| Apnea | to stop breathing intermittently, may be due to compression on the brainstem. |
| Arachnoid | one of the three membranes covering the brain, appears to resemble a spider web. There are 3 layers of covering of the brain and spinal cord- the pia, arachnoid and dura. |
| Arachnoiditis | inflammation of the arachnoid |
| Aspirate | inhaling fluids into the lungs. (Inability to swallow properly can allow fluid into the lungs.) |
| Ataxia | inability to coordinate movement. Stumbling when walking, or inability to maintain a fluid gait when asked to walk a straight line. |
| Atrophic | changes in the muscle that create an appearance of being wasted away |
| Atrophy | wasting away of the muscle, often due to lack of use or inability to exercise a muscle. |