent of the caudal tip of cerebellar tonsils past the
foramen magnum.
• Chiari II: brainstem, fourth ventricle, and >5 mm descent of the caudal tip of
cerebellar tonsils past the foramen magnum with spina bifida.
• Chiari III: herniation of the cerebellum with or without the brainstem through
a posterior encephalocele.
• Chiari IV: Cerebellar hypoplasia or aplasia with normal posterior fossa and
no hindbrain herniation.

HISTORICAL PERSPECTIVE
• In the early 1890s, Professor Hans Chiari (1851–1916) (Fig. 1),
collected and analyzed data from over 40 post mortem
examinations of hindbrain malformations.
• In 1891, he published his first paper analyzing these data and
described what is now known as the Chiari malformations
types I, II, and III (Chiari, 1891(1); Bejjani, 2001(2)).
• The type IV malformation was added later in Chiari’s 1895
publication (Chiari, 1895)(3).

HISTORICAL PERSPECTIVE (cont’d)
• Chiari described the malformations as follows:
• Type I was defined as cerebellar tonsils that lie below the plane of the foramen
magnum.
• Type II was described as caudal descent of the cerebellar vermis, brainstem, and
fourth ventricle below the plane of the foramen magnum.
• Type III was described as a posterior cranial fossa encephalocele that contains
hindbrain structures.
• Type IV was defined as a hypoplastic/aplastic cerebellum with tentorial
hypoplasia.

• In Observationes Medicare written by Nicholas Tulp (1593–1674),
hindbrainherniation was observed in a myelodysplastic individual (Koehler,
1991)(4).
• In 1829, Cruveilhier (1791–1874) described a patient with a
myelomeningocele and a “considerably dilated cervical region that contained
both the medulla oblongata and the corresponding part of the cerebellum,
which was elongated and covered the fourth ventricle, itself enlarged and
elongated” (Koehler, 1991)(4).

• In 1883, John Cleland (1835–1925) described a hindbrain hernia in a post
mortem examination of a patient with myelodysplasia (Koehler, 1991)(4).
• In 1894, Julius Arnold (1835–1915) described a single patient with hindbrain
herniation and myelodysplasia (Koehler, 1991)(4).
• Although Chiari type II malformations are occasionally referred to as Arnold–
Chiari or Cleland–Chiari malformations, it is reasonable to use only Chiari’s
name and classification method (Oakes and Tubbs, 2004(5); Wellons et al.,
2005(6)).

RECENT PERSPECTIVE
• Two additional classifications of Chiari malformations have been defined; Chiari type 0 and
Chiari type 1.5.
• Chiari type 0 is defined as syringomyelia that responds to posterior cranial fossa
decompression without the associated tonsillar herniation seen with the type I malformation
(Tubbs et al., 2001)(7).
• Newton(8) first described this in 1969 and then in 1998, Iskandar et al.(9) identified five similar
patients with syringomyelia, no evidence of tonsillar herniation where posterior cranial fossa
decompression decreased or resolved their syringomyelia.
• Chiari type 1.5 describes patients that have tonsillar tissue herniation through the foramen
magnum (i.e., Chiari I malformation) associated with the additional finding of caudal
displacement of the brainstem as evident by an inferiorly located obex (Tubbs et al., 2004)(10).

GROSS ANATOMY OF
THE CHIARI
MALFORMATIONS

CHIARI TYPE I MALFORMATION
(Introduction)
• Chiari type I is currently described as caudal displacement of the cerebellar
tonsils more than 3–5 mm inferior to the plane of the foramen magnum (Tubbs
et al., 2004)(10) (Figs. 2 and 3).
Fig. 2. Postmortem view of an individual with a Chiari I
malformation. Note the ectopic cerebellar tonsils (black arrows).
Fig. 3. Sagittal MRI of the Chiari I malformation
(arrow A) with concomitant syringomyelia (arrow).

