Hydrocephalus Imaging

Introduction

Hydrocephalus is derived from the Greek word hydro-water & cephalus - brain, referring to an increase in water content of the brain i.e. an increase in the ventricular size. It results when there is an imbalance between the production of CSF & its drainage by the arachnoid villi. Three mechanisms account for the development of hydrocephalus.

Figure 1: Pathogenesis of hydrocephalus

Classification of Hydrocephalus

It can be classified depending on the level of obstruction.

Depending on physiological seepage via damaged subependymal lining resulting in periventricular ooze, it can be classified as:

CT Findings

Figure: Normal CT scan

Figure: Meningitis with Hydrocephalus

Figure: Choroid plexus carcinoma with hydrocephalus

Causes of Hydrocephalus

The various causes of hydrocephalus are as follows:

Ventricular Dilatation Versus Hydrocephalus

Cerebral atrophy produces dilatation of the ventricular system with proportionated dilatation of cortical sulci & CSF spaces. (Sylvian fissure) Any obvious disproportionality should point towards abnormal enlargement. In hydrocephalus, the frontal horn of the lateral ventricles become rounded, balloon-shaped, whereas, in atrophy, the ventricular shape does not change. The anterior angle between two lateral ventricles may increase in hydrocephalus & subependymal ooze also may be present. All these features, with clinical correlation, will differentiate between atrophy & hydrocephalus.

Ultrasound

Ultrasound is a safe, quick, non-invasive & repeatable modality, has a definite role in the diagnosis of hydrocephalus. However, the ultrasound waves cannot penetrate the bony skull. It is still used in neonatal brain imaging where the open anterior fontanelle is the acoustic window. Hence, its use is limited between the age group 6 months-2 years.

When hydrocephalus is diagnosed in intrauterine life, associated CNS/extracranial anomalies should be looked for such as - meningomyelocele, other neural tube defects like spina bifida, or Chiari malformation. Often hydrocephalus can be diagnosed in utero by 15 weeks gestation. In utero, an upper limit of 10 mm for the ventricular atrium is considered significant and hydrocephalus can be suspected.

Neonatal hydrocephalus is easy to recognize by routine coronal & sagittal imaging. Thus, diagnosis & progression can be evaluated. Care must be taken so that changes in ultrasound sector depth do not result in apparent enlargement or decompression of ventricles related to magnification difference when different depth scales are used. Failure to do so may result in a false impression of changing hydrocephalus.

Ventricular/Hemispheric Ratio:

Figure: Normal USG

Figure: USG Hydrocephalus

Severity of Hydrocephalus

Though the degree of ventricular dilatation is estimated on imaging, clinical implications are unpredictable. Even with mild hydrocephalus, there may be more neurological damage than expected.

On CT scan, prominent temporal horns are amongst the first indicators. The transverse diameter of the third ventricle >5 mm is considered abnormal. Ballooning of the frontal horn with periventricular hypodensity is seen in obstructive hydrocephalus. In addition, the ventricular SRC index may be used.

Ventricular SRC Index= Distance between Anterior tips of the frontal horn/Bifrontal diameter at the same level (from the inner table of the skull) (Normal = 30%)

Ultrasound uses the same principle. However, grading as mild, moderate, severe is subjective.

Normal Pressure Hydrocephalus

This is an idiopathic form of communicating hydrocephalus, which features clinically as a triad of dementia, incontinence & gait disturbances. Symptoms may be relieved by ventricular shunting. Imaging appearances are nonspecific. The patient improves after repeated lumbar punctures. The persistence of intrathecal contrast for 48 hrs on CT cisternography is also suggestive of this condition.

Shunt Complications And Imaging

Following shunting, there is a drop in the size of the ventricles. However, even when the tip of the shunt tube is seen outside ventricles, ventricular size is not enlarged as this is a tube with many side holes.

Complications of shunt surgery:

• Blocked shunt tube: Shunt tube is seen in situ the ventricles, but ventricles are still dilated.

  Shunt-tube migration - Migration of shunt tube may be extracranial or intracranial (It occurs due to the motion of the patient's head).

• Subdural hematoma: These may occur when large ventricles are shunted - due to a decrease in ventricular size, subarachnoid spaces increase, stretching the vein which tears, resulting in subdural hematomas.
• Slit ventricle syndrome: Also called shunt dependency syndrome. It is seen due to the inability of the ventricles to re-expand after shunting due to lack of compliance or adhesions.
• Isolated the fourth ventricle: This occurs when there are exudates at the aqueduct & fourth ventricular outlet or aqueductal stenosis. Following shunting, there is dilation of the fourth ventricle due to the aqueductal stenosis. Hence, the fourth ventricle does not get decompressed.

Infection - Following shunting, there may be infection along the shunt tract leading to ventriculitis, which is seen as an enhancement along the ventricular wall.

Figure: Congenital Hydrocephalus with shunt

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Definition of Various Terminologies Used

Arrested hydrocephalus: This is a condition, in which the ventricular system is not actively dilating. A more appropriate term is compensated hydrocephalus.

Active hydrocephalus: Active hydrocephalus is marked by an increase of ventricular volume. The activity is recognized by pronounced clinical symptoms and by progression in follow up CT studies.

Hydrocephalus ex vacuo:

Figure: Exvacuo dilation of the ventricle

Colpocephaly