Arnold–Chiari Type II Malformation in Adults
Overview
Arnold–Chiari type II malformation (CM‑II) is a congenital hindbrain herniation almost always associated with myelomeningocele (open neural tube defect) and hydrocephalus. With advances in pediatric neurosurgical care, a growing cohort of patients survives to adulthood and presents with a diverse range of neurological, orthopedic, and urological sequelae — whether or not they underwent posterior fossa decompression as children.
CM‑II is distinct from the more common Chiari type I (tonsillar herniation without myelomeningocele). Type II involves herniation of the cerebellar vermis, brainstem, and fourth ventricle through the foramen magnum, producing a characteristically crowded posterior fossa.
Pathophysiology of Late Symptoms
Late-deteriorating symptoms in adults arise from multiple concurrent mechanisms:
- Crowded posterior fossa → ongoing mechanical stress on brainstem and spinal cord
- Syringomyelia (40–80% of adults)
- Hydrocephalus / shunt issues (malfunction, overdrainage, slit-ventricle syndrome)
- Tethered spinal cord (scarring at the original myelomeningocele closure site)
- Musculoskeletal deterioration (scoliosis, contractures, osteoporosis from insensate limbs)
Symptom Categories
Headache & Craniocervical Pain
Occipital/suboccipital headache exacerbated by Valsalva maneuvers (coughing, straining), reported in 30–60% of symptomatic adults. May indicate progressive brainstem impaction. Recurrence after prior decompression suggests inadequate bony removal, dural scarring, or re-herniation.
Cranial Nerve & Brainstem Dysfunction
- Lower CN deficits (IX–XII): dysphagia, aspiration, dysarthria, vocal cord paralysis (stridor)
- Central sleep apnea from medullary respiratory centre compression
- Ocular motor abnormalities: downbeat nystagmus, oscillopsia, impaired smooth pursuit
- Facial numbness, hearing loss, tinnitus (CN V, VIII)
Cerebellar & Vestibular Signs
Truncal/appendicular ataxia (multifactorial: cerebellar + proprioceptive + paraplegic contributions), intention tremor, dysmetria, titubation, episodic vertigo, and nystagmus — may persist even after decompression.
Syringomyelia
See dedicated syringomyelia page. Key features: dissociated sensory loss (pain/temperature) in a “cape” distribution, segmental weakness, hand amyotrophy, spastic paraparesis, and worsening neurogenic bladder/bowel. Progression more likely in unoperated patients; decompression with duraplasty can collapse the syrinx.
Hydrocephalus & Shunt Issues
Shunt malfunction presents with headache, vomiting, lethargy, cognitive slowing, Parinaud’s syndrome, and carries risk of sudden death. Overdrainage/slit-ventricle syndrome causes positional headache and chronic subdural haematomas. Even rare unshunted adults can develop late-onset symptomatic hydrocephalus.
Tethered Cord Syndrome
Back/leg pain (non-dermatomal, worsened by flexion), progressive lower limb weakness, gait decline, worsening scoliosis, and loss of bowel/bladder control. Surgical untethering can stabilise function but recovery is often incomplete if long-standing. A common cause of late deterioration even in previously operated patients.
Orthopedic & Musculoskeletal Deterioration
Progressive scoliosis/kyphosis (neuromuscular + syrinx + tethering), joint contractures, osteoporosis, pathological fractures, pressure ulcers, and Charcot neuroarthropathy in insensate extremities.
Urological & Gastrointestinal Dysfunction
Neurogenic bladder (UTIs, vesicoureteral reflux, renal scarring, CKD), neurogenic bowel (constipation, faecal incontinence, prolonged colonic transit), and late complications from bladder augmentation.
Cognitive & Psychosocial Manifestations
Executive dysfunction, visuospatial impairment, attention deficits; verbal IQ relatively preserved while performance IQ depressed. High rates of depression, anxiety, and social isolation. Worse outcomes correlate with multiple shunt revisions or episodes of raised intracranial pressure.
Impact of Prior Surgical Treatment
Posterior Fossa Decompression (PFD)
Patients with prior PFD have fewer life-threatening lower cranial nerve crises and better syrinx stabilisation, but remain at risk for late re-stenosis. Patients without PFD may have milder initial hindbrain herniation but can become symptomatic in adulthood (ataxia, headache, syrinx progression).
Shunting
Shunt-dependent patients require lifelong surveillance. Slit ventricles make malfunction diagnosis challenging. Never-shunted patients (rare) may develop late-onset hydrocephalus presenting as chronic headache, cognitive decline, and gait apraxia.
Tethered Cord Release
Timely childhood release may preserve motor/urological function for decades, but re-tethering is common. First presentation in adulthood often involves irreversible deficits, though surgery can prevent further decline.
Acute Deterioration Triggers
Minor head/neck trauma, general anaesthesia with neck extension, rapid CSF loss (lumbar puncture), and systemic infection/shunt colonisation.
Surveillance Recommendations for Adults
- Neurological exam (lower CN, cerebellar, motor/sensory level, anal reflex)
- Spine MRI (craniocervical junction + whole spine) for syrinx and brainstem herniation monitoring
- Brain imaging (CT/fast MRI) for ventricular size assessment
- Urodynamics + renal ultrasound for neurogenic bladder surveillance
- Sleep study if central sleep apnea suspected
- Neuropsychology assessment for cognitive deficits
- Multidisciplinary care: neurosurgery, urology, orthopaedics, physiatry, psychology
References
- DeepSeek chat summary (2026). Clinical Summary: Arnold–Chiari Type II Malformation in Adults. 1