Year : 2020 | Volume
: 25 | Issue : 6 | Page : 419--420
Use of train of four as a guide to neurological recovery: What is the rationale?
N Kalita, Sandip Chatterjee
Park Clinic, Kolkata, West Bengal, India
Dr. Sandip Chatterjee
Park Clinic, Kolkata, West Bengal
|How to cite this article:|
Kalita N, Chatterjee S. Use of train of four as a guide to neurological recovery: What is the rationale?.J Indian Assoc Pediatr Surg 2020;25:419-420
|How to cite this URL:|
Kalita N, Chatterjee S. Use of train of four as a guide to neurological recovery: What is the rationale?. J Indian Assoc Pediatr Surg [serial online] 2020 [cited 2021 Jul 27 ];25:419-420
Available from: https://www.jiaps.com/text.asp?2020/25/6/419/299208
The letter highlights that the use of train of four (TOF) as a predictor of the neurological outcome is rather original and interesting. However, a few basic facts need to be considered before drawing the conclusions which the authors have hinted at. In the above study, the use of succinylcholine (SCh) that is a depolarizing neuromuscular blocker is mentioned. SCh is structurally two molecules of acetylcholine bound together and depolarizes (opens) the ion channels. Because SCh is not hydrolyzed by acetylcholinesterase, the channel remains open for a longer period of time than would be produced by acetylcholine, resulting in a depolarizing block. Sustained depolarization prevents propagation of an action potential. SCh has a short duration of action due to its rapid hydrolysis by plasma cholinesterase. Recovery from SCh-induced blockade occurs as SCh diffuses away from the neuromuscular junction, down a concentration gradient as the plasma concentration decreases.
The TOF pattern of twitch stimulation involves stimulation of the peripheral nerve with the purpose to determine the degree of muscle relaxation by interpretation of muscle response. For upper limbs, the ulnar nerve, and for lower limb monitoring, the posterior tibial nerve are generally used. Stimulus is given with a frequency of 2 Hz, that is, four stimuli, each separated by 0.5 s. The TOF is then repeated every 10 s (train frequency of 0.1 Hz). It enables the observer to compare first twitch of the TOF (T1) to control (T0), and it also enables comparison of T4 to T1. This is known as the TOF ratio. When a nondepolarizing agent is given, a typical pattern is observed. There is a reduction in the amplitude of the evoked responses, with T4 affected first, then T3, followed by T2, and finally T1. This decrement in twitch height is known as fade. One of the most useful clinical applications of the TOF ratio is in monitoring recovery from the neuromuscular block. The TOF pattern is less useful in monitoring depolarizing neuromuscular block. During onset of depolarizing block, each of the four twitches is decreased equally in size, that is, there is no fade. This is also observed during recovery.
In the case described, baseline TOF response was obtained after administering induction dose of propofol and fentanyl. There are studies , that show that propofol administered even in small doses during recovery from neuromuscular blocking drugs resulted in marked decreases of both T1 and the TOF ratio. The baseline response of 50% TOF can thus be attributed to propofol administration and not due to the pathology of compressive myelopathy.
Indeed, intraoperative neurophysiological monitoring is the standard monitoring technique in spinal surgery. The motor evoked potential (MEP) is the most recent addition to the somatosensory evoked potential (SSEP) and electromyography in surgeries where the motor tracts are at risk. The MEP is produced by transcranial multipulse electrical stimulation of the motor cortex using scalp electrodes. MEP is relevant in this case since the child presented with sudden-onset weakness of the bilateral lower limbs, which was progressive in nature. MEP monitoring is favored because intraoperative changes correlate with postoperative motor outcome. This is due to the monitoring of the corticospinal tract and because the motor pathways are more sensitive to ischemic insults caused by stretch, compression, vascular disruption, or direct trauma. MEP is more sensitive than the SSEP in this regard.
The gradual recovery in the TOF ratio toward the completion of procedure in this case can be the normal recovery from the depolarizing block itself. As propofol and fentanyl infusions were used intraoperatively, the TOF could have been low, but the serial values have not been mentioned. Moreover, complete recovery occurred as the infusions were titrated toward the end of surgery and stopped as it is the standard method of total intravenous anesthesia.
However, the idea propounded by the authors highlights their concern in predicting neurological recovery and may pave the way for their routine use of IONM (Intra Operative Neuro Monitoring) in the future.
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Conflicts of interest
There are no conflicts of interest.
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