This study will measure brain excitability in patients with succinic semialdehyde dehydrogenase (SSADH) deficiency, and in their parents. SSADH is a rare inherited disease in which changes in certain brain chemicals affect brain cell activity. Symptoms vary greatly among patients, and may include mental retardation, impaired ability to coordinate movements, and delays in language and speech development. Other symptoms may include poor muscle tone, uncontrolled seizures and other neurological or behavioral abnormalities. Test findings in patients and their parents will be compared with those of healthy normal volunteers.

The following individuals may be eligible for this study: patients with SSADH who are between 5 and 24 years of age; parents of patients who are between 18 and 55 years of age; healthy normal children who are between 10 and 17 years of age; and healthy normal adults who are between 18 and 55 years of age. Candidates are screened with blood and urine tests.

All participants undergo the following:

• Transcranial magnetic stimulation (TMS): This procedure maps brain function. A wire coil is held on the scalp, and a brief electrical current is passed through the coil, creating a magnetic pulse that stimulates the brain. During the stimulation, the subject may be asked to tense certain muscles slightly or perform other simple actions to help position the coil properly. The stimulation may cause a twitch in muscles of the face, arm, or leg, and the subject may hear a click and feel a pulling sensation on the skin under the coil. During the test, electrical activity of muscles is recorded with a computer or other recording device, using electrodes attached to the skin with tape.
• Magnetic resonance imaging (MRI): This test combines a powerful magnet with an advanced computer system and radio waves to produce accurate, detailed pictures of organs and tissues. During the scan, the subject lies on a table in a narrow cylinder containing a magnetic field, wearing ear plugs to muffle loud noises that occur with electrical switching of the magnetic fields. He or she can speak with a staff member via an intercom system at all times during the procedure. In addition to standard MRI, subjects may have newer MRI tests, such as diffusion tensor MRI and magnetic resonance spectroscopy.

In addition to the above, patients and their parents may also undergo the following tests:

• Electroencephalography (EEG): This test records brain waves (electrical activity of the brain). Electrodes are placed on the scalp and brain electrical activity is recorded while the subject lies quietly, breathes deeply, watches flashes of light, or sleeps. Some patients may also have video-EEG monitoring, which involves simultaneous clinical recording using a video camera along with brain wave recording.
• Sleep study and multiple sleep latency onset testing (MSLT): Electrodes are placed on the subject's scalp and remain there while the subject sleeps in the hospital overnight. MSLT is done the next day, starting 2 hours after the subject wakes up from the night's sleep. A total of five 20- to 30-minute naps are recorded, each every 2 hours, to access daytime sleepiness.
• Nerve conduction studies: This test measures the speed with which nerves conduct electrical impulses and the strength of the connection between the nerve and the muscle. A probe is placed on the skin to deliver a small electrical stimulus, and wires taped on the skin record the impulses.