(Skull Abnormalities)
• Basilar skull and cranio-cervical junction anomalies are a fairly common finding, seen in
roughly 50% of cases.
• Another frequent finding in patients with Chiari type I is underdevelopment of the occipital
bone.
• The foramen magnum is usually increased in size and the clivus is shorter than normal.
• The slope of the clivus in relation to the floor of the anterior cranial fossa may be greater in
the Chiari type I malformation.
• Additionally, the anterior cranial fossa can be longer in Chiari type I.
• Sgouros et al. (2007)(11) found the mean angle between the left anterior clinoid process, crista
galli, and the right anterior clinoid process to be 34° in controls and 29° in Chiari type I
patients.

(Skull Abnormalities) (cont’d)
• The posterior cranial fossa of many patients is smaller and shallower and the
anterior cranial fossa may be longer.
• Sgouros et al. (2007)(11) found the mean angle between left internal acoustic
meatus, the foramen magnum, and the right internal acoustic meatus to be
110° in controls and 122–123° in Chiari type I patients.
• Other basilar skull abnormalities found are clival concavities, basilar
invagination, midline occipital keels, platybasia, and remnants of the pro-atlas
such as an accessory occipital condyle.

(Spine Abnormalities)
• A common spinal defect in Chiari type I is the Klippel–Feil deformity, which is
a maldeveloped vertebra and/or a fusion between two or more cervical
vertebrae.
• Another common finding in these patients is atlantoaxial assimilation.
• Occasionally, retroflexion of the odontoid process and thickening of the
ligamentum flavum are seen.
• Additionally, scoliosis is a common finding in Chiari type I malformation,
usually due to an underlying syrinx, which is a cerebrospinal fluid filled cavity
within the spinal cord (Figs. 3–5). This scoliosis is often a levoscoliosis.

Fig. 4. Axial T2-weighted MRI of the
cervical spine noting the syringomyelia
(denoted by the arrow) in a patient with
Chiari I malformation.
Fig. 5. Sagittal T2-weighted MRI noting the Chiari
1.5 malformation (tonsillar ectopia and caudal ectopia of
the medulla oblongata (B)). Also note the syringomyelia
(C) and retroverted odontoid process (A).

(Ventricular/Cisternal Abnormalities)
• Ventricular anatomy is generally normal in Chiari type I malformation
except for the occasional elongated fourth ventricle.
• Only <10%of patients with Chiari type I malformation present with
hydrocephalus.
• The retrocerebellar cerebrospinal fluid space is often diminished in
these patients.

(Meningeal Abnormalities)
• The slope of the tentorium cerebelli is usually elevated in Chiari type I
malformation.
• The arachnoid mater at the level of the foramen magnum is commonly
thickened and a constricting dural band may also be seen at this level.
• The outlets of the fourth ventricle are often obstructed by arachnoid veils.
• Occasionally, a thick constricting band can be found at the level of the
posterior arch of the atlas.

(Spinal Cord Abnormalities)
• The most prominent spinal cord finding in Chiari type I malformation is the
presence of a syrinx, which is reported in 50–75% of these patients (Figs. 3
and 4).
• These syringes are usually found in the upper thoracic or lower cervical spine.
• Sometimes a syrinx is found, which may extend the entire length of the spinal
cord and thus be termed “holocord” in nature.
• Routinely, there is a segment of cord between the fourth ventricle and the
syrinx that is spared from cavitation and usually at the first cervical segment of
the cord.

(Brain Abnormalities)
• The brain of Chiari type I malformation is generally normal except for the
cerebellar tonsillar herniation and the uncommon cases that also have
hydrocephalus.
• There are rare cases where the midbrain, pons, and medulla are flattened.
• The tonsils of the cerebellum generally lose their folia and become atrophic
from chronic compression giving them a smooth appearance.
• They are usually described as peg-like or pointed and are often
asymmetrically herniated.

