Inability to site

It may not be possible to access the subarachnoid space with the needle. This may be because of inability to find a pathway through the bony 'window' formed by the vertebrae. In turn, this is most commonly attributable to the inability of the patient to flex sufficiently, inability of the operator to palpate the bony landmarks, or operator inexperience. Re-positioning the patient and/or redirecting the needle may help. Occasionally a 'dry tap' can occur, where the needle is in the subarachnoid space, but cerebrospinal fluid cannot be aspirated. However, successful block can still be achieved.

Pain on insertion

Superficial pain is most likely due to insufficient local infiltration. It is important to distinguish this from the lancinating pain in a discrete nerve root distribution which would suggest the needle has contacted a nerve root. If this occurs before drug injection, the needle should be withdrawn until the pain disappears completely. If this results in loss of cerebrospinal fluid flow, it will be necessary to reinsert in a different orientation. If it occurs during drug injection, this may indicate intraneural placement. Injection should be stopped immediately and the needle withdrawn. In either case, pain that persists raises the possibility of root injury.

Vasovagal episode

Some patients are susceptible to this complication, particularly if insertion has been prolonged, difficult or painful. This is one of the reasons for continuous monitoring during this time.

Hypotension

The commonest complication of spinal anaesthesia is hypotension, as a result of the interruption to sympathetic innervation to the lower body. Generally, the effect is proportional to the height of the block. It is generally more common at a younger age and in pregnancy.

Bradycardia

If the block reaches the T2-T4 dermatomes, the cardiac sympathetic efferent nerves become blocked and bradycardia can further reduce blood pressure. Patients can also have delayed vasovagal episodes.

High spinal

Occasionally a block may rise high enough to cause actual or imagined difficulty breathing and/or weakness in the upper limbs. Reassurance may be all that is required. Conversely, reasons to consider taking over airway control in this situation would include: inability to breathe adequately, loss of protective airway reflexes and patient distress.

Total spinal

It is rare for a single shot spinal to cause a high enough block to cause unconsciousness, but it can happen. Treatment is centred on airway control, ventilation and treatment of hypotension and bradycardia.