Increased intracranial pressure

About 150 ml of CSF is present in the ventricles at any time. About 700 ml of CSF is produced each day. Normal CSF pressure is about 100-180 mm of H2O (8-15 mm Hg) with the patient lying on the side and 200-300 mm with the patient sitting up. Intracranial pressure is lower in infants and children. Raised intracranial pressure or tension can be due to brain tumors, aneurysms, hydrocephalus, intracerebral hemorrhage, meningitis, head injury, etc. The combination of headache, papilloedema, and vomiting is generally considered indicative of raised intracranial pressure (ICP). Pressure headaches are often described as throbbing or bursting and are exacerbated by any factors that further increase ICP such as coughing, sneezing, recumbency or exertion. Cushing’s reflex of bradycardia and hypertension occurs in response to raised ICP. Classically the headache of raised ICP is worse in the morning.

Papilledema is seen due to elevated ICP. Ophthalmoscopy shows swollen optic disc with blurring of disc margins. Raised ICP can lead to intracranial herniation with catastrophic results. The tentorium is a dural structure that separates the cerebrum from the brainstem and cerebellum. The midbrain passes through the tentorial incisura, which is an opening in the tentorium. Lumbar puncture should be avoided in patients with raised ICP as it may precipitate herniation of brain structures.

(I) Hydrocephalus: The term hydrocephalus is derived from the Greek words “hydro” meaning water and “cephalus” meaning head. It is a condition in which the primary characteristic is excessive accumulation of CSF in the brain causing dilation of the ventricles and often, but not always, leading to increased intracranial pressure.

It can be either primary or secondary. In primary hydrocephalus there is an actual increase in CSF volume along with elevated intracranial pressure, while in secondary hydrocephalus there is a compensatory increase in CSF without increase in intracranial pressure due to cerebral atrophy. Secondary hydrocephalus is seen in Alzheimer’s disease, called “hydrocephalus ex-vacuo”. The brain parenchyma is thinned and ventricles are dilated.

Types of primary hydrocephalus:

i) Obstructive or non-communicating hydrocephalus: This type is due to obstruction of CSF flow out of the ventricles. It is seen in congenital stenosis of the cerebral aqueduct of Sylvius, Arnold -Chiari malformations etc. or is acquired due to tumors, hemorrhages etc. that block the flow of CSF.

ii) Non-obstructive or communicating hydrocephalus: In communicating hydrocephalus, CSF can easily flow from ventricles to the subarachnoid space. It occurs due to overproduction or reduced absorption of CSF like in choroid plexus papillomas, scarring post-meningitis, SAH, dural sinus thrombosis etc.

In the early stages of hydrocephalus, the periventricular white matter loses myelin and axons. If pressure is not relieved, permanent atrophy of initially, white matter followed by grey matter occurs. This causes spastic paralysis, loss of bladder function, and dementia. In young children, before closing of fontanelles, hydrocephalus causes an increase in head circumference while in adults, head circumference does not change.

Normal pressure hydrocephalus or NPH: NPH is an abnormal increase of cerebrospinal fluid that may result from a subarachnoid hemorrhage, head trauma, infection, tumor, or complications of surgery. Most cases are idiopathic. It presents with wide based gait, urinary incontinence and dementia. Patients may complain of general slowing of movements or that his or her feet feel "stuck."CSF pressure is normal. Removal of CSF relieves symptoms including dementia. Imaging shows dilated ventricles and atrophy of sulci.

Pseudotumor cerebri or benign intracranial hypertension: Pseudotumor cerebri literally means "false brain tumor”. Intracranial pressure is high due to the buildup or poor absorption of CSF. The ventricles are not dilated. The disorder is most common in women between the ages of 20 and 50. Symptoms of pseudotumor cerebri, which include headache, nausea, vomiting, and pulsating sounds within the head, closely mimic symptoms of large brain tumors. Obesity, hypothyroidism, and some medications such as oral contraceptives, steroids, tetracyclines, all -trans retinoic acid, tamoxifen, can cause pseudotumor cerebri. Repeated ophthalmologic exams are required to monitor any changes in vision. Weight loss through dieting or weight loss surgery and cessation of predisposing drugs may lead to improvement. Therapeutic shunting is done in resistant cases. Flattening of the posterior lobe is seen on MRI.

(II) Cerebral edema: It is swelling of the brain. It can be vasogenic, cytotoxic or interstitial. Vasogenic edema is caused by cerebral infarcts, contusions, tumors etc. which increase the net filtration pressure across the vessel. Cytotoxic edema results from hypoxia or toxins that cause direct damage to the cells. Interstitial edema occurs when excess fluid e.g. from a hydrocephalus, crosses the ependymal lining and accumulates in the periventricular white matter. The intracranial pressure rises. On gross examination, the brain looks swollen with flattened gyri and narrowed sulci.