Oxycodone and Tramadol Pharmacology

Quick Answer

Oxycodone and tramadol represent two distinct approaches to opioid analgesia with fundamentally different pharmacological profiles essential for ANZCA Primary examination. Oxycodone is a semi-synthetic opioid derived from thebaine (not morphine), acting as a full mu-receptor agonist with additional kappa-receptor activity, characterised by high oral bioavailability (60-87%) compared to morphine (20-40%), hepatic metabolism via CYP3A4 (major) and CYP2D6 (minor) to noroxycodone (inactive) and oxymorphone (active but clinically insignificant quantities), and elimination half-life of 3-5 hours. Tramadol is a synthetic opioid with a unique dual mechanism: weak mu-receptor agonism combined with serotonin-norepinephrine reuptake inhibition (SNRI activity), requiring CYP2D6 metabolic activation to its active metabolite O-desmethyltramadol (M1), which has 200-300× greater mu-affinity than the parent compound. Key clinical distinctions include tramadol's seizure risk (dose-dependent, enhanced by SSRIs/SNRIs), serotonin syndrome potential, and controversial "ceiling effect" for respiratory depression, versus oxycodone's predictable dose-response relationship, established equianalgesic dosing (oral oxycodone 20 mg = oral morphine 30 mg), and availability as controlled-release (OxyContin) and immediate-release formulations including parenteral preparations in some countries. Both drugs demonstrate significant CYP2D6 pharmacogenetic variability affecting clinical response. [1-8]

Oxycodone Pharmacology

Chemical Structure and Classification

Oxycodone (4,5α-epoxy-14-hydroxy-3-methoxy-17-methylmorphinan-6-one) is a semi-synthetic opioid derived from thebaine, a minor alkaloid constituent of opium poppy (Papaver somniferum). This derivation from thebaine rather than morphine is clinically significant—it accounts for oxycodone's distinct pharmacological properties including higher oral bioavailability and different metabolic pathway. [9-12]

Molecular characteristics:

• Molecular formula: C18H21NO4
• Molecular weight: 315.4 Da
• pKa: 8.5
• Percentage unionised at pH 7.4: ~10%
• Log P (octanol/water partition coefficient): 0.7 (low lipophilicity, similar to morphine)

Structural relationship to morphine:

The 3-methoxy group (rather than free phenolic hydroxyl) contributes to oxycodone's improved oral bioavailability by reducing first-pass glucuronidation. The 14-hydroxyl group affects receptor binding characteristics and contributes to kappa-receptor activity. [13-16]

Mechanism of Action

Opioid Receptor Profile

Oxycodone is a full agonist at opioid receptors with the following affinity profile:

The kappa-receptor activity distinguishes oxycodone from morphine and may contribute to its efficacy in visceral pain. Some studies suggest oxycodone produces superior analgesia for visceral pain compared to morphine, possibly related to kappa-receptor-mediated spinal mechanisms. [17-20]

G-Protein Signalling

Like all opioid agonists, oxycodone activates inhibitory G proteins (Gi/Go) coupled to mu-receptors:

1. Adenylyl cyclase inhibition: Reduced intracellular cAMP
2. GIRK channel activation: Potassium efflux causing membrane hyperpolarisation
3. Calcium channel inhibition: Reduced presynaptic neurotransmitter release
4. Descending inhibition: Activation of periaqueductal grey (PAG) and rostral ventromedial medulla (RVM) pathways

The downstream effects at nociceptive pathways produce:

• Reduced transmission at spinal dorsal horn (laminae I, II)
• Enhanced descending inhibition from brainstem
• Modification of affective pain response (limbic system)

Pharmacokinetics

Absorption

Oral bioavailability:

• Oxycodone: 60-87% (mean ~75%)
• This compares favourably to morphine (20-40%) and is the primary reason for oxycodone's widespread oral use
• The higher bioavailability results from reduced first-pass hepatic metabolism (3-methoxy group prevents glucuronidation at this position)

Formulations:

Important: Controlled-release formulations must be swallowed whole. Crushing destroys the matrix and releases the full dose immediately (dose-dumping), causing potentially fatal overdose. [21-24]

