Hemianopia:  The loss of vision of one-half of the visual field.
Hemiplegia:  The paralysis of one side of the body.
Hemorrhage:  Bleeding due to the escape of blood from a blood vessel.
Herniated nucleus pulposus (HNP):  The extrusion of the central portion (nucleus) of an intervertebral disc through the outer cartilaginous ring (annulus). The herniated portion can compress the spinal cord or nerves in or exiting the spinal canal.
Hydrocephalus:  A condition, often congenital, marked by abnormal and excessive accumulation of cerebrospinal fluid in the cerebral ventricles. This dilates the ventricles and increases intracranial pressure. More about hydrocephalus.
Hydromyelia:  Expansion of the spinal cord due to increased size of the central canal of the cord, which is filled with cerebrospinal fluid.
Hyperesthesia:  Excessive sensibility to touch, pain, or other stimuli.
Hypertension:  High blood pressure.
Hypophysectomy:  Surgical removal of the hypophysis (pituitary gland).
Hypothalamus:  A collection of specialized nerve cells at the base of the brain that controls the anterior and posterior pituitary secretions, and is involved in other basic regulatory functions such as temperature control and attention.
Ictal:  Related to a seizure (as in, the ictal focus is the location where a seizure originates). Find out more about Epilepsy Surgery.
Instrumentation:  Metal supports sometimes used in spinal fusions to help steady the spine while it fuses. Common types of support include plates, rods, or screws.
Internal fixation:  The insertion of metal rods, wires, pins, screws, or plates (or a combination of these) into bone fragments.
Intra-arterial:  Delivered into an artery (as opposed to intravenous, which is delivered into a vein).
Intra-arterial catheterization angiography:  An invasive study in which a catheter (a small tube) is placed in an artery (usually the femoral artery) and guided into the arteries in the neck and head. Contrast material is injected, which makes the blood vessels visible on an x-ray image.
Intracerebral hematoma:  A blood clot within the brain.
Intracranial pressure (ICP):  The overall pressure inside the skull.
Intrathecal:  Within a theca or the dura mater membrane that surrounds the spinal canal.
Intravenous:  Delivered into a vein.
Ischemia:  Inadequate circulation of blood, generally due to a blockage of an artery. More about ischemic stroke.

Weill Cornell Medicine Brain & Spine Center 525 East 68 Street, Box 99 New York, NY 10065 Phone: 866-426-7787