In tethered cord syndrome, the spinal cord gets tethered or stuck to the sides of the spinal canal. This usually happens in the caudal region of the spine--the part that's right above the tail bone and extends up into the lower back.

Syringomyelia (sih-ringo-my-EE-lee-uh) is a neurological disorder. It is caused when a syrinx--a small cyst or collection of fluid--forms within the spinal cord.(1)

Cranio-cervical instability--alternately punctuated as craniocervical instability, abbreviated as CCI, and known as occipitoatlantialaxial hypermobility--is a disorder of the head (cranio-) and the upper vertebra (back bones) of the spine closest to the head.

The Ehlers-Danlos Syndromes (EDS) are a group of related genetic conditions that affect various types of connective tissue in the human body.(1) There are 13 subtypes that are defined by the type of collagen affected and the symptoms that are caused.(2)

Basilar (meaning "situated at the base, often of the skull") invagination (meaning "the action or process of being turned inside out or folded back on itself to form a cavity or pouch") is a rare condition that affects the vertebrae--the 33 bones that make up the spine. These bones and the discs that provide cushioning between them make up a passage for the spinal cord and nerves.

Intracranial hypertension is increased pressure (hypertension) inside the cranium, or skull. Inside the cranium sits the brain, surrounded by cerebrospinal fluid (CSF) and the various blood vessels that supply the brain.

Postural orthostatic tachycardia syndrome or POTS is a blood circulation and autonomic nervous system disorder that can be understood by breaking down the four words of its name:(1)

Postural: relating to the position of the body, in this case, moving from lying down to sitting, or from lying down to standing

Orthostatic: related to standing upright

Tachycardia: a fast heartbeat

Syndrome: a collection of symptoms that happen together (so POTS is not a disease, but a cluster of symptoms that are all probably caused by something else(8))