Distribution

• Volume of distribution (Vd): 2.6-3.0 L/kg
• Plasma protein binding: 38-45% (primarily albumin)
• CNS penetration: Good, despite low lipophilicity
• Crosses placenta: Yes
• Breast milk excretion: Yes (M:P ratio ~3.4)

The moderate Vd reflects distribution to muscle, visceral organs, and brain. Lower protein binding than fentanyl (84%) means more free drug is available for effect. [25-28]

Metabolism

Oxycodone undergoes extensive hepatic metabolism via cytochrome P450 enzymes:

Primary pathway (CYP3A4):

• N-demethylation to noroxycodone (~45% of dose)
• Noroxycodone is pharmacologically inactive (negligible mu-affinity)
• This is the major metabolic pathway

Secondary pathway (CYP2D6):

• O-demethylation to oxymorphone (~11% of dose)
• Oxymorphone is a potent mu-agonist (10× morphine potency)
• However, plasma concentrations are clinically insignificant (~1% of parent drug levels)
• Unlike codeine (where CYP2D6 activation is essential), oxycodone is intrinsically active

Further metabolism:

• Noroxycodone → noroxymorphone (via CYP2D6)
• Oxymorphone → oxymorphone-3-glucuronide (via UGT2B7)
• All metabolites are renally excreted

CYP2D6 Pharmacogenetics:

Unlike codeine (prodrug), oxycodone's analgesic effect does NOT critically depend on CYP2D6 status:

The clinical significance of CYP2D6 polymorphisms for oxycodone is less pronounced than for codeine because oxycodone is intrinsically active at mu-receptors. [29-35]

Elimination

• Elimination half-life: 3-5 hours (immediate-release)
• Clearance: 0.8 L/min
• Renal excretion: ~10% unchanged drug; ~70% as metabolites
• Hepatic extraction ratio: Low-intermediate (~0.5)

Renal impairment:

• Oxycodone and metabolites accumulate in renal impairment
• Unlike morphine (M6G accumulation), oxycodone metabolites are less potent
• However, dose reduction (25-50%) is still recommended for eGFR <30 mL/min
• Generally considered safer than morphine in renal impairment, but not completely safe

Hepatic impairment:

• Reduced clearance in cirrhosis
• AUC increased 95% and Cmax increased 50% in hepatic impairment
• Reduce dose by 50% and extend interval in moderate-severe hepatic disease [36-40]

Clinical Pharmacology

Dosing and Equianalgesic Conversions

Equianalgesic dose ratios (approximate):

Oral morphine:oxycodone ratio = 1.5:1 (oral oxycodone is 1.5× more potent than oral morphine)

Typical dosing:

OxyContin prescribing:

• Start with lowest dose (10 mg BD) in opioid-naïve patients
• Titrate by 25-50% every 3-7 days
• Breakthrough: Immediate-release oxycodone 10-15% of total daily dose PRN
• Maximum: No defined ceiling, but doses >400 mg/day require specialist review [41-45]

Clinical Applications

Postoperative analgesia:

• Widely used for transition from IV to oral opioids
• Typical conversion: IV morphine → oral oxycodone at 1:1 mg ratio (accounting for morphine bioavailability)
• Example: Patient on morphine PCA 30 mg/24hr → oxycodone 5 mg PO every 4 hours (30 mg/24hr)

Cancer pain:

• Oxycodone is a WHO Step 3 strong opioid
• Controlled-release formulation preferred for background pain
• Immediate-release for breakthrough
• Effective for nociceptive and mixed pain; less evidence for pure neuropathic pain

Chronic non-cancer pain:

• Increasingly restricted due to abuse potential
• Requires careful patient selection, monitoring, and opioid contract
• Australian TGA and NZ Medsafe recommend caution and specialist involvement

Procedural sedation:

• IV oxycodone used in Australia/NZ for procedural analgesia
• 2.5-5 mg IV titrated to effect
• Similar profile to IV morphine but may have faster onset [46-50]

