Methaemoglobinaemia is a rare disorder
where haemoglobin is oxidized to methaemoglobin, which cannot
carry oxygen efficiently. In pregnancy, unrecognized
methaemoglobinaemia may pose risks to both mother and foetus.
Congenital methaemoglobinaemia poses unique anaesthetic
challenges, particularly in pregnant patients undergoing
surgical interventions. We present the successful anaesthetic
management of a young female at 32 weeks gestation who was
diagnosed intraoperatively with congenital methaemoglobinaemia
during an emergency lower segment caesarean section for
preeclampsia with placental abruption.
Introduction
Methaemoglobinaemia is a rare haematological condition
characterized by the presence of methaemoglobin (MetHb), an
oxidized form of haemoglobin in which iron exists in the
ferric (Fe
3+) state rather than the normal ferrous
(Fe
2+) state. This altered form of haemoglobin is
unable to effectively bind and transport oxygen, leading to
impaired oxygen delivery despite normal arterial oxygen
tension and haemoglobin concentration. Clinically, patients
may present with cyanosis that does not improve with oxygen
supplementation, and in severe cases, symptoms of tissue
hypoxia such as headache, fatigue, dyspnoea, and altered
mental status may occur [1,2].
Methaemoglobinaemia may be acquired, most commonly due to
exposure to oxidizing agents such as nitrates, local
anaesthetics (e.g., benzocaine, prilocaine), sulfonamides, or
certain antibiotics. Alternatively, it may be congenital,
resulting from inherited defects in enzymes responsible for
reducing methaemoglobin to haemoglobin, most notably
cytochrome b5 reductase (also known as NADH-dependent
methaemoglobin reductase) [3,4]. Congenital
methaemoglobinaemia is further classified into Type I (limited
to erythrocytes) and Type II (generalized, with neurological
involvement), with the former typically presenting as lifelong
cyanosis without systemic symptoms [5].
During pregnancy, physiological changes in plasma volume,
oxygen consumption, and respiratory function increase maternal
and foetal vulnerability to hypoxia. Additionally, foetal
haemoglobin is more susceptible to oxidative stress, although
the foetal NADH-dependent reductase system is generally
sufficient to maintain MetHb at safe levels unless a
congenital enzyme deficiency is present [6,7]. In such cases,
both maternal and foetal outcomes may be jeopardized if the
condition goes unrecognized.
Anaesthetic management of patients with methaemoglobinaemia
presents specific challenges. Pulse oximetry becomes
unreliable due to the unique light absorption spectrum of
MetHb, typically resulting in SpO₂ readings around 85%
regardless of the actual oxygenation status. Arterial blood
gas (ABG) analysis with co-oximetry is required for accurate
measurement of MetHb and oxygenation parameters [8]. Avoidance
of oxidative stress, careful drug selection, and appropriate
oxygen delivery strategies are vital. Regional anaesthesia,
when feasible, is often preferred over general anaesthesia in
stable patients to minimize oxidative drug exposure and airway
manipulation.
This report presents the anaesthetic management of a
parturient with undiagnosed congenital methaemoglobinemia who
underwent emergency lower segment caesarean section (LSCS)
under spinal anaesthesia. The case highlights the importance
of clinical vigilance, timely diagnosis, and safe anaesthetic
planning in patients with this rare condition.
Case Description
A young female, at 32 weeks of gestation, presented with
complaints of mild vaginal bleeding and abdominal pain of
one-day duration. She had no significant past medical history
and was only on routine antenatal iron and calcium
supplements. Antenatal course until then was uneventful. Her
previous pregnancy was uneventful.
On evaluation in the labour room, her BP was elevated (160/100
mmHg), and oxygen saturation (SpO₂) was noted to be 88% on
room air. Notably, she showed no clinical signs of respiratory
distress. She was diagnosed with preeclampsia with suspected
placental abruption and was posted for emergency LSCS.
She received a loading dose of 4 grams of magnesium sulphate
intravenously and was shifted to the Operating Room with
oxygen via face mask at 6 litres per minute. On arrival, her
vitals were stable: HR 88 bpm, RR 20/min, BP 140/90 mmHg, and
SpO₂ 91% on 6 L/min oxygen. Clinical examination revealed
peripheral cyanosis involving fingers and toes, with warm
peripheries and bilateral pedal oedema. Chest auscultation
revealed normal vesicular breath sounds, with no added sounds.
Administration of 100% oxygen marginally improved SpO₂ to 93%.
Foetal Heart Rate was 140 beats per minute.
Upon further history taking, a sibling was reported to have
similar finger discoloration, raising suspicion of congenital
methaemoglobinaemia.
Anaesthetic Management
In this patient, multiple factors influenced the choice of
anaesthetic technique, including her pregnancy, the urgency of
the Caesarean section, and the suspicion of
methaemoglobinaemia based on unexplained cyanosis and low
pulse oximetry readings unresponsive to supplemental oxygen.
Preoperative Assessment and Planning
On arrival in the operating room, the patient's vital signs
were stable, but her peripheral cyanosis and SpO₂ of 91% on
supplemental oxygen were concerning. There were no signs of
respiratory distress, and auscultation of the lungs was
normal, which made intrinsic pulmonary pathology less likely.
The differential diagnosis at this stage included
methaemoglobinaemia and dyshaemoglobinaemias, with further
suspicion supported by a family history of similar bluish
discoloration of the fingers in a sibling.
Given the non-progressive desaturation and preserved
oxygenation on ABG, the team prioritized a regional technique
over general anaesthesia to avoid the use of oxidizing agents
and to minimize foetal drug exposure. A high-risk consent was
obtained, clearly explaining the possibility of rare
congenital blood disorders, risk of intra-operative blood loss
and risk of further worsening with administration of
anaesthetic agents.
