Hydrocephalus

Definition

Hydrocephalus is an abnormal expansion of cavities (ventricles) within the brain caused by the accumulation of cerebrospinal fluid.

Description

Hydrocephalus is the result of an imbalance between the formation and drainage of cerebrospinal fluid (CSF). Approximately 500 milliliters (about a pint) of CSF is formed within the brain each day, by structures called choroid plexus, with epidermal cells lining chambers called ventricles. Once formed, CSF usually circulates among all the ventricles before it is absorbed and returned to the circulatory system. The normal adult volume of circulating CSF is 150 ml, so that the CSF turn-over rate is more than three times per day. Production is independent of absorption, and reduced absorption causes CSF to accumulate within the ventricles.

Reduced absorption most often occurs when one or more passages connecting the ventricles become blocked, preventing movement of CSF to its drainage sites in the subarachnoid space just inside the skull. This type of hydrocephalus is called "noncommunicating." Reduction in absorption rate can also be caused by damage to the absorptive tissue. This type is called "communicating hydrocephalus."

Both of these types lead to an elevation of the CSF pressure within the brain. This increased pressure squeezes the soft tissues of the brain, distorting and damaging them. In infants whose skull bones have not yet fused, the intracranial pressure is partly relieved by expansion of the skull, so that symptoms may not be as dramatic. Both types of elevated-pressure hydrocephalus may occur from infancy to adulthood.

A third type of hydrocephalus, called "normal pressure hydrocephalus," is marked by ventricle enlargement without an apparent increase in CSF pressure. This type affects mainly the elderly.

Causes & symptoms

Hydrocephalus may be caused by:

• Congenital brain defects
• Hemorrhage, either in the ventricles or the subarachnoid space
• Infection of the central nervous system (syphilis, herpes, meningitis, encephalitis, or mumps)
• Tumor.

Symptoms of elevated-pressure hydrocephalus include:

• Headache
• Nausea and vomiting, especially in the morning
• Lethargy
• Gait disturbance
• Double vision
• Subtle difficulties in learning and memory
• Delay in achievement of developmental milestones in children.

Irritability is the most common sign of hydrocephalus in infants and, if untreated, this may lead to lethargy. Bulging of the fontanelle, the soft spot between the skull bones, may also be an early sign. Hydrocephalus in infants prevents fusion of the skull bones, and causes expansion of the skull.

Symptoms of normal pressure hydrocephalus include dementia, gait abnormalities, and incontinence (involuntary urination or bowel movements).

Diagnosis

Imaging studies--x ray, computed tomography scan (CT scan), ultrasound, and especially magnetic resonance imaging (MRI)--are used to assess the presence and location of obstructions, as well as changes in brain tissue that have occurred as a result of the hydrocephalus. Lumbar puncture (spinal tap) may be performed to aid in determining the cause.

Treatment

The primary method of treatment for both elevated- and normal pressure hydrocephalus is surgical installation of a shunt. The shunt is a tube connecting the ventricles to an alternative drainage site, usually the abdomen. The shunt contains a one-way valve to prevent reverse flow. In some cases of non-communicating hydrocephalus, a direct connection can be made between one of the ventricles and the subarachnoid space, allowing drainage without a shunt.

Installation of a shunt requires lifelong monitoring by the patient or family members for signs of recurring hydrocephalus due to obstruction or failure of the shunt.

Some drugs may postpone the need for surgery by inhibiting the production of CSF. These include acetazolamide and furosemide. Other drugs used to delay surgery include glycerol, digoxin, and isosorbide.

Prognosis

Prognosis for elevated-pressure hydrocephalus depends on a wide variety of factors, including the cause, age of onset, and the timing of surgery. Studies indicate that about half of all children who receive appropriate treatment and follow-up will develop IQs greater than 85. Those with hydrocephalus at birth do better than those with later onset due to meningitis. For patients with normal pressure hydrocephalus, shunt installation may lead to improvement in approximately half.

Prevention

Some cases of elevated pressure hydrocephalus may be preventable by preventing or treating the infectious diseases which precede them. Prenatal diagnosis of congenital brain malformation is often possible, offering the option of family planning.

Key Terms

Subarachnoid space

The space between two membranes surrounding the brain, the arachnoid and pia mater.

Further Reading

For Your Information

Books

• Bradley, et al. Neurology in Clinical Practice, 2nd ed. Butterworth-Heinemann, 1996.

  Other

• The Hydrocephalus Association. 870 Market Street, Suite 955 San Francisco, CA 94102. 415-732-7040. Fax: 415-732-7044.

Gale Encyclopedia of Medicine. Gale Research, 1999.