Adverse Effects

Common Opioid Class Effects

Oxycodone-Specific Considerations

• Constipation: Possibly more severe than morphine (some studies)
• Less histamine release: Oxycodone causes minimal histamine release compared to morphine
• Euphoria: May be more pronounced than morphine (higher abuse potential)
• Hallucinations/delirium: Reported, especially in elderly [51-54]

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Tramadol Pharmacology

Chemical Structure and Classification

Tramadol hydrochloride ((1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexan-1-ol) is a synthetic opioid with a unique dual mechanism of action. It is structurally unrelated to morphine or the phenanthrene opioids. [55-58]

Molecular characteristics:

• Molecular formula: C16H25NO2
• Molecular weight: 263.4 Da (299.8 Da as hydrochloride)
• pKa: 9.4
• Log P: 1.35 (moderate lipophilicity)

Stereochemistry:

• Tramadol exists as a racemic mixture of (+)- and (-)-enantiomers
• (+)-Tramadol: Preferentially inhibits serotonin reuptake; metabolised to (+)-M1 (mu-agonist)
• (-)-Tramadol: Preferentially inhibits norepinephrine reuptake
• The racemic mixture provides synergistic analgesia through multiple mechanisms

Mechanism of Action

Dual Mechanism

Tramadol's analgesia arises from two distinct mechanisms:

1. Opioid receptor agonism (weak):

• Tramadol parent compound: Weak mu-affinity (Ki ~2,400 nM, ~6,000× weaker than morphine)
• O-desmethyltramadol (M1): Active metabolite with 200-300× greater mu-affinity than parent
• M1 is the primary contributor to opioid-mediated analgesia
• This CYP2D6-dependent activation is critical for tramadol's analgesic effect

2. Monoamine reuptake inhibition (SNRI activity):

• Inhibition of serotonin (5-HT) reuptake by (+)-tramadol
• Inhibition of norepinephrine (NE) reuptake by (-)-tramadol
• These actions enhance descending inhibitory pain pathways (similar to duloxetine, venlafaxine)
• This mechanism is independent of opioid receptors

The dual mechanism explains why naloxone only partially reverses tramadol analgesia and why tramadol provides modest benefit in neuropathic pain. [59-64]

Pharmacokinetics

Absorption

• Oral bioavailability: 68-75% (first dose); increases to ~90% with repeated dosing (saturable first-pass)
• Tmax: 1.6-2.0 hours (immediate-release)
• Food effect: Minimal effect on absorption
• Formulations: Immediate-release (50-100 mg), sustained-release (100-300 mg)

Administration routes:

Distribution

• Volume of distribution: 2.6-2.9 L/kg
• Plasma protein binding: 20% (low)
• CNS penetration: Good
• Crosses placenta: Yes (neonatal withdrawal reported)
• Breast milk: Excreted; use with caution

Metabolism

Tramadol metabolism is complex and highly dependent on CYP2D6:

Phase I metabolism:

1. CYP2D6 → O-desmethyltramadol (M1): The active metabolite responsible for opioid effects
2. CYP3A4/CYP2B6 → N-desmethyltramadol (M2): Inactive metabolite

Phase II metabolism:

• M1 undergoes glucuronidation and sulfation
• Excreted renally

CYP2D6 Pharmacogenetics (CRITICAL for Tramadol):

Unlike oxycodone, tramadol's analgesic efficacy is highly dependent on CYP2D6 status:

CYP2D6 poor metabolisers (~5-10% of Caucasians, ~1% of Asians) may experience analgesic failure with tramadol. Ultra-rapid metabolisers (1-10%, higher in some populations) are at increased risk of toxicity. [65-70]

Elimination

• Elimination half-life: Tramadol 5-7 hours; M1 7-9 hours
• Clearance: 6 mL/min/kg
• Renal excretion: 90% (30% unchanged, 60% metabolites)

Renal impairment:

• CrCl <30 mL/min: Extend interval to every 12 hours
• Dialysis: Give after dialysis session; tramadol is partially removed
• M1 accumulation can occur

Hepatic impairment:

• Reduced M1 formation (reduced analgesia)
• Prolonged half-life
• Reduce dose 50% in cirrhosis [71-74]

Clinical Pharmacology

Dosing

Standard dosing:

Equianalgesic dosing (approximate):

