R. Muralidharan2, MD
A. Dutta3 MD
1. Additional Professor
2, 3 Senior Resident
Department of Anaesthesia and Intensive
Care
Postgraduate Institute of Medical Education and Research,
Chandigarh-160012, India
Author for Correspondence:
Dr. S.K. Malhotra , M.D.
Additional Professor
Department of Anaesthesia and Intensive Care
Postgraduate Institute of Medical Education and Research
Chandigarh - 160012, INDIA
Myelomeningocele (MMC) repair is a common paediatric surgical procedure in our country. The standard protocol involves a preoperative cranial CT scan to evaluate the ventricles, insertion of ventriculo-peritoneal (VP) shunt, if indicated, before undertaking the repair of MMC. If the ventricles are not dilated the said surgical repair is undertaken straightaway. We report a case of a large lumbar MMC without any associated hydrocephalus in a child who failed to emerge from anaesthesia after an otherwise uneventful procedure. A typical sequence of pupillary changes was the only positive finding in the immediate postoperative period indicating possible acute transtentorial herniation ('coning'). He required prolonged postoperative ventilation and died after 2 weeks. The possible mechanisms involved and various clinical aspects are discussed.
Keywords: Myelomeningocele repair, Hydrocephalus, General Anaesthesia, Delayed
awakening, Pupillary changes
MMC is mostly associated with hydrocephalus or Arnold-Chiari malformation (ACM)
i.e. caudal migration of brainstem, which may be present at birth or manifests
a few days after surgery. ACM type II may also present postoperatively in the
form of tense and bulging fontanelles with features of lethargy, irritability,
vomiting or even respiratory depression. MMC associated with hydrocephalus requires
insertion of VP shunt prior to or at the time of surgical repair (Martz DG et
al, 1990). We report a case of acute transtentorial herniation ('coning'), a
rare and fatal perioperative complication of MMC repair in a child where preoperative
computerized tomography (CT) scan showed absence of hydrocephalus.
A 18 month-old male child weighing 10 kg. was listed for surgical repair of
lumbar MMC. A routine CT scan did not show any associated hydrocephalus. On
preoperative visit, the child was conscious, slightly irritable and had weakness
of lower limbs and incontinence of urine and faeces. No history of any seizure
or neck stiffness was detected. The clinical examination of the cardiovascular
and the respiratory systems did not reveal any abnormality. An echocardiography
ruled out the presence of any congenital cardiac lesion. Haematological and
biochemical investigations were within normal limits. After premedication with
syrup diazepam 0.1 mg/kg, the child was shifted to OR and monitoring of ECG,
NIBP and SpO2 started. After halothane mask induction, I.V. access was secured
followed by administration of atropine 0.2 mg and morphine 1 mg. Endotracheal
intubation was facilitated with succinylcholine 20 mg and anaesthesia was maintained
with 66% nitrous oxide in oxygen and halothane 0.5-1 percent. The child was
positioned prone and the ventilation was controlled with atracurium using modified
Jackson-Rees circuit and EtCO2 monitored. Intraoperative fluid administration
included isolyte paediatric solution.
The procedure lasted 90 minutes and was uneventful. The patient was turned to
supine position. Respiratory efforts or response to oral suction were not present,
and pupils were constricted and non-reacting. Naloxone, 25 µg in divided
dosage was administered for suspected morphine sensitivity, but pupils as well
as, respiratory efforts showed no change. Also, there was no change in the level
of consciousness or motor activity. After 20 minutes of ventilation some limb
movements in response to painful stimuli appeared. Hence, the reversal of neuromuscular
blockade with atropine 20 µg/kg and neostigmine 50µg/kg was attempted
without any success. Hypothermia, hypoglycemia and hypocalcaemia were ruled
out.
After 90 minutes, pupils suddenly got fully dilated and continued non-reacting.
Acute rise in intracranial pressure (ICP) was suspected and 20% mannitol i.v.
0.5 gm/kg was administered, but without any pupillary change. Arterial blood
gas analysis was suggestive of mild metabolic acidosis, which did not need correction.
Patient was shifted to the Intensive Care Unit for elective ventilation. A CT
scan repeated after 24 hours showed diffuse cerebral oedema, small capacity
ventricles and no skeletal malformation of the base of skull, foramen magnum
or cervical vertebrae. Fundus examination on second postoperative day revealed
no papilloedema and the eye movements were lost on cold caloric test. The child
remained comatose and developed hypotension over the next 24 hours requiring
inotropic support. The situation continued for 2 weeks and eventually resulted
in cardiac arrest and the resuscitation was not successful.
Delayed awakening from anaesthesia may be due to various causes which include
hypoxia, hypothermia, hypoglycemia, metabolic and endocrine abnormalities, paradoxical
air embolism into cerebral circulation, intravenous and volatile anaesthetics,
opioids or neuromuscular blocking agents (Denlinger JK, 1983; Clayton DG et
al, 1985; Delisle S et al, 1982; Leila GW, 1996; Miller RD et al, 1975; Mcleod
K et al, 1976). In addition, a patient may fail to emerge from anaesthesia if
pre-existing occult or overt intracranial hypertension leads to herniation of
cerebral cortex across the tentorium cerebelli or the brainstem though the foramen
magnum.
Overdose of anaesthetic agents in our case was ruled out to be the cause as
the accidental administration or drug overdosage was cross-checked. Hypoxia
during the procedure is unlikely as the ETT position and bilateral air entry
was checked before and after turning the patient into prone position. Continuous
oxygen saturation monitoring, too excluded any desaturation.
Hypothermia and hypoglycaemia were also excluded as possible causes. Metabolic
acidosis detected was mild and does not explain the catastrophe. Although, venous
air embolism could be a possibility, but is usually associated with decrease
in EtCO2 and brady- arrhythmias, not observed in this case.
Acute transtentorial herniation or 'coning' is a possible explanation as it
occurs following a sudden change in the pressure gradient across the tentorium.
An increase in the pressure in the supratentorial or a decrease in the infratentorial
compartment could lead to this condition. Tentorial herniation is of two types,
central and uncal. The latter refers to the impaction of the uncinate gyrus
into the anterior portion of the tentorial opening initially causing oculomotor
nerve irritation resulting in pupillary constriction, followed by compression
of the oculomotor nerve resulting in pupillary dilatation, a typical finding
in our case. Subsequently, coma can develop due to compression of the midbrain
reticular formation by the para-hippocampal gyrus. Herniation of the intracranial
contents through the foramen magnum can clinically manifest as tachycardia and
hypertension (pressure over trigeminal or any sensory nerve nucleus), reflex
bradycardia, sudden cessation of spontaneous respiratory efforts (pressure over
respiratory centre), pupillary constriction (oculomotor nerve irritation), pupillary
dilatation (oculomotor nerve paralysis) and papilloedema (defective drainage
of CSF) (Ropper AH et al, 1998).
Central herniation of the brainstem through the tentorial hiatus reflects a
pressure gradient between a supratentorial mass containing compartment and the
posterior fossa. Such a pressure gradient is dramatically and dangerously increased
by an ill-advised lumbar puncture. Central herniation causes progressive brainstem
compression with sequential loss of pupillary responses and eye movements. Patients
in deep metabolic coma, conversely, may show total loss of eye movements, but
retain pupillary reflexes. With central herniation, pupillary reflexes are typically
lost and eye movements become abnormal or absent (Harrison MJG, 1996). Central
transtentorial herniation denotes symmetric downward movement of the upper diencephlon
(thalamic region) through the tentorial opening in the midline and is heralded
by miotic pupils (Ropper AH et al, 1998).
ACM, a common associated anomaly of MMC might be present preoperatively without
clinical features of raised ICP (Martz DG et al, 1990). Postoperatively, sudden
changes in transtentorial gradient across foramen magnum may lead to acute herniation
or 'coning' and if gradual, may lead on to postoperative progressive hydrocephalus
as observed in the present case. Moreover, there was no evidence of raised ICP
in the preoperative period. The other common causes of delayed awakening from
anaesthesia were ruled out. The presence of constricted pupils on turning the
patient supine followed by a sudden spontaneous dilatation 90 minutes later
was the only positive observation. Moreover, the eye movements were completely
lost by 24 hours. This is consistent with transtentorial uncal herniation which
might have been precipitated by the sudden fall in infratentorial pressure following
sudden release of CSF from the tense sac of MMC.
Postoperative CT scan may not be confirmative because a coning event may be
transient with long lasting effects. Moreover, a CT scan may not pick up even
a minimal displacement of brainstem, as is possible with Magnetic Resonance
Imaging (MRI).
To conclude, a preoperative CT scan/MRI and fundus examination should be
considered in all the patients with MMC to exclude elevated ICP, even in the
absence of clinical features. Control of ICP through a shunt or drug therapy
should be achieved before undertaking the excision of MMC. Measures to prevent
the increase in the transtentorial pressure gradient should be considered, such
as, selection of appropriate anaesthetic agents and techniques, avoidance of
sudden release of pressure from the sac and adhering to a head-low position.
Coning, as a possible cause of delayed awakening following anaesthesia in susceptible
children should always be anticipated so that immediate measures to decrease
the ICP may be timely initiated.
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