(Symptomatology, Presentation)
• The most common symptom is pain which occurs in approximately 60–70% of patients
and is often localized to the occiput or upper cervical region.
• This pain is generally exacerbated by Valsalva maneuver and laughing, coughing, or
sneezing.
• The scoliosis that these patients may present is often a left thoracic scoliotic curve and if
due to syringomyelia, patients may present with abnormal abdominal reflexes, loss of
pain and temperature sensation or diffuse non-dermatomal pain in the flank or back.
• Symptoms that can be localized to the brainstem include extraocular muscle changes,
tongue atrophy, down-beat-nystagmus, gagging, sleep apnea, and dysphagia.

CHIARI TYPE II MALFORMATION
(Introduction)
• The primary finding in the Chiari type II malformation is herniation of the cerebellar
vermis with caudal descent of the brainstem and fourth ventricle (Figs. 6 and 7).
Fig. 6. Sagittal section of a fetus with the Chiari II
malformation. Note the displacement of the lower brainstem
(arrow) and cerebellum into the neck.
Fig. 7. Specimen shown in Figure 6 with the brain removed.
Note the low position of the tentorium cerebelli (arrow) and the
very small posterior cranial fossa.

CHIARI TYPE II MALFORMATION (cont’d)
• It is almost always seen with a myelomeningocele (Fig. 8), but rarely has
been reported without this.
Fig. 8. Typical myelomeningocele (lumbosacral) found in almost all
patients with the Chiari II malformation.

• A common finding of the calvaria in Chiari type II is a “beaten copper”
appearance of the bone, termed craniolacunia or lückenschädel.
• This phenomenon may affect both the inner and outer tables of the skull and
usually decreases in severity with age.
• When present, it is more prominent over the upper half of the calvaria.
• The anterior portion of the frontal bone may have a scalloped appearance on
axial imaging and is termed the “lemon sign”.

• The posteromedial aspect of the petrous part of the temporal bones and
jugular tubercles may also show scalloping.
• This scalloping phenomenon shortens the internal acoustic canal.
• As often seen in type I malformations, Chiari type II malformations also
usually have an enlarged foramen magnum. The posterior cranial fossa is
smaller and has a flattened floor.
• The opisthion (the mid-point on the posterior margin of the foramen magnum)
may be notched and cranioschisis (congenital failure of the skull to completely
close) is sometimes seen.

• The concavity is increased in the basioccipital portion of the clivus with partial
or full obliteration of the diploic space.
• The inion is usually inferiorly displaced and there may be a midline occipital
keel.
• Lastly, assimilation of the atlas and basilar invagination are occasionally seen
but not as often as with the Chiari type I malformation

• Deformities of the spine in Chiari type II are usually seen in the
cervical region.
• Some of the general findings are scalloping of the odontoid
process, Klippel–Feil anomaly, enlarged cervical spinal canal, and
incomplete formation of the posterior arch of the atlas.

(Ventricular/Cisternal Abnormalities)
• Approximately 90% of patients with Chiari type II malformation
present with hydrocephalus.
• The lateral ventricles also show many intrinsic malformations and
the left and right sides are often asymmetric.
• The frontal horns are pointed laterally in the portion that is superior
to the head of the caudate nucleus.

(Ventricular/Cisternal Abnormalities) (cont’d)
• Additional lateral pointing of the frontal horn may be present near the
interventricular foramen (of Monro).
• This lateral pointing is directed towards the striothalamic groove, which is
located posteroinferiorly to the head of the caudate nucleus.
• There is also medial pointing of the inferior margins of the floor of the lateral
ventricles near the interventricular foramen (of Monro).
• The atria and occipital horns can be disproportionately enlarged, which is
termed colpocephaly.

• The third ventricle may have two diverticula; a posterior and an anterior one
termed the shark tooth deformity.
• Occasionally, a commissure of Meynert may be seen bridging the floor of the
third ventricle. This commissure connects nonthalamic deep gray matter on
either side.
• The fourth ventricle is commonly flat, small, and elongated with ill-defined
lateral recesses.
• The choroid plexus of the fourth ventricle is usually found outside of the
ventricle near its caudal pole, which is its embryological location.