• Oral tramadol 100 mg ≈ Oral morphine 10 mg (controversial, highly variable)
• Tramadol is approximately 1/10th the potency of morphine

Ceiling Effect Controversy

The concept of a "ceiling effect" for tramadol respiratory depression is debated:

Arguments FOR ceiling effect:

• Respiratory depression appears less dose-dependent than pure mu-agonists
• Fatalities from tramadol alone (without co-ingestants) are relatively rare
• The SNRI component does not cause respiratory depression

Arguments AGAINST ceiling effect:

• Respiratory depression and deaths DO occur, especially in:
  • Ultra-rapid metabolisers
  • Renal impairment (M1 accumulation)
  • Combination with other CNS depressants
• At high doses (>400 mg), opioid effects predominate

Clinical conclusion: While tramadol may have a relative safety margin compared to pure opioids, it should NOT be considered safe in overdose. Respiratory depression can occur, particularly in vulnerable populations. [75-78]

Seizure Risk

Tramadol lowers the seizure threshold through multiple mechanisms:

Mechanisms:

1. Serotonin effects: Serotonin excess can cause seizures
2. Inhibition of GABA-A receptors: Direct pro-convulsant effect
3. Opioid withdrawal: In dependent patients, abrupt cessation causes seizures

Risk factors:

Incidence: Seizures occur in 1-2% of patients at therapeutic doses, higher at supratherapeutic doses. Most seizures are single, generalised tonic-clonic seizures occurring within 24 hours of starting or increasing dose.

Contraindication: Tramadol is contraindicated in uncontrolled epilepsy. Use with caution in patients with seizure history. [79-82]

Serotonin Syndrome Risk

Tramadol's SNRI activity creates significant serotonin syndrome risk when combined with other serotonergic drugs:

Hunter Criteria for Serotonin Syndrome:

• Spontaneous clonus, OR
• Inducible clonus + agitation/diaphoresis, OR
• Ocular clonus + agitation/diaphoresis, OR
• Tremor + hyperreflexia, OR
• Hypertonia + temperature >38°C + ocular/inducible clonus

High-risk combinations with tramadol:

Management of serotonin syndrome:

• Stop all serotonergic drugs
• Supportive care (cooling, benzodiazepines for agitation/myoclonus)
• Cyproheptadine 12 mg initially then 2 mg every 2 hours (serotonin antagonist)
• ICU admission if severe (hyperthermia >41°C, rhabdomyolysis) [83-86]

Adverse Effects

Common Effects

Tramadol-Specific Serious Effects

1. Seizures (1-2%): See above
2. Serotonin syndrome: With serotonergic drug combinations
3. Hypoglycaemia: Rare but reported, especially in diabetics
4. Respiratory depression: Possible, especially in UM or overdose
5. Dependence/withdrawal: Despite "weak" opioid status, dependence occurs
6. QT prolongation: Minor effect, rarely clinically significant [87-90]

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Comparative Pharmacology

Morphine vs Oxycodone vs Tramadol

Equianalgesic Dose Comparison

Important caveats:

• Equianalgesic ratios are approximations based on single-dose studies
• Individual variability is substantial (especially for tramadol)
• Cross-tolerance may be incomplete—use 50-75% of calculated dose when rotating
• Tramadol conversions are particularly unreliable due to dual mechanism [91-95]

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Drug Interactions

CYP2D6 Interactions (Both Drugs)

Clinical pearl: CYP2D6 inhibition significantly impairs tramadol analgesia but has minimal effect on oxycodone because oxycodone is intrinsically active while tramadol requires CYP2D6 activation.