Intraoperative Course
The patient was coloaded with 500 mL of Ringer's lactate to
reduce the risk of post-spinal hypotension. Under strict
asepsis, spinal anaesthesia was administered at the L3-L4
interspace using 2.2 mL of 0.5% hyperbaric bupivacaine without
adjuvants. Of note, the blood withdrawn during needle
placement appeared chocolate-brown, which was a critical
visual clue supporting the diagnosis of methaemoglobinaemia.
Sensory block to T4 was achieved within 5 minutes. The patient
remained hemodynamically stable throughout the surgery, with a
mean arterial pressure maintained above 70 mmHg. Prophylactic
phenylephrine boluses (50-100 mcg IV) were administered
intermittently to counteract mild hypotension.
SpO₂ levels remained between 91-93% throughout the procedure
despite administration of 100% oxygen via facemask.
Capnography and electrocardiography remained normal. Estimated
blood loss was approximately 800 mL, and urine output was
adequate. No sedatives, opioids, or vasodilators were
administered intraoperatively to avoid exacerbation of
methaemoglobinaemia or foetal depression.
A male neonate weighing 1.4 kg was delivered with Apgar scores
of 8 and 9 at 1 and 5 minutes, respectively. No resuscitation
was required, and the baby was transferred to the NICU for
preterm care. No signs of neonatal cyanosis were noted.
Postoperative Management
Postoperatively, the patient was monitored in the intensive
care unit. Pain management was achieved with intravenous
paracetamol (1g every 6 hours). She remained hemodynamically
stable with no evidence of neurologic deficits, worsening
cyanosis, or lactic acidosis.
A confirmatory arterial blood gas analysis with co-oximetry
revealed a methaemoglobin level of 34%, consistent with
moderate methaemoglobinaemia. As the patient remained
asymptomatic with no hypoxia or organ dysfunction,
conservative management was chosen. She was started on oral
vitamin C (ascorbic acid 500 mg twice daily) and riboflavin
supplementation to promote endogenous reduction of
methaemoglobin.
Genetic counselling and testing were advised for confirmation
of congenital cytochrome b5 reductase deficiency, the most
common cause of hereditary methaemoglobinaemia.
Follow-up at two weeks showed a decreasing trend in
methaemoglobin levels upto 13% and continued clinical
stability.
Discussion
Methaemoglobinaemia is a rare but potentially serious
condition resulting from the oxidation of haemoglobin's iron
molecule from Fe
2+ to Fe
3+, forming
methaemoglobin (MetHb), which cannot bind oxygen. This impairs
oxygen delivery despite normal arterial oxygen tension,
resulting in a functional anaemia. Clinically, it presents
with cyanosis, low SpO₂ on pulse oximetry, and a lack of
response to oxygen therapy. When severe, it can lead to
hypoxic symptoms such as headache, fatigue, dyspnoea,
arrhythmias, and seizures [1,9].
The diagnosis is often delayed because standard pulse
oximeters cannot accurately detect methaemoglobin. They
typically show saturation readings around 85%, regardless of
the true oxygenation status. In our patient, cyanosis was
limited to extremities, and she was otherwise asymptomatic
with no signs of respiratory distress, suggesting a
functional, rather than hypoxic, cause of desaturation [10].
Definitive diagnosis requires co-oximetry, which uses multiple
light wavelengths to directly measure methaemoglobin levels.
This was performed postoperatively in our case, confirming the
diagnosis [11].
Anaesthetic implications are significant. Many common
anaesthetic agents, such as benzocaine, prilocaine, nitrites,
and nitroprusside, can induce or worsen methaemoglobinaemia
[9]. General anaesthesia involves the risk of such drug
exposure and may also impair oxygenation due to airway
manipulation and sedative effects. In contrast, regional
anaesthesia minimizes these risks and preserves patient
consciousness for early detection of symptoms [12].
Our patient underwent spinal anaesthesia with bupivacaine,
which is considered safe. Spinal anaesthesia preserves
spontaneous ventilation and maintains stable cardiovascular
status. It gives rapid onset and adequate anaesthesia suitable
for emergency Caesarean section. It allowed us to avoid
polypharmacy. No sedatives or other oxidizing agents were
administered. Intraoperative monitoring and oxygen
supplementation ensured maternal and foetal stability
throughout the procedure.
Treatment of methaemoglobinaemia depends on severity. In mild
or asymptomatic cases (MetHb <20%), conservative management
is often sufficient. In symptomatic or severe cases, methylene
blue is the treatment of choice, acting as an electron donor
to reduce MetHb back to functional haemoglobin via the
NADPH-methaemoglobin reductase pathway. However, it is
contraindicated in patients with G6PD deficiency [13,14].
In pregnancy, foetal MetHb levels should also be considered,
though foetal reductase systems are usually sufficient unless
there is a congenital enzyme deficiency. Our patient's neonate
was clinically stable at birth and did not require any
intervention.
This case reinforces the importance of clinical suspicion in
the presence of low SpO₂ with normal respiratory examination
and highlights the role of regional anaesthesia in managing
such cases safely. It also emphasizes the importance of
postoperative monitoring and appropriate use of diagnostic
tools like co-oximetry to confirm the diagnosis.
Conclusion
Methaemoglobinaemia, though rare, should be considered in
pregnant patients presenting with unexplained low oxygen
saturation, especially when accompanied by peripheral cyanosis
and minimal respiratory symptoms. Early recognition, avoidance
of oxidizing agents, and safe use of regional anaesthesia are
key to successful management. Co-oximetry is essential for
diagnosis, and most mild cases can be managed conservatively.
With appropriate perioperative planning, excellent maternal
and foetal outcomes can be achieved.
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