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2Brain

pressure, some patients’ symptoms mayimprove with therapeutic lumbar puncture,and patients in this group sometimes benefit from ventriculoperitoneal shunt placement.

Obstructive hydrocephalus, or noncom-municating hydrocephalus, refers to dila-tion of ventricles proximal to a mechanical block by tumor, blood clot, developmental web, periventricular parenchymal hemor-rhage, or other mass. The most frequent sites of obstruction are the foramina of Monro, the third ventricle, the sylvian aq-ueduct, and the fourth ventricle.

Obstructive hydrocephalus may be acute or chronic. In acute obstructive hy-drocephalus, CSF passes through small tears in the stretched ventricular ependy-mal lining and is absorbed by capillaries in the adjacent brain parenchyma (transep-endymal CSF resorption). This appears as low-attenuation parenchymal changes ad-jacent to the dilated lateral ventricles and does not occur in chronic or slowly devel-oping hydrocephalus.

Temporal horn enlargement may be the earliest manifestation of acute ventricular obstruction. The width of the third ven-tricle is a sensitive and reliable indicator of changes in ventricular volume on serial examinations (Fig. 2.35).

◆Hydrocephalus

Communicating hydrocephalus consists of enlargement of all cerebral ventricles due to impairment of CSF resorption by dural arachnoid granulations. It may be acute or chronic. Causes include trauma, subarach-noid hemorrhage, meningitis, and prior surgery. In patients with chronic commu-nicating hydrocephalus, a cause may not be identified, and patients come to clinical attention when hydrocephalus is inciden-tally discovered on studies obtained for minor trauma or in the evaluation of cog-nitive impairment.

Distinguishing between communicat-ing hydrocephalus and global cerebral atrophy may be dicult and depends on estimation of ventricular size in relation to sulcal enlargement. Generalized cere-bral atrophy may be due to chronic alcohol or anticonvulsant use, prior trauma, and neurodegenerative disorders such as Par-kinson disease, Alzheimer dementia, and long-standing multiple sclerosis.

Normal-pressure hydrocephalus is aclinical syndrome, usually seen in patientsover 50 years old, in which communicatinghydrocephalus is associated with gradualdevelopment of urinary incontinence, gaitdisturbance, and memory loss. Even thoughthese patients have a normal CSF opening

Fig. 2.35a–fa,b Chronic communicating hydrocephalus. Ventricular enlargement out of proportion to sulcal size. This appearance would be characteristic of a patient with clinical ndings of normal-pressure hydrocepha-lus or could be due to remote meningeal inammation, most often from subarachnoid hemorrhage or meningitis.

c,d Chronic obstructive hydrocephalus due to a cerebellar medulloblastoma. A hyperdense mass lls the fourth ventricle. The third and lateral ventricles are enlarged but do not show transependymal CSF resorption.

e,f Acute obstructive hydrocephalus due to craniopharyngioma. Partially calcied, solid, and cystic su-prasellar mass. Lateral ventricular enlargement; normal third ventricle and cortical sulcal eacement are due to obstruction at the foramina of Monro. Subtle low-attenuation parenchymal changes adjacent to the ventricles indicate transependymal CSF resorption.