Most of the contra-indications to spinal anaesthesia apply equally to other forms of regional anaesthesia. These include:

Inadequate resuscitative drugs and equipment. No regional anaesthetic technique should be attempted if drugs and equipment for resuscitation are not immediately to hand.

Clotting disorders. If bleeding occurs into the epidural space because an epidural vein has been punctured by the spinal needle, a haematoma could form and compress the spinal cord. Patients with a low platelet count or receiving anticoagulant drugs such as heparin or warfarin are at risk. Remember that patients with liver disease may have abnormal clotting profiles whilst low platelet counts as well as abnormal clotting can occur in pre-eclampsia.

Hypovolaemia from whatever cause e.g. bleeding, dehydration due to vomiting, diarrhoea or bowel obstruction. Patients must be adequately rehydrated or resuscitated before spinal anaesthesia or they will become very hypotensive. Any sepsis on the back near the site of lumbar puncture.

Patient refusal. Patients may be understandably apprehensive and initially state a preference for general anaesthesia, but if the advantages of spinal anaesthesia are explained they may then agree to the procedure and be pleasantly surprised at the outcome. If, despite adequate explanation, the patient still refuses spinal anaesthesia, their wishes should be respected.

Uncooperative patients. Although spinal anaesthesia is suitable for children, their cooperation is necessary and this must be carefully assessed at the pre-operative visit. Likewise, mentally handicapped patients and those with psychiatric problems need careful pre-operative assessment.

Septicaemia. Due to the presence of infection in the blood there is a possiblity of such patients developing meningitis if a haematoma forms at the site of lumbar puncture and becomes infected.

Anatomical deformities of the patient's back. This is a relative contraindication, as it will probably only serve to make the dural puncture more difficult.

Neurological disease. The advantages and disadvantages of spinal anaesthesia in the presence of neurological disease need careful assessment. Any worsening of the disease postoperatively may be blamed erroneously on the spinal anaesthetic. Raised intracranial pressure, however, is an absolute contra-indication as a dural puncture may precipitate coning of the brain stem.

 
Physiology of Spinal Anaesthesia.

Local anaesthetic solution injected into the subarachnoid space blocks conduction of impulses along all nerves with which it comes in contact, although some nerves are more easily blocked than others.

There are three classes of nerve: motor, sensory and autonomic. The motor convey messages for muscles to contract and when they are blocked, muscle paralysis results. Sensory nerves transmit sensations such as touch and pain to the spinal cord and from there to the brain, whilst autonomic nerves control the calibre of blood vessels, heart rate, gut contraction and other functions not under conscious control.

Generally, autonomic and pain fibres are blocked first and motor fibres last. This has several important consequences. For example, vasodilation and a drop in blood pressure may occur when the autonomic fibres are blocked and the patient may be aware of touch and yet feel no pain when surgery starts.

There are practical implications associated with these physiological phenomena.

• The patient should be well hydrated before the local anaesthetic is injected and should have an intravenous infusion in place so that further fluids or vasoconstrictors can be given if hypotension occurs.

• The site to be operated on should not be repeatedly touched and the patient asked "Can you feel this?" as this increases the patient's anxiety. Often some sensation of touch or movement remains and yet no pain is felt. It is better to pinch the skin gently either with artery forceps or fingers and ask if it is painful. If it is not then surgery can begin.

 
Anatomy.

The spinal cord usually ends at the level of L2 in adults and L3 in children. Dural puncture above these levels is associated with a slight risk of damaging the spinal cord and is best avoided. An important landmark to remember is that a line joining the top of the iliac crests is at L4 to L4/5

Remember the structures that the needle will pierce before reaching the CSF (fig 1.). The skin. It is wise to inject a small bleb of local anaesthetic into the skin before inserting the spinal needle. Subcutaneous fat. This, of course, is of variable thickness. Identifying the intervertebral spaces is far easier in thin patients.

The supraspinous ligament which joins the tips of the spinous processes together.

The interspinous ligament which is a thin flat band of ligament running between the spinous processes.

The ligamentum flavum is quite thick, up to about 1cm in the middle and is mostly composed of elastic tissue. It runs vertically from lamina to lamina. When the needle is within the ligaments it will feel gripped and a distinct "give" can often be felt as it passes through and into the epidural space.

The epidural space contains fat and blood vessels. If blood comes out of the spinal needle instead of CSF when the stylet is removed, it is likely that an epidural vein has been punctured. The needle should simply be advanced a little further.

The dura. After feeling a "give" as the needle passes through the ligamentum flavum, a similar sensation may be felt when the needle is advanced a short distance further and pierces the dural sac.

The subarachnoid space. This contains the spinal cord and nerve roots surrounded by CSF. An injection of local anaesthetic will mix with the CSF and rapidly block the nerve roots with which it comes in contact.

(Continued...)

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