CYP3A4 Interactions

Pharmacodynamic Interactions

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Abuse Potential and Regulatory Considerations

Oxycodone Abuse

Oxycodone has significant abuse potential:

Factors contributing to abuse liability:

• High oral bioavailability enables oral abuse
• Euphoria may be more intense than morphine
• Controlled-release formulations can be manipulated (crushing, dissolving)
• "OxyContin" became associated with prescription opioid epidemic

Abuse-deterrent formulations:

• OxyContin reformulation (2010) contains polymer that resists crushing and gelling
• However, these formulations can still be abused by oral route
• Efficacy in reducing abuse is debated

Regulatory status (Australia):

• Schedule 8 (Controlled Drug)
• Authority prescription required in most states for quantities >8 days
• Real-time prescription monitoring (RTPM) in all states

Tramadol Abuse and Rescheduling

Tramadol was historically considered "low abuse potential" but this is now recognised as incorrect:

Evidence of abuse potential:

• Dependence syndrome identical to other opioids
• Withdrawal symptoms include seizures (unique)
• Increasing reports of misuse and diversion
• Deaths from tramadol overdose (particularly in combination with other CNS depressants)

Regulatory changes:

• USA: Rescheduled to Schedule IV (2014)
• Australia: Remains Schedule 4, but under review
• Some countries have implemented stricter controls

Prescribing recommendations:

• Do NOT prescribe tramadol as a "safe" alternative to other opioids
• Same precautions as for other opioids regarding abuse risk
• Avoid in patients with history of substance use disorder [101-105]

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Indigenous Health Considerations

Aboriginal and Torres Strait Islander Considerations

Culturally safe prescribing of oxycodone and tramadol for Aboriginal and Torres Strait Islander peoples requires consideration of several important factors that affect both pharmacokinetics and access to care.

Pharmacogenetic considerations: Limited data exists on CYP2D6 polymorphism frequencies specifically in Aboriginal and Torres Strait Islander populations. However, global Indigenous population studies suggest potential variation in metaboliser phenotypes. This has particular relevance for tramadol, where CYP2D6 poor metabolisers experience reduced analgesia and ultra-rapid metabolisers face toxicity risk. When prescribing tramadol to Indigenous patients, clinicians should be vigilant for signs of non-response (possible poor metaboliser) or unexpected toxicity (possible ultra-rapid metaboliser) and consider alternative analgesics if response is suboptimal.

Chronic disease burden: Higher rates of chronic kidney disease in Indigenous Australians (3-4× non-Indigenous prevalence) affect both oxycodone and tramadol dosing. While neither drug is as hazardous as morphine in renal impairment, dose reduction is required for eGFR <30 mL/min. Diabetes prevalence is also elevated, relevant given tramadol's rare association with hypoglycaemia.

Access considerations: Remote and rural Indigenous communities may have limited access to controlled medications (Schedule 8) due to pharmacy logistics and storage requirements. Tramadol (Schedule 4) may be more readily available in remote health services, though this should not drive prescribing decisions over clinical appropriateness. Pain management plans should account for medication access when patients return to community.

Cultural safety: Family (mob) involvement in pain management discussions is culturally appropriate. Aboriginal Health Workers can assist in medication counselling and monitoring. Pain expression varies across cultures—assessment should use culturally validated tools where available, and clinicians should avoid assumptions about pain tolerance or medication-seeking behaviour based on cultural stereotypes.

Maori Health Considerations (Aotearoa New Zealand)

Whanau (extended family) involvement in healthcare decisions reflects Maori cultural values and should be supported in pain management planning. Health equity principles require that effective analgesics, including oxycodone when indicated, are accessible to Maori patients regardless of prescriber concerns about abuse that may reflect unconscious bias. Monitoring for opioid safety should be applied equitably across all patient populations.

Traditional medicine (rongoa Maori) may be used alongside prescribed analgesia. A non-judgmental approach that acknowledges traditional practices while ensuring patient safety is recommended. No direct interactions between tramadol/oxycodone and traditional remedies are established, but comprehensive medication reconciliation should include rongoa use. [106-108]

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ANZCA Primary Exam Focus

High-Yield MCQ Topics

1. Oral bioavailability comparison:

   • Oxycodone 60-87% vs Morphine 20-40%
   • Explains oxycodone's oral utility

2. Tramadol dual mechanism:

   • Weak mu-agonism via M1 metabolite (CYP2D6)
   • SNRI activity (independent of opioid receptors)
   • Only ~30-40% of analgesia reversed by naloxone

3. CYP2D6 pharmacogenetics:

   • Critical for tramadol (poor metabolisers = analgesic failure)
   • Minimal clinical significance for oxycodone

