Backflow of cerebrospinal fluid through a 25 gauge spinal needle after puncture of the arachnoid mater during initiation of spinal anaesthesia
Spinal anaesthesia (or spinal anesthesia), also called spinal analgesia, spinal block or sub-arachnoid block (SAB), is a form of regional anaesthesia involving injection of a local anaesthetic into the subarachnoid space, generally through a fine needle, usually 9 cm long (3.5 inches). For extremely obese patients, some anaesthesiologists prefer spinal needles which are 12.7 cm long (5 inches). The tip of the spinal needle has a point or small bevel. Recently, pencil point needles have been made available (Whitacre, Sprotte, Gertie Marx & others).
Difference from epidural anesthesia 
Epidural anesthesia is a technique whereby a local anesthetic drug is injected through a catheter placed into the epidural space. This technique has some similarity to spinal anesthesia, and the two techniques may be easily confused with each other. Differences include:
- The involved space is larger for an epidural, and consequently the injected dose is larger, being about 10–20 mL in epidural anesthesia compared to 1.5–3.5 mL in a spinal.
- In an epidural, an indwelling catheter may be placed that avails for additional injections later, while a spinal is almost always a one-shot only.
- The onset of analgesia is approximately 15–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.
- An epidural often does not cause as significant neuromuscular block unless specific local anesthetics are used which block motor fibres as readily as sensory nerve fibres, while a spinal more often does.
- An epidural may be given at a cervical, thoracic, or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.
Injected substances 
Bupivacaine (Marcaine) is the local anaesthetic most commonly used, although lignocaine (lidocaine), tetracaine, procaine, ropivacaine, levobupivicaine and cinchocaine may also be used. Sometimes a vasoconstrictor such as epinephrine is added to the local anaesthetic to prolong its duration. Of late, many anaesthesiologists are preferring to add opioids like morphine, fentanyl, or buprenorphine, or non-opioids like clonidine, to the local anaesthetic used in a spinal injection, to give a smoother effect and to provide prolonged pain relief once the action of the spinal local anaesthetic has worn off.
Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Usually, the hyperbaric, (for example, hyperbaric bupivacaine) is chosen, as its spread can be effectively and predictably controlled by the anaesthesiologist, by tilting the patient. Hyperbaric solutions are made more dense by adding dextrose to the mixture.
Regardless of the anaesthetic agent (drug) used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anaesthetic used and the properties of the axon. Thin unmylenated C-fibres associated with pain are blocked first, while thick, heavily mylenated A-alpha motor neurons are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechanoreceptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.
Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake.
Spinal anaesthetics are typically limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralysing the intercostal respiratory muscles, or even the diaphragm in extreme cases (called a "high spinal", or a "total spinal", with which consciousness is lost), as well as the body's ability to control the heart rate via the cardiac accelerator fibres. Also, injection of spinal anaesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.
The first spinal analgesia was administered in 1885 by James Leonard Corning (1855–1923), a neurologist in New York. He was experimenting with cocaine on the spinal nerves of a dog when he accidentally pierced the dura mater.
The first planned spinal anaesthesia for surgery in man was administered by August Bier (1861–1949) on 16 August 1898, in Kiel, when he injected 3 ml of 0.5% cocaine solution into a 34-year-old labourer. After using it on 6 patients, he and his assistant each injected cocaine into the other's spine. They recommended it for surgeries of legs, but gave it up due to the toxicity of cocaine.
Worldwide usage 
Current usage of this technique is waning in the developed world, with epidural analgesia or combined spinal-epidural anaesthesia emerging as the techniques of choice where the cost of the disposable 'kit' is not an issue.
However spinal analgesia is the mainstay of anaesthesia in countries like India, Pakistan and parts of Africa, excluding the major centres. Thousands of spinal anaesthetics are administered daily in hospitals and nursing homes. At a low cost, a surgery of up to two hours duration can be performed below the umbilicus of the patient.
This technique is very useful in patients having an irritable airway (bronchial asthma or allergic bronchitis), anatomical abnormalities which make endotracheal intubation very difficult (micrognathia), borderline hypertensives where administration of general anaesthesia or endotracheal intubation can further elevate the blood pressure, procedures in geriatric patients.
Non-availability of patient's consent, local infection or sepsis at the site of lumbar puncture, bleeding disorders, space occupying lesions of the brain, disorders of the spine and maternal hypotension.
All surgical interventions below the umbilicus, is the general guiding principle:
- Abdominal & vaginal hysterectomies
- Laparoscopy Assisted Vaginal Hysterectomies (LAVH) combined with general anaesthesia
- Caesarean sections
- Hernia (inguinal or epigastric)
- Piles fistulae and fissures
- Orthopaedic surgeries on the pelvis, femur, tibia and the ankle
- Trauma surgery on the lower limbs, especially if the patient is full-stomach
- Open tubectomies
- Transurethral resection of the prostate
Can be broadly classified as immediate (on the operating table) or late (in the ward or in the P.A.C.U. post-anaesthesia care unit):
- Spinal shock.
- Cauda equina injury.
- Cardiac arrest.
- Broken needle.
- Bleeding resulting in haematoma, with or without subsequent neurological sequelae due to compression of the spinal nerves
- Infection: immediate within six hours of the spinal anaesthetic manifesting as meningism or meningitis or late, at the site of injection, in the form of pus discharge, due to improper sterilization of the LP set.
- post dural puncture head ache (PDPH) or post spinal head ache
See also 
- Lumbar puncture
- Combined spinal and epidural anaesthesia
- Transparent reality simulation of spinal anaesthesia
- Various diagrams of needles for Lumbar puncture, Epidural, Spinal Anesthesia, etc
- Corning J. L. N.Y. Med. J. 1885, 42, 483 (reprinted in 'Classical File', Survey of Anesthesiology 1960, 4, 332)
- Bier A. Versuche über Cocainisirung des Rückenmarkes. Deutsch Zeitschrift für Chirurgie 1899;51:361. (translated and reprinted in 'Classical File', Survey of Anesthesiology 1962, 6, 352)