• The inferior medullary velum is commonly absent, which distorts the shape of
the fourth ventricle.
• Although the shape of the fourth ventricle is distorted, there are no abnormal
communications between it and the surrounding spaces.
• Lastly, the median aperture (of Magendie) may have a cyst associated with it
or may be absent.
• The cisterns anterolateral to the pons and around the cerebellum are enlarged

(Meninges Abnormalities)
• In Chiari type II, the tentorium cerebelli is usually heart or V-shaped, widened,
hypoplastic, and low-lying.
• Sometimes, large venous lakes may be present between the leaves of the
tentorium cerebelli, especially near the foramen magnum.
• The straight sinus is more vertical because of the low-lying tentorium
cerebelli.
• Additionally, the low tentorium cerebelli causes the confluence of sinuses and
lateral venous sinuses to lie very close to the foramen magnum.

(Meninges Abnormalities) (cont’d)
• The incisura is elongated in the sagittal plane.
• The falx cerebri can be fenestrated or hypoplastic, with the under developed
portion generally lying anteriorly.
• The leptomeninges are noted to be vascular and thickened at the level of the
foramen magnum.
• At times, arachnoid cysts are seen anterior to the upper portion of the cervical
cord.
• Finally, the upper dentate ligaments may be thickened.

(Spinal Cord Abnormalities)
• Almost all patients with Chiari type II malformation present with a
myelomeningocele.
• Approximately 6% of these patients have an associated split cord
malformation that historically, was called diastematomyelia.
• This literature reports the incidence of an associated
syringohydromyelia with Chiari type II malformation to be 20–95% of
patients.
• Exophytic syringes may be associated with the spinal cord.

(Spinal Cord Abnormalities) (cont’d)
• The cervical spinal cord is shorter and caudally displaced.
• Finally, there is a reduction in the neuronal counts in the upper
cervical cord accompanied with a reduction in myelinization.
• The corticospinal tracts have the greatest reduction of
myelinisation.

• Chiari type II malformations have multiple brain anomalies.
• These anomalies will be described based on the embryological divisions
of the brain.

(Brain Abnormalities - Telencephalon)
• The corpus callosum may present with partial or
complete agenesis.
• This is accompanied with partial or complete
absence of the septum pellucidum especially
anterior to the interventricular foramen (of
Monro).
• With absence of the corpus callosum,
interdigitation (Chinese lettering) of the left and
right cerebral hemispheres may occur (Fig. 9).
Fig. 9. Axial, FLAIR MRI of the
brain noting the cerebral
interdigitation (arrow) in a patient
with Chiari II malformation. Note
the lack of a falx cerebri.

(Brain Abnormalities - Telencephalon) (cont’d)
• The anterior commissure is generally prominent.
• The cortex has specific sulcal deformities without microscopic layering abnormalities termed polygyria.
• The olfactory bulb and tract may be completely or partially absent.
• The cingulate gyrus may be absent.
• The head of the caudate nucleus is usually prominent.
• Lastly, the internal aspect of the lateral ventricles may have a granular appearance due to heterotopic
gray matter bulging into them.
• This gray matter consists of neurons that are separated from the cortex by bands of white matter and
from the ventricles by ependymal.

(Brain Abnormalities - Diencephalon)
• The massa intermedia is enlarged
in 75–90% of patients with Chiari
type II malformation (Fig. 10) and
is also often anteriorly displaced.
• The hypothalamus may be
elevated.
• The habenular commissure and
the pineal gland can be elongated
Fig. 10. Sagittal, T1-weighted MRI of the brain of a
patient with Chiari II malformation. Note the
constellation of brain malformations in addition to
the vermian herniation.

(Brain Abnormalities - Mesencephalon)
• The midbrain is usually elongated with a shortened quadrigeminal plate.
• Tectal beaking (Fig. 10) may be present, which is the fusion of the colliculi.
• Usually, the inferior colliculi contribute more to the tectal beaking. The cerebral
aqueduct (of Sylvius) can be stretched, laterally compressed, forked,
• stenotic, or posteriorly kinked.
• The tegmental nuclei may have differing degrees of dysgenesis.
• Finally, hypoplasia or aplasia cranial nerve nuclei may be occur

(Brain Abnormalities - Metencephalon)
• In Chiari type II malformation, the cerebellum is grossly smaller and may
upwardly herniate over the tentorium cerebelli.
• The herniating portion may appear bullet-shaped.
• The cerebellum is also laterally displaced, which allows the lower cranial
nerves to transverse the folia to reach the respective foramina.
• The vermis is herniated through the foramen magnum.
• There may be absent folia and dysplasia.

(Brain Abnormalities - Metencephalon) (cont’d)
• The cerebellum may exhibit the “banana sign” on axial imaging, where the
anterior portion of the cerebellum is pointed, and its overall appearance is
curved.
• Cerebellar inversion may be present, where the lateral cerebellar edges touch
anteriorly to the brainstem and basilar artery.
• Also seen are cerebellar heterotopias and the herniated cerebellum may be
covered dorsally with ependyma.
• Additionally, there may be a reduction in cell count with dysplastic deep gray
matter.

(Brain Abnormalities - Metencephalon) (cont’d)
• The pons is generally flattened and elongated (Fig. 10).
• Indentations of the ventral pontine surface may be present.
• These indentions are formed by either a displaced vertebral artery
or the anterior rim of the foramen magnum.
• The ponto-medullary junction is usually difficult to distinguish.
Basal pontine nuclei, nuclei of the tegmentum, and cranial nerve
nuclei can all be dysplastic

(Brain Abnormalities - Myelencephalon)
• On imaging, the medulla oblongata may
be interpreted as trumpet-like because it is
elongated and flattened in the sagittal
plane.
• In 70% of patients with Chiari type II
malformation, a protuberance can be seen
just caudal to the gracile and cuneate
tubercles and is usually seen between C2
and C4 spinal levels (Fig 11).
Fig. 11. Operative example of the
Chiari II malformation.

(Brain Abnormalities - Myelencephalon) (cont’d)
• This protuberance has
been described as the
cervicomedullary kink,
buckle, hump, or spur. (Fig.
12)
• Additionally, the pyramidal
decussation may occur
more cephalad than normal.
Fig. 11. Gross specimen illustrating the
medullary kinking (arrow) seen in the Chiari II
malformation.

(Additional Abnormalities)
• Chiari type II malformations are associated with cranial and upper
cervical nerves that take a vertical versus horizontal course.
• In addition, the upper cranial nerves generally have a longer
intracranial course.
• The vertebrobasilar system and its branches may be caudally
displaced.
• Lastly, the great cerebral vein (of Galen) tends to be elongated.

(Symptamatology, Presentation)
• With Chiari type II malformations, the common symptoms tend to be
associated within an age group.
• Neonates and infants can present with the most lifethreatening symptoms.
• These symptoms include apnea, dysphagia, bradycardia, and inspiratory
stridor.
• Older children, however, most commonly present with symptoms associated
with spinal cord dysfunction. These range from hyporeflexia to weakness.

CHIARI TYPE III MALFORMATION
(Introduction)
• Chiari type III malformations are defined
as findings of a Chiari II malformation
with the addition of a posterior cranial
fossa encephalocele that contains
• cerebellar and brainstem tissue (Figs.
13 and 14).
• It is the rarest hindbrain hernia,
accounting for less than 1% of all Chiari
malformations.
Fig. 13. Posterior view
of a neonate with Chiari
III malformation. Note
that the head is to the
left and the large
encephalocele is
interposed between it
and the scapulae.
Fig. 14. Intraoperative
view of the herniated
posterior cranial fossa
tissue in the Chiari III
malformation.

• Scalloping of the dorsal clivus and petrous ridge is commonly
seen. Lückenschädel of the skull is also commonly present.
• Occasionally, the parietal bones are hypoplastic.
• There is usually an enlarged foramen magnum.

• Generally, patients with Chiari type III malformation have osseous
deformities of the upper cervical spine.
• Some patients have posterior cervical agenesis.

• In all cases, the cerebellum is herniated into a low occipital or high
cervical encephalocele.
• Some patients also have herniation of the brainstem, occipital
lobes, dural venous sinuses, and subarachnoid or ventricular
cerebrospinal fluid spaces.
• Additionally, partial or complete dysplasia of the corpus callosum is
seen.

(Symptamatology, Presentation)
• Chiari type III malformations present with the same symptoms
seen in Chiari type II malformations.
• Additionally, they have more severe neurological sequelae that
include cortical, visual, cognitive, and intellectual impairment

CHIARI TYPE IV MALFORMATION
• Chiari type IV malformations do not have hindbrain herniation.
• They are defined as hypoplasia or aplasia of the cerebellum. This
is occasionally associated with tentorial hypoplasia.
• Chiari type IV patients, despite a striking radiologic appearance,
tend to appear amazingly well and may have only mild to
moderate neurologic deficits.

CHIARI TYPE 1.5 MALFORMATION
• The term Chiari 1.5 malformation was recently coined to
specifically address patients with Chiari type I malformation with an
added component of brainstem and fourth ventricle
elongation/caudal descent.
• Chiari type 1.5 patients have symptoms similar to Chiari type I
patients and may have a higher incidence of developing
syringomyelia.

CHIARI TYPE 0 MALFORMATION
• This group of patients have syringomyelia but no associated findings of
hindbrain herniation.
• Additionally, these patients’ symptoms improve following posterior cranial
fossa decompression.
• Many of these patients also present with craniocervical anomalies such as
those seen in Chiari type I malformation.
• Lastly, crowding of the foramen magnum and obstruction of the median
aperture (of Magendie) by arachnoid veils and adhesions are seen.

CHIARI TYPE 0 MALFORMATION (cont’d)
• By definition, Chiari type 0 patients present with symptoms
associated with syringohydromyelia, and these symptoms are like
those seen in patients with Chiari type I malformation and
syringomyelia.

EMBRYOLOGY & PATHOLOGY OF CHIARI
MALFORMATIONS
• Chiari malformations may be acquired or congenital.
• Acquired Chiari malformations are due to raised intracranial pressure caused
by trauma or intracranial masses.
• One of the more notably recognized examples of acquired Chiari
malformation is the association of Chiari malformations and craniosynostosis
in syndromic and nonsyndromic patients

EMBRYOLOGY & PATHOLOGY OF CHIARI
MALFORMATIONS (cont’d)
• Leikola et al. (2010)(12) state the incidence of Chiari malformation in
nonsyndromic single suture craniosynostosis patients to be 5.6%, while
incidence of Chiari malformation in syndromic and multisutural
craniosynostosis ranges from 50 to 100% (Cinalli et al., 2005)(13).
• The premature suture fusions in craniosynostosis result in a diminished
cranial volume.
• This decreased volume couple with hindbrain growth leads to increased
intracranial pressure and the eventual promotion of cerebellar herniation.

EMBRYOLOGY & PATHOLOGY OF
CONGENITAL CHIARI MALFORMATIONS
• No single unified hypothesis for the congenital forms.
• These differing theories attribute possible etiologies which may involve
hydrodynamics, small posterior cranial fossa, hindbrain overgrowth caudal
traction, hindbrain dysgenesis, underdevelopment of the basiocciput, lack of
embryological ventricular distention, and genetic factors.
• It is not at all clear that the 4 types of Chiari malformation represent a disease
continuum corresponding to a single disorder.
• The 4 types (particularly types III and IV) are increasingly believed to have
different pathogenesis and share little in common other than their names.

