craniotomy

craniotomy By derivation, this word covers any surgical opening into the skull, but it is now usually used only to describe the fashioning of a hinged flap of bone which allows the intracranial cavity to be reached. Opening the skull (trepanning) in the belief that this would let out evil spirits is one of the oldest operations in the history of surgery, as evidenced by the finding of man-made holes in skulls from very early periods. In these cases, pointed flints were probably used to bore out the hole, and some skulls show evidence of subsequent healing with new bone formation — indicating that people survived such procedures. Until the middle of the twentieth century, skull openings were still being made by some tribes in Kenya. Indications probably included mental illness, epilepsy, and perhaps severe, recurrent headache such as migraine. Recently an eccentric small group have advocated do-it-yourself drilling of the skull, supposedly to restore the alleged benefit of allowing a pressure outlet similar to that in infancy before the fontanelle closes. Technical instructions for this bizarre procedures have even appeared on the Internet.

By the seventeenth century the ‘trephine’ was an instrument in the surgical armamentarium for opening the skull. This was a circular crown saw with a central pin, which would cut out a disc or button of bone. It could vary in diameter from 1 cm or so up to 8 cm; sometimes more than one disc was cut to give adequate access. The larger discs would be put back in place, but smaller holes were left as permanent defects. A further development was to use a brace and bit to ream out (as bone dust) burr holes about 1 cm in diameter. Through trephine or burr holes, blood clots that were threatening life by compression of the brain could be let out, and lives were saved. Burr holes are still used today for releasing surface clots and also to allow probes to be passed into deeper parts of the brain. These may be for measuring the pressure in, or draining fluid from, the cerebral ventricles; for obtaining biopsy specimens of brain for microscopic examination; to record electrically from the deeper parts of the brain; or to produce controlled damage in parts of the brain for the correction of abnormal functioning (e.g. epilepsy or tremor). The limited access provided by a burr hole may be increased by nibbling away more bone with forceps, the resulting procedure being termed craniectomy. This leaves a permanent defect in the skull, with the brain covered only by the skin and other soft tissues.

The Gigli saw, invented by this Florentine obstetrician in 1894 to enlarge the pelvic outlet in obstructed labour, made the craniotomy flap possible. This saw is a serrated wire that can be passed under the bone from one burr hole to the next one and used to saw the bone in between. This is done along the lines joining each pair of burr holes, except the line which underlies the fold of skin and other tissues that has been turned back. This base section of unsawn bone is then cracked as the bone flap is raised. The bone flap remains hinged on the soft tissues attached to it, preserving its blood supply (see figure). This allows major intracranial surgery to be performed, such as the removal of a tumour, and when this has been completed the bone flap can be turned back into place and the soft tissues and skin also replaced. The use of a brace and bit and Gigli saw has recently been replaced by power driven drills and saws.

An unusual use of the term ‘craniotomy’ is in obstetrics. This is when a dead baby with an abnormally large head has caused obstructed labour, and the obstetrician has to reduce the size of the head by perforating it in order to allow delivery.

Bryan Jennett