4. Tramadol seizure risk:

   • Dose-dependent, enhanced by serotonergic drugs
   • Contraindicated in uncontrolled epilepsy

5. Serotonin syndrome:

   • Tramadol + MAOI = contraindicated
   • Tramadol + SSRI/SNRI = high risk
   • Hunter criteria for diagnosis

6. Equianalgesic dosing:

   • Oral morphine:oxycodone = 1.5:1
   • Oral morphine:tramadol = 1:10 (approximately)

7. Renal impairment:

   • Morphine: High risk (M6G accumulation)
   • Oxycodone: Moderate risk (safer than morphine)
   • Tramadol: Moderate risk (M1 accumulation)

Viva Question Themes

• Compare and contrast tramadol and oxycodone mechanisms of action
• A patient taking an SSRI requires postoperative analgesia—discuss opioid selection
• Explain why naloxone only partially reverses tramadol analgesia
• Discuss the clinical significance of CYP2D6 polymorphisms for opioid prescribing
• A patient has chronic renal impairment (eGFR 25)—compare analgesic options

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Assessment Content

SAQ Practice Question (15 marks)

Question: A 58-year-old woman with type 2 diabetes, depression (on sertraline 100 mg daily), and chronic knee osteoarthritis presents for total knee replacement. Her eGFR is 45 mL/min/1.73m². Her usual analgesia is tramadol 50 mg TDS.

(a) Discuss the pharmacological concerns regarding her current tramadol therapy given her comorbidities and co-medications. (6 marks)

(b) Outline an appropriate postoperative analgesic plan, justifying your opioid selection. (6 marks)

(c) Describe the key monitoring requirements for your chosen opioid regimen. (3 marks)

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Model Answer:

(a) Pharmacological concerns with tramadol (6 marks)

Serotonin syndrome risk [2 marks]:

• Tramadol has SNRI activity (serotonin reuptake inhibition)
• Sertraline is an SSRI (potent serotonin reuptake inhibitor)
• Combination significantly increases serotonin syndrome risk
• Symptoms include hyperthermia, hyperreflexia, clonus, agitation
• Postoperative setting may mask early signs (sedation attributed to anaesthesia)

CYP2D6 drug interaction [2 marks]:

• Sertraline is a moderate CYP2D6 inhibitor
• Tramadol requires CYP2D6 activation to O-desmethyltramadol (M1) for opioid effect
• This interaction may be reducing her analgesic response to tramadol
• May partly explain need for regular tramadol for "moderate" osteoarthritis pain

Renal impairment [1 mark]:

• eGFR 45 mL/min represents moderate CKD (Stage 3b)
• M1 metabolite accumulates in renal impairment
• Standard tramadol dosing may lead to accumulation and toxicity
• Dose extension (every 8-12 hours) recommended for CrCl <60 mL/min

Diabetes consideration [1 mark]:

• Tramadol rarely associated with hypoglycaemia
• Risk may be increased in diabetic patients on hypoglycaemic agents
• Requires blood glucose monitoring

(b) Postoperative analgesic plan (6 marks)

Multimodal approach [1 mark]:

• Paracetamol 1g every 6 hours (regular, renal dose adjustment not required until eGFR <30)
• Avoid NSAIDs (relative contraindication in CKD)
• Consider regional technique (peripheral nerve block for TKR—adductor canal or femoral nerve block)

Opioid selection [3 marks]:

I would select oxycodone rather than tramadol or morphine:

Oxycodone provides:

• Good oral bioavailability (60-87%)
• No active metabolites that significantly accumulate in moderate CKD
• No serotonin syndrome risk with sertraline
• Minimal CYP2D6 dependence

Dosing regimen [2 marks]:

• PCA initially: Oxycodone 1 mg bolus, 5-minute lockout (if IV available) OR morphine PCA with caution
• Transition to oral oxycodone 5-10 mg every 6 hours (extended interval for CKD)
• Reduce doses by 25-50% from standard given moderate renal impairment
• Avoid tramadol due to serotonin syndrome risk with sertraline

(c) Monitoring requirements (3 marks)

Respiratory monitoring [1 mark]:

