A congenital hindbrain malformation with caudal displacement of the cerebellar vermis/tonsils, brainstem, and fourth ventricle through the foramen magnum due to a small posterior fossa. It is almost always associated with a lumbosacral myelomeningocele and frequently with hydrocephalus; classic MRI features include medullary kinking, beaked tectum (tectal beaking), low tentorium, and a crowded posterior fossa.
CM‑II drives much of the morbidity in open spina bifida: neonatal respiratory/feeding problems from brainstem compression, frequent hydrocephalus needing CSF diversion, and long‑term motor/sensory deficits. Prenatal recognition changes counseling and eligibility for fetal myelomeningocele repair, which reduces shunt need and improves motor outcomes.
Prenatal ultrasound: cranial markers of open spina bifida—'lemon sign' (frontal bone scalloping, common ≤24 weeks) and 'banana sign' (curved, small cerebellum/obliterated cisterna magna); often ventriculomegaly.
Neonate/infant with myelomeningocele plus signs of hydrocephalus (rapid head growth, bulging fontanelle, vomiting, sunsetting eyes).
MRI head/spine: small posterior fossa, downward herniation of vermis/tonsils & brainstem, medullary kinking, tectal beaking, low‑lying tentorium/torcula; associated supratentorial anomalies (e.g., corpus callosum dysgenesis) may be present.
If myelomeningocele is present, assume CM‑II and screen for hydrocephalus (cranial US/MRI).
In a symptomatic infant (apnea/stridor/dysphagia), treat hydrocephalus first (VP shunt or ETV±CPC when appropriate). Hindbrain symptoms often improve after CSF diversion.
Reserve posterior fossa decompression for refractory brainstem compression after adequate CSF diversion or in the rare infant without hydrocephalus who remains symptomatic.
For prenatal diagnosis at specialized centers, discuss MOMS‑criteria fetal repair (typically 19–26 weeks): lowers shunt requirement and lessens hindbrain herniation but carries maternal/fetal risks.
Think about sleep‑disordered breathing (often central) in infants with CM‑II—obtain sleep study if suggestive symptoms or unexplained desaturations.
Always evaluate the whole neuraxis: associated tethered cord, syringomyelia, and orthopedic sequelae are common in spina bifida.
Acquired tonsillar descent from skull base anomalies; look for synostosis and skull base metrics.
Occipital encephalocele (Chiari III)
Herniation into an occipital/cervical encephalocele sac; extremely rare.
MMC closure: postnatal closure within 24–48 h or fetal repair in selected candidates (19–26 weeks) to reduce hindbrain herniation and shunt need.
Hydrocephalus management: VP shunt is standard; ETV ± choroid plexus cauterization (ETV/CPC) is an alternative in selected infants depending on anatomy/center expertise.
Posterior fossa decompression only for persistent, clinically significant hindbrain/brainstem compression after CSF diversion is optimized.
Multidisciplinary care (neurosurgery, neonatology, urology, rehab, pulmonology/sleep) and long‑term surveillance for scoliosis, tethered cord, and SDB.
Type II = think two big associations: (1) myelomeningocele and (2) hydrocephalus.
MRI buzzwords for CM‑II: medullary kinking + tectal beaking + low tentorium + crowded small posterior fossa.
Ultrasound fruit salad: lemon sign early (≤24 wk) and banana sign across gestation in open spina bifida.
Hydrocephalus often drives symptoms—divert CSF first before considering decompression.
After prenatal MMC repair, hindbrain herniation can partially reverse—don't be surprised if postnatal imaging looks better.
Recurrent apnea, stridor, bradycardia, or aspiration in an infant with MMC—evaluate urgently for hydrocephalus or hindbrain compression.
Rapid head growth, sunsetting eyes, or a tense fontanelle → CSF diversion may be emergently needed.
Any decline after shunt placement (vomiting, lethargy, irritability) → shunt malfunction until proven otherwise.
Prenatal US/MRI suggests open spina bifida → counsel & refer for fetal surgery evaluation if within window.
At birth: protect MMC sac → postnatal closure (if not repaired in utero) within 24–48 h; baseline head US.
If ventriculomegaly/ICP signs → VP shunt or ETV±CPC depending on anatomy/center.
