Hydrocephalus

Hydrocephalus is the abnormal accumulation of cerebrospinal fluid (CSF) in the ventricles
Hydrocephalus can be obstructive or non-obstructive.

Relevant anatomy

Lateral ventricles: bilateral - 1 in each cerebral hemisphere. Have frontal, occipital and temporal horns. The lateral ventricles are connected to the third through the foramen of monroe.
Third ventricle: located between the thalami. Connected to the fourth ventricle through the cerebral aqueduct.
Fourth ventricle: Located at the junction between the pons and the medulla oblongata. From here, CSF drains into the central spinal canal and subarachnoid cisterns.
Choroid plexus: Choroid plexi are located in the lateral ventricles.

Pathophysiology

- Features of acute obstructive hydrocephalus:
  - CSF cannot pass through the ventricular system, causing dilation proximal to obstruction.
  - Lateral ventricles: enlarged temporal horns and transependymal oedema.
  - Third ventricle: bowing of lateral walls and floor.
  - Fourth ventricle: enlarged fourth suggests obstruction at foramina of Luschka or Magendie.
  - Can be caused by a phsyical stenosis (e.g. at various foramina) or pathological - i.e. obstruction secondary to mass effect from tumour/haemorrhage/infarct
- Features of non-obstructive (communicating) hydrocephalus:
  - Passage of CSF through ventricular system is unimpeded.
  - Can be caused by subarachnoid haemorrhage, infection, leptomeningeal infiltration, normal pressure hydrocephalus.

CT findings of hydrocephalus

Transependymal oedema is pathognomic of severe obstructive hydrocephalus.
Other findings include dilated ventricles, visible temporal horns of the lateral ventricles, bulging third/fourth ventricle.
Alternative imaging modalities include T2 CISS/FIESTA midline sequences to identify abnormalities in CSF flow such as aqueductal stenosis, webs or membranes.

CT examples

OBSTRUCTIVE HYDROCEPHALUS

CT shows dilated lateral ventricles with transependymal oedema and sulcal effacement.
Ventricular catheter is in situ (likely as part of a ventriculoperitoneal shunt or external ventricular drain).

obstructive hydrocephalus secondary to mass effect

Sagittal MRI shows a posterior fossa lesion causing compression of the fourth ventricle and CSF outflow, resulting in an obstructive hydrocephalus.

References

Obstructive hydrocephalus: Case courtesy of Ian Bickle, Radiopaedia.org, rID: 55488Obstructive hydrocephalus secondary to medulloblastoma compressing CSF outflow (MRI-T1): Case courtesy of Ahmed Abdrabou, Radiopaedia.org, rID: 22688

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