• Continuous pulse oximetry for 24-48 hours
• Respiratory rate hourly (target >10/min)
• Sedation scoring (e.g., Pasero scale) every 2-4 hours
• Naloxone immediately available

Renal function monitoring [1 mark]:

• Daily creatinine and eGFR (oxycodone accumulates if AKI develops)
• Fluid balance chart
• Urine output monitoring

Additional monitoring [1 mark]:

• Blood glucose (diabetes + surgical stress)
• Pain scores using validated tool
• Nausea/vomiting and bowel function
• Watch for confusion/delirium (opioid-related, especially in elderly diabetics)

Total: 15 marks

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Viva Scenario (20 marks)

Stem: A 45-year-old man is brought to the emergency department with reduced level of consciousness. His partner reports he has chronic back pain and takes tramadol 200 mg three times daily. Today he started fluoxetine prescribed by his GP for depression. He has been increasingly agitated and confused over the past 6 hours.

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Examiner: What is your differential diagnosis?

Candidate: Given the history of tramadol use and recent introduction of fluoxetine (an SSRI), my primary concern is serotonin syndrome. Other differentials include:

1. Serotonin syndrome (most likely):

   • Tramadol has SNRI activity
   • Fluoxetine is a potent SSRI
   • Combination significantly increases serotonin toxicity risk
   • Onset within hours of adding second serotonergic agent is typical

2. Tramadol overdose (less likely given context):

   • Could cause CNS depression, but partner reports agitation initially
   • Respiratory depression would be expected

3. Other causes to exclude:

   • Sepsis/meningitis
   • Other toxic ingestion
   • Metabolic disturbance (hypoglycaemia)
   • Intracranial pathology

(3 marks)

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Examiner: On examination, his temperature is 39.2°C, heart rate 125 bpm, BP 160/95 mmHg. He has dilated pupils, diaphoresis, generalised hyperreflexia, and inducible clonus at the ankles. How do you confirm the diagnosis?

Candidate: These clinical findings strongly support serotonin syndrome using the Hunter Criteria:

Hunter Serotonin Toxicity Criteria (requires serotonergic drug exposure PLUS one of):

• Spontaneous clonus
• Inducible clonus + agitation OR diaphoresis
• Ocular clonus + agitation OR diaphoresis
• Tremor + hyperreflexia
• Hypertonia + temperature >38°C + ocular clonus OR inducible clonus

This patient has:

• Serotonergic drug exposure (tramadol + fluoxetine) ✓
• Inducible clonus ✓
• Agitation ✓
• Diaphoresis ✓
• Also: Hyperthermia, hypertension, tachycardia, mydriasis, hyperreflexia

Diagnosis: Serotonin syndrome (Hunter criteria met)

This is a clinical diagnosis—there is no confirmatory laboratory test. It is distinguished from neuroleptic malignant syndrome by:

• Rapid onset (hours) vs NMS (days)
• Hyperreflexia/clonus vs rigidity/bradyreflexia in NMS
• Recent serotonergic drug change vs antipsychotic exposure

(4 marks)

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Examiner: Describe your management of this patient.

Candidate:

Immediate management (ABC approach):

Airway and breathing [1 mark]:

• Supplemental oxygen
• Prepare for intubation if deteriorating consciousness or severe rigidity
• Respiratory depression unlikely but possible from tramadol component

Circulation [1 mark]:

• IV access, fluid resuscitation
• Monitor for arrhythmias (serotonin-induced)
• Hyperthermia management is priority

Stop serotonergic drugs [1 mark]:

• Cease tramadol immediately
• Cease fluoxetine (though long half-life means effects persist)

Active cooling [1 mark]:

• Target temperature <38.5°C
• External cooling (ice packs, cooling blankets)
• Avoid antipyretics (ineffective, hyperthermia is muscular not hypothalamic)
• Consider intubation and paralysis if severe hyperthermia (>41°C)

Benzodiazepines [1 mark]:

• Diazepam 5-10 mg IV or midazolam 2.5-5 mg IV
• Controls agitation
• Reduces myoclonus and muscle hyperactivity
• Reduces heat generation

Cyