During labour, assessment of the level of the block is an important monitor of the spread of local anaesthetic solution. A "high spinal" may occur as a result of inadvertent subarachnoid or subdural spread of solution (Chapter 15), or excessively high epidural spread. The former situations may result from epidural catheter migration or catheter malposition and may occur with both intermittent dose or infusion techniques. Conversely, the mechanisms of an apparently unsatisfactory block may be better evaluated. For instance, inappropriate doses of local anaesthetic versus malpositioned catheter. Ensuring that an appropriate sensory level has been achieved is an important prelude to surgery for caesarean section.

The three neural modalities affected by neuraxial blockade (sensory, motor, sympathetic) may be assessed individually (Chapter 10).

Methods of assessing the sensory level of the block include:
1. analgesia to pin prick,
2. loss of temperature sensation to ice, and
3. anaesthesia to soft touch.

Methods of measurement of motor blockade include:
1. the Bromage scoring method, (Table 8.1) and
2. dynamometry.

The degree of sympathetic blockade may be assessed by observing changes in:
1. haemodynamic status, and
2. temperature regulation.

Assessing the adequacy of the block for Caesarean section:
Perception of pain to pin prick, temperature change, or paraesthesiae (loss of sensation to soft touch) are all methods used for determining the upper sensory level of a neuraxial block.

A zone of differential blockade is observed between analgesia to pin prick and paraesthesiae - the former typically several segments more cephalad. Temperature perception extends one or two segments further cephalad than pin prick and reflects sympathetic blockade (1). There is no constant relationship between the width of this differential zone for any of these modalities (1, 2).

A recent study has concluded that the upper sensory level which is required for satisfactory conditions during caesarean section can be defined accurately if a standardised approach to the measurement of this level is used (3). In this study, an upper level of anaesthesia to soft touch at or above the T5 dermatome was found to be the most reliable index of a satisfactory block.

In contrast, levels of analgesia to pin prick which were subsequently associated with pain during the operation, varied from T1-T9. Interestingly, 25% of patients who experienced pain had a sensory level to pinprick of T3 or T4 which is commonly recommended as the sensory level which will provide adequate conditions for caesarean section. A level of anaesthesia to soft touch at or above the T5 dermatome was never associated with pain. This applied to both epidural and spinal blockade. For levels of anaesthesia below T4, pain was more likely to occur with an epidural than a spinal block ( 46% vs 22% ).

References:
1. Brull SJ, Greene NM. Zones of differential sensory block during extradural anaesthesia Br. J. Anaesth 1991; 66: 651-55.

2. Rocco AG, Raymond SA, Murray E, Dhingra U, Freiberger D. Differential spread of blockade of touch, cold, and pinprick during spinal anesthesia . Anesth Analg 1985; 64: 917-23.

3. Russell IF. Levels of anaesthesia and intraoperative pain at caesarean section under regional block . International Journal of Obstetric Anesthesia. 1995; 4: 71-77.

4. Pedersen H, Santos AC, Steinberg ES, Schapiro HM, Harmon TW, Finster M. Incidence of visceral pain during cesarean section: The effect of varying doses of spinal bupivacaine Anesth Analg 1989; 69: 46-49.

5. Alahuhta S, Kangas - Saarela T, Hollmén AI, Edström HH. Visceral pain during cesarean section under spinal and epidural anaesthesia with bupivacaine Acta Anaesthesiol Scand. 1990; 34: 95-98