The filum terminale, or terminal filament, is a slender flexible strand that attaches the bottom of the spinal cord to a bone called the coccyx at the lower end of the vertebral column. Nerve tissue makes up the spinal cord, and this transmits nerve impulses allowing communication between brain and body. The cord is wrapped inside three membranes, known as meninges, and the innermost one, the pia mater, is full of blood vessels and is continuous with the covering of the brain. It is an extension of the pia mater that forms the filum terminale. The spinal cord tapers at its lower end into the conus medullaris and from the pointed end a strand of the pia mater protrudes, becoming the filum terminale, which runs down to attach to the coccyx.
Normally, the spinal cord is suspended freely, surrounded by cerebrospinal fluid and protected within the bony column formed by the vertebrae. The filum terminale serves to attach the cord to the end of the bony spine, while still allowing some movement due to its elastic nature. Sometimes the spine fails to develop properly in the embryo, leading to an abnormal terminal filament, which is shorter than usual and fibrous rather than flexible. This condition is known as tight filum terminale syndrome.
In tight filum terminale syndrome, the spinal cord is tightly tethered. When a person moves the spine to stretch and bend, as happens regularly during daily activities, the abnormal filum terminale cannot flex to accommodate this and the spinal cord, together with its nerves, ends up stretching instead. Over time this causes damage to the nerves, especially those arising from the lower part of the spinal cord, and symptoms arise affecting the parts of the body they supply. Pain in the lower back can occur, and a curved spine may be seen, together with deformed feet and weak legs. There can be problems with walking — the legs and feet might feel numb — with one limb growing more than the other, and bladder and bowel control may deteriorate over time.
Treatment of the syndrome involves a surgical operation to sever the filum terminale completely, leaving the spinal cord detached and freely suspended. The outcome of the procedure depends on how far nerve damage has already progressed, but generally the symptoms are prevented from becoming any worse, and often they improve. Bladder symptoms are least likely to show an improvement, but often any pain in the back and legs disappears after treatment.