EMBRYOLOGY & PATHOLOGY OF CONGENITAL
CHIARI MALFORMATIONS (Hydrodynamic Theory)
• The hydrodynamic theory was initially postulated by Chiari and was
expounded by Gardner in 1965(14) and then by Williams (1993)(15) and Oldfield
et al. (1994)(16).
• Chiari initially believed that the various degrees of hindbrain herniation were
due to chronic supratentorial hydrocephalus.
• Gardner tried to explain the relationship between Chiari malformations and
syringomyelia.
• He proposed that choroid plexus pulsations and cerebrospinal fluid flow were
critical for normal brain development.

• Gardner stated that an overactive supratentorial choroid plexus pulsation would push
down the tentorium cerebelli and cause the contents of the posterior cranial fossa to
herniate through the foramen magnum.
• Therefore, a syrinx would result from redirected cerebrospinal fluid into the central
canal.
• Williams (1993) and Oldfield et al. (1994) expanded the hydrodynamic theory
suggesting that Valsalva maneuvers increase intracranial pressure and create a
negative intraspinal pressure because the epidural venous congestion fails to
equalize. The negative pressure then draws the cerebrospinal fluid into the central
canal. This mechanism worsens hindbrain impaction and syringomyelia.

• The opposition to these theories is that they do not explain the
smaller posterior cranial fossa, the upward herniation of the
posterior cranial fossa contents, and the slit-like fourth ventricle
seen in Chiari malformations.
• Additionally, these theories require a connection between the cyst
and the fourth ventricle, which is not always present.

Small Posterior Fossa Theory and Hindbrain
Overgrowth Theory
• Initially demonstrated in the study conducted by Nyland and Krongness
(1978).(17)
• They showed that patients with Chiari type I malformations had a smaller
posterior cranial fossa.
• The hindbrain overgrowth theory postulated that asymmetrical and continued
growth of the hindbrain in a confined space could lead to its herniation
through the foramen magnum (Panteliadis and Darras, 1999).(18)
• This may be evident in cases of craniosynostosis, where the cranial volume is
reduced due to the premature suture fusions.

Small Posterior Fossa Theory and Hindbrain
Overgrowth Theory
• The criticism of these theories lies in the fact that there is evidence
in some Chiari patients with a normal posterior cranial fossa at
birth that the cerebellum is often hypoplastic.

Caudal Traction Theory
• Distal fixation and tethering of the spinal cord may result in inappropriate
tension on the developing hindbrain with resultant herniation (Penfield and
Cobum, 1938).(19)
• However, Goldstein and Kepes (1966)(20) were able to show in an animal
model that distal traction of the spinal cord was dissipated over three to four
spinal segments.

Hindbrain Dysgenesis Theory
• Abnormal neural tissue and not mechanical forces are the main cause of
Chiari malformations (Keens and Ward, 2000).(21)
• This theory supports the observations of abnormal brainstem nuclei,
cerebellum, thalamus, and corpus callosum in Chiari patients.
• However, this theory does not explain the constellation of mesodermal
abnormalities seen in Chiari patients.

Chiari II Unified Theory
• McLone and Knepper (1989)(22) proposed that the occurrence of Chiari type II malformations in early
• embryonic life precedes the development of hydrocephalus.
• According to their theory, if the neural tube failed to close with reopening of the neurocele, then
cerebrospinal fluid would drain out from the spinal cord and cranial ventricular system.
• The loss of fluid and distension would deprive the developing neuroectoderm and skull mesenchyme
from developing normally.
• This would lead to the formation of a small posterior cranial fossa, rostral and distal displacement of
posterior cranial fossa contents, dysgenesis of the corpus callosum, brainstem kinking, and abnormal
cerebral cortex and skull.
• The criticism of this theory is that it does not explain the development of Chiari malformations without
neural tube defects.

Chiari I Chromosome Abnormality Theory
• Boyles et al. (2006)(23) ran a link analysis of families with Chiari I malformations suspecting to find a
genetic component.
• They identified regions on chromosomes 15q21.1–22.3 and 9q21.33-33.1 that were linked to this
malformation.
• The former locus contains the fibrilin-1 gene, which is affected in Marfan’s syndrome and could
potentially explain the association between this syndrome and Chiari type I malformations.
• As shown above, Chiari malformations are a heterogeneous entity. So far, no single theory or
hypothesis has been able to explain all aspects of the multiple presentations.
• Until a unifying etiology is discovered, it appears as though congenital Chiari malformations are
distinctly different conditions that happen to have one similar clinical feature.

DIAGNOSIS
• MRI is the most useful and most widely used imaging study for diagnosing Chiari
malformation.
• CSF flow analysis through foramen magnum with phase-contrast cine MRI helps
distinguish symptomatic Chiari I from asymptomatic cerebellar ectopia and helps
predict response to surgical decompression.
• Other potentially useful tests include myelography as an alternative in patients in
who cannot undergo MRI, and CT or radiographs of the neck and head, which
may help reveal common associated bony defects, particularly of the
craniocervical junction.

TREATMENT
• The only effective treatment for symptomatic Chiari malformations is
decompressive surgery.
• The goal of surgical decompression is to enlarge the posterior fossa, recreate the
cisterna magna, and allow normal cerebral spinal fluid (CSF) follow to the cervical
subarachnoid space.
• Complications with posterior fossa decompressive surgeries are uncommon, but
hemorrhaging, injury to the posterior inferior cerebellar artery, pseudomeningocele
formation, reclosure of the outlet foramen, and hydrocephalus secondary to
posterior fossa CSF hygromas may occur.

TREATMENT – Chiari Type I
• With Chiari type I malformations, the first step is to determine if there is an
underlying cause of the disease such as a brain tumor, craniosynostosis, or
hydrocephalus.
• Patients with Chiari malformation related to craniosynostosis benefit from
posterior occipital decompression with cranial vault remodelling.
• If it is found that they have idiopathic Chiari type I malformation, then the next step
is to determine if they have a syrinx. If they do, then surgery is usually offered.
• The idea behind immediate surgery is to alleviate pathological forces acting on the
spinal cord.

TREATMENT – Chiari Type I
• In Chiari type I patients without syringes, the decision of whether or not to operate
is more difficult.
• If the patient’s symptoms are minimal and not lifestyle limiting, then they can be
clinically followed.
• However, if the symptoms are life altering, then surgery is the best option

TREATMENT – Chiari Type II
• With Chiari type II malformations, the natural history can be disastrous
• Therefore, surgical intervention early in a symptomatic child is widely accepted.
• Pollack et al. (1996)(24) concluded from their study that early recognition and
surgical treatment of brainstem compromise in Chiari type II patients produces
long term clinical recovery.
• Such studies demonstrate that patients who are symptomatic, especially with
signs of medullary compromise, should have surgery.

TREATMENT – Chiari Type III, IV & 0
• Chiari type III malformations are so severe that the surgery is the only viable
option
• Whereas for Chiari type IV, posterior fossa decompression is not done.
• The patient is followed, while the malformation resolve naturally.
• As Chiari type 0 may hold similar presentation to syringomyelia or Chiari type 1
with syringomyelia, surgical decompression is the therapeutic treatment and may
hold diagnostic value as well.

CONCLUSION
• Chiari malformations represent a wide range of anatomical anomalies and clinical
symptoms that may or may not have a common embryological origin.
• The subgroups of Chiari type I malformations can be caused by other conditions
such as a tumor or trauma, or they can be congenital.
• Therefore, any proposed theory must address all of the pathological findings in
each subgroup.
• A bettering understanding of the anatomy and embryology of the Chiari
malformations should lead to earlier diagnoses, improved treatments, and over-all
better clinical outcome for patients.

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the Chiari 0 malformation. Neurosurgery 48:1050–1054.
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and Practice of Pediatric Neurosurgery. New York: Thieme. p 165–187.

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