FRCA Notes

Pyloric Stenosis

This topic featured as a CRQ in 2023 (71% pass rate) but is also 'classic' Final FRCA SBA/MTF fodder.

Resources

  • Pyloric stenosis occurs due to hypertrophy of the smooth muscle of the pylorus, which forms part of the gastric outlet
  • Its incidence is 1.5/1000 live births
  • The precise aetiology in pyloric stenosis is unclear, with a number of hereditary and environmental factors contributing
  • Higher rates of concordance in monozygotic vs. dizygotic twins certainly points to some genetic component

Risk factors

Maternal Patient
Young age Males (4 - 5x)
Smoking First born child (OR 1.9)
Maternal family history Born in autumn or spring
Post-natal erythromycin therapy
There is an association with bottle feeding


  • Typically presents in the second, or third, month of life
  • 30% are age 7 - 28 days at the time of surgery

  • Symptoms
    • Projectile vomiting of non-bilious stomach contents
    • Failure to gain weight or weight loss e.g. crossing centiles on growth chart
    • Dehydration

  • Signs
    • Palpation of 'olive' sized 2-3cm mass in the abdomen, typically right side of epigastrium
    • Visible peristalsis (typically crossing abdomen from left to right)

  • The majority of patients are diagnosed (earlier) with ultrasound

Metabolic alkalosis

  • There is a loss of gastric secretions through vomiting and thus a loss of:
    • Hydrogen ions
    • Chloride ions
    • Some sodium and potassium
    • Water
  • Thus leading to the characteristic hypochloraemic, hypokalaemic metabolic alkalosis

  • The fluid loss, dehydration and reduction in plasma volume lead to ADH and renin secretion
  • This causes renal sodium and water retention
  • However, the effect on the Na/K exchange mechanism causes further renal potassium loss, exacerbating the existing hypokalaemia

Compensation

  • Initially, bicarbonate is excreted in the urine to compensate for the metabolic alkalosis
    • There is thus alkaline urine

  • Eventually the maintenance of plasma volume becomes the priority over acid/base:
    • Aldosterone acts on principal cells in the DCT to cause renal excretion of hydrogen ions in exchange for sodium and water
    • This leads to a paradoxical production of acidic urine despite a metabolic alkalosis

Respiratory effects

  • Medullary chemoreceptors in the brain stem sense CSF H+ concentrations
  • I.e. an increased PaCO2 causes an increased CSF H+ concentration and thus stimulates ventilation
  • Conversely, metabolic alkalosis can cause respiratory depression and apnoea, which is what happens in pyloric stenosis

  • There is an increased risk of post-operative apnoea, owing to the above effect and the CNS-depressant effect of GA
    • Pre-term infants of <60weeks postmenstrual age are at even higher risk

  • In order to minimise the risk of post-operative apnoea, the alkalosis must be corrected before surgery
    • However, equilibration of plasma pH and CSF pH takes several hours
    • Even if plasma pH has normalised, it is possible there is still a CSF alkalosis
    • Therefore all infants should be monitored for apnoea post-pyloromyotomy

Correcting dehydration

  • Keep NBM
  • Calculate the degree of dehydration and correct slowly
  • E.g. with a fluid such as 0.45% NaCl + 5% dextrose + 10mmol KCl/500ml
  • Rate: 150ml/kg/24hrs until the alkalosis has been corrected; 100ml/kg/24hrs thereafter

Targets

Variable Target range
pH 7.35 - 7.45
Chloride 95 - 112mmol/L
Potassium 3.5 - 5.5mmol/L
Base excess -4 to +2.5mmol/L

Surgery

  • Pyloric stenosis is a medical, not surgical, emergency; patients should only proceed to surgery once their dehydration, alkalosis and electrolyte abnormalities have been corrected
  • Both open (curved circum-umbilical incision) and laparoscopic (~25%) pyloromyotomy techniques are described

  • The laparoscopic technique is associated with:
    • Less PONV
    • Less post-operative pain
    • Earlier return to full feeds
    • Shorter duration of stay
    • The possibility of inadequate myotomy and need for re-operation

