The use of mandibular nerve block to predict safe anaesthetic induction in patients with
acute trismus
A.M.B. Heard,1 R.J. Green,2 D.A. Lacquiere,3 and P. Sillifant.4
1 Department of Anaesthesia and Pain Medicine, and 4 Department of Maxillofacial Surgery,
Royal Perth Hospital, Perth, Australia
2 Department of Anaesthesia, Royal Bournemouth NHS Foundation Trust, Bournemouth, UK
3 Department of Anaesthesia, Taunton and Somerset NHS Foundation Trust, Taunton, UK
Acute trismus can be caused by pain, muscle spasm, swelling or mechanical obstruction.
Trismus caused by either pain or muscle spasm can be reversed with induction of general
anaesthesia whereas trismus caused by swelling or other mechanical obstruction cannot.
However, currently it is not possible to predict which will occur during the preoperative
airway assessment. In this pilot study we prospectively evaluated mandibular nerve block as
a preoperative tool to identify patients with reversible causes, namely pain or spasm, in order
to allow safe anaesthetic induction.
Methods. With Local Research Ethics Committee approval, six patients with unilateral
fractured mandibles and trismus received a mandibular nerve block prior to induction of
general anaesthesia. Inter-incisor distance and pain scores were measured before, and 30
minutes after the blocks. Inter-incisor distance was measured again after induction of general
anaesthesia. Data were evaluated using the Wilcoxon Signed Rank test.
Results. There was an increase in maximal inter-incisor gap after the blocks (median
distance in mm (range): pre-block 16.5 (14-30); post-block 34 (32-35); p = 0.027), and no
further improvement after induction of general anaesthesia (post-GA 37 (30-40) p = 0.276
compared with post-block). There was an improvement in pain scores (p = 0.027), and no
side-effects were detected.
Discussion. Preoperative mandibular nerve blockade appears to reverse trismus caused by
pain and muscle spasm. Reversal of trismus would allow the anaesthetist to carry out a
standard airway assessment, and then confidently perform an intravenous induction and direct
laryngoscopy if no other predictors of airway difficulty are revealed. Conversely, if there is
no reversal of trismus our findings suggest that the limited mouth opening is not due
primarily to pain or spasm, but may be due to swelling or other mechanical obstruction. In
this case mouth opening is unlikely to increase after the onset of general anaesthesia, and an
awake fibreoptic intubation is genuinely indicated.
Reference for full paper:
“The use of mandibular nerve block to predict safe anaesthetic induction in patients with
acute trismus”
A. M. B. Heard, R. Green, D. A. Lacquiere, P. Sillifant. Anaesthesia 2009; 64(11):1196-98