Chiari II Malformation: Pathophysiology, Imaging Features, Diagnosis, and Treatment – A Comprehensive Review

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Introduction

Chiari II malformation, also referred to as Arnold–Chiari malformation, represents a complex congenital hindbrain anomaly with significant clinical implications. It is most often associated with myelomeningocele and manifests with a combination of structural, neurological, and developmental abnormalities. Understanding Chiari II malformation requires an in-depth examination of its pathophysiology, epidemiology, clinical presentation, imaging features, differential diagnosis, treatment strategies, and prognosis. This article aims to provide a comprehensive, expert-level review of Chiari II malformation, supported by the latest evidence-based research, while optimizing for academic and clinical relevance.

Pathophysiology

Chiari II malformation is characterized by the downward displacement of the cerebellar vermis, tonsils, medulla, and fourth ventricle through the foramen magnum. The condition is thought to arise due to abnormal development of the posterior fossa. A small posterior fossa, combined with tethering from a myelomeningocele, creates a craniospinal pressure gradient that draws hindbrain structures caudally. This displacement disrupts cerebrospinal fluid (CSF) dynamics, frequently resulting in hydrocephalus. Additionally, alterations in cortical development, such as periventricular and subependymal heterotopias, are often observed and may explain the frequent occurrence of epilepsy in patients with Chiari II.

Key pathological findings include:

  • Herniation of the vermis, medulla, and fourth ventricle

  • Elongation of the pons and medulla (beaked tectum)

  • Small posterior fossa with crowding

  • Associated spinal dysraphism, most commonly myelomeningocele

Epidemiology

Chiari II malformation is relatively rare, with an estimated prevalence of approximately 0.02% of live births. Epidemiological studies demonstrate a female predominance, with affected females twice as likely to be diagnosed compared to males. Almost all cases occur in association with open neural tube defects, particularly myelomeningocele. Advances in prenatal imaging, particularly ultrasound and fetal MRI, have allowed for earlier detection, often in the second trimester of pregnancy.

Clinical Presentation

The clinical spectrum of Chiari II malformation is broad and age-dependent.

  • Neonates and Infants: Commonly present with signs of brainstem dysfunction, including stridor, apnea, dysphagia, aspiration, and arm weakness. Myelomeningocele is usually evident at birth.

  • Children: Hydrocephalus is frequently diagnosed during infancy or early childhood, often requiring shunt placement. Developmental delay and scoliosis are common.

  • Adults: Rarely, adults with Chiari II may present with seizures secondary to cortical malformations, progressive neurological deficits, or complications from untreated hydrocephalus.

Seizures, while not the primary feature, have been strongly correlated with heterotopias and other malformations of cortical development,

Imaging Features

Magnetic Resonance Imaging (MRI) remains the gold standard for diagnosing Chiari II malformation. The hallmark imaging findings include:

Posterior fossa findings:

  • Herniation of the cerebellar tonsils, vermis, and medulla through the foramen magnum

  • Small posterior fossa with crowding

  • Beaked tectum

  • Low-lying and elongated fourth ventricle

Supratentorial findings:

  • Hydrocephalus with dilated lateral and third ventricles

  • Thin corpus callosum with interhemispheric cysts

  • Luckenschädel skull changes

  • Periventricular and subependymal heterotopias

Spinal findings:

  • Myelomeningocele (nearly universal)

  • Syringomyelia and spinal cord kinking

Figures

Figure 1. Sagittal T1 MRI (non-contrast): Demonstrates downward displacement of the cerebellar tonsils and vermis through the foramen magnum.

Figure 2. Axial T2 MRI (non-contrast): Shows ventricular dilatation with periventricular nodular heterotopia.

Figure 3. Coronal T1 STIR MRI: Reveals corpus callosum thinning and interhemispheric cysts.

Figure 4. Sagittal T1 STIR MRI: Illustrates small posterior fossa and beaked tectum.

Differential Diagnosis

Several entities must be distinguished from Chiari II malformation:

  1. Chiari I malformation – involves isolated herniation of cerebellar tonsils without brainstem involvement.

  2. Dandy-Walker malformation – characterized by cystic dilation of the fourth ventricle with hypoplasia of the cerebellar vermis.

  3. Joubert syndrome – features the molar tooth sign on MRI due to midbrain–hindbrain malformation.

  4. Pontocerebellar dysplasia – involves underdevelopment of the pons and cerebellum【6†Case Review.docx】.

Diagnosis

Diagnosis relies on neuroimaging, particularly MRI, combined with clinical history and physical examination. Prenatal ultrasound can suggest Chiari II by detecting lemon-shaped skull, banana-shaped cerebellum, or ventriculomegaly. Confirmation is typically achieved postnatally with MRI.

Treatment

There is no definitive cure for Chiari II malformation; management is primarily symptomatic and tailored to clinical severity:

  • Neurosurgical interventions:

    • Ventriculoperitoneal (VP) shunt placement for hydrocephalus

    • Posterior fossa decompression in cases of brainstem compression

    • Myelomeningocele repair (typically performed neonatally)

  • Supportive care:

    • Nutritional and feeding support for dysphagia

    • Respiratory support in cases of apnea

    • Orthopedic management for scoliosis

  • Epilepsy management:

    • Antiepileptic medications for seizures associated with heterotopias.

Prognosis

The long-term prognosis of Chiari II malformation depends on the severity of associated anomalies and complications. With appropriate neurosurgical and supportive interventions, survival into adulthood is possible, although many patients experience lifelong neurological deficits. Quality of life is strongly influenced by the degree of hydrocephalus, presence of cortical malformations, and functional impairment from spinal dysraphism.

Quiz Section

Question 1: Which imaging finding best distinguishes Chiari II malformation from benign tonsillar ectopia?

A. Tonsillar descent <5 mm with rounded tonsils

B. Inferior displacement of cerebellar tonsils ≥5 mm below foramen magnum with pointed morphology

C. Cystic dilation of fourth ventricle

D. Hypoplasia of the pons

Question 2: What is the most common spinal abnormality associated with Chiari II malformation?

A. Syringomyelia

B. Myelomeningocele

C. Tethered cord

D. Spina bifida occulta

Question 3: In Chiari II malformation, seizures are most strongly associated with:

A. Hydrocephalus

B. Subependymal heterotopia

C. Syringomyelia

D. Brainstem compression

Answer & Explanation

1. Answer: B. Inferior displacement of cerebellar tonsils ≥5 mm below foramen magnum with pointed morphology. Explanation: Benign tonsillar ectopia typically demonstrates <5 mm descent with rounded morphology

2. Answer: B. Myelomeningocele. Explanation: Nearly all patients with Chiari II malformation have associated open neural tube defects, most often myelomeningocele.

3. Answer: B. Subependymal heterotopia. Explanation: Cortical malformations, particularly periventricular and subependymal heterotopias, are strongly linked with epilepsy in Chiari II patients.

References

[1] L. T. Holly and U. Batzdorf, "Chiari malformation and syringomyelia," Journal of Neurosurgery: Spine, vol. 31, no. 5, pp. 619–628, 2019.

[2] J. Kuhn and P. D. Emmady, "Chiari II Malformation," in StatPearls, Treasure Island, FL: StatPearls Publishing, 2020.

[3] R. Nadgir and D. M. Yousem, Neuroradiology: The Requisites, 4th ed., Philadelphia, PA: Elsevier, 2016, pp. 284–285.

[4] R. S. Tubbs and W. J. Oakes, "Treatment and management of the Chiari II malformation: An evidence-based review of the literature," Child’s Nervous System, vol. 20, no. 6, pp. 375–381, 2004.

[5] M. J. Strahle, H. J. Muraszko, and C. M. Garton, "Chiari malformation Type II: Clinical and radiological findings," Neurosurgical Focus, vol. 41, no. 5, pp. E2, 2016.

[6] A. M. McLone and D. H. Knepper, "The cause of Chiari II malformation: A unified theory," Pediatric Neuroscience, vol. 15, no. 1, pp. 1–12, 1989.

[7] M. A. Dias and D. J. Partington, "Embryology of myelomeningocele and anencephaly," Neurosurgical Focus, vol. 16, no. 2, pp. 1–16, 2004.

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