Perioperative management of the child undergoing pyloromyotomy for pyloric stenosis


  • There is a risk of pulmonary aspiration due to gastric outlet obstruction
    • The stomach may contain significant volumes of gastric acid secretions
    • The risk is highest during laryngoscopy due to airway stimulation

  • Even though the patient may already have an NG tube in situ, maintain a high index of suspicion that the tube may be blocked or malpositioned
    • Should be inserted to >20cm depth of insertion
    • Check fluid balance chart for evidence of regular aspirates
    • Suction the NG tube to check it isn't blocked
    • If in doubt, remove and re-site

  • If not present, an NG/OG tube is required to empty the stomach prior to induction of anaesthesia
  • There is some evidence to suggest that earlier NG tube insertion (e.g. at diagnosis rather than immediately before anaesthesia) reduces risk of vomiting at induction

  • The stomach should be emptied via said gastric tube using 'four quadrant aspiration'
    • The infant is rotated: from supine, to left lateral, to prone, to right lateral
    • Aspirate the NG tube in each position
  • Ultrasound assessment of gastric contents can be used to provide a qualitative assessment of stomach contents

  • The risk of aspiration can be further reduced by:
    • Smooth induction
    • Avoidance of hypoxia
    • Only instrumenting the airway with adequate depth of anaesthesia and complete neuromuscular block

Induction

  • Patients with pyloric stenosis have a higher incidence of difficult airways compared to a general cohort (BJA, 2022)
  • The choice of induction technique is, naturally, controversial; both inhalational and intravenous techniques are described

  • Modified RSI technique
    • Avoid cricoid pressure
      • Cricoid ring more difficult to identify
      • May distort the more compressible infant airway
      • May make intubation more difficult
    • Gentle BVM ventilation after induction may be required to prevent hypoxia and bradycardia, which arises due to:
      • Reduced FRC from more elastic chest wall and diaphragmatic splinting by relatively large abdomen
      • Greater oxygen consumption vs. older children
    • Avoid vigorous BVM ventilation as risk inflating the stomach and increasing risk of aspiration
    • The effect of rocuronium is prolonged in neonates compared with other age groups so use a lower (0.3 - 0.7mg/kg) dose or atracurium (0.5mg/kg)

Maintenance

  • Maintenance with sevoflurane or desflurane
    • Nitrous oxide is avoided because it causes expansion of bowel gas
    • Isoflurane is associated with post-operative apnoea and therefore avoided

  • Temperature control is important, with active warming and raised ambient temperature
  • Glucose homeostasis with monitoring and glucose-containing fluids

Laparoscopy

  • Be vigilant during laparoscopic cases during abdominal insufflation
    • Keep pressure <10mmHg
    • May need to adjust ventilatory requirements as CO2 absorption can lead to hypercapnoea

  • Towards the end of the case may need to insufflate a volume of air into the stomach through the NG tube, to:
    • Test the mucosa has remained intact
    • Check passage of air into duodenum is seen, suggesting pyloric division is satisfactory

Intra-operative analgesia

  • IV paracetamol - care with dosing (see paediatric pain section)
  • Local anaesthetic - surgical infiltration, TAP or rectus sheath blocks
  • Consider rectal NSAID
  • Dealer's choice of opioid e.g. fentanyl

Analgesia

  • Post-operative pain is typically not severe
  • Paracetamol ± NSAID, in addition to the LA techniques used intra-operatively, is often sufficient
  • European guidelines recommend fentanyl, nalbuphine or tramadol for breakthrough pain (ESPA, 2018)

Monitoring

  • Aspirate NG and remove prior to end of surgery
  • Extubate in left lateral position
  • Recover in post-operative area until fully awake
  • Continuous pulse oximetry and apnoea monitoring for 12hrs for all children regardless of age
    • If no apnoeas, discontinue after 12hrs / if apnoeas, continue for a further 12hrs after the apnoea

  • Post-operative feeding practice varies
    • Some surgeons advocate 2hrs NBM period
    • Trend towards earlier feeding; typically water or electrolyte solution first, then milk