| Hemisphere | one half of the upper portion of the brain. |
| Herniation | abnormal protrusion (or overhang) of tissue. Some MRI reports will read herniation of the tonsils- this means the tonsils are hanging down too far. |
| Hoffman's sign | (or reflex) an abnormal reflex that is tested by placing the fingers in a neutral position, and (painlessly) flicking the nail of the middle finger. If positive, the other fingers will abnormally flex in response. |
| Hydrocephalus | Abnormal, over filling of the spinal fluid pockets (ventricles) in the brain. Can cause symptoms such as headache, balance problems, incontinence and memory problems. |
| Hydrosyringomyelia | a fluid cavity in the spinal cord. May also be referred to as syrinx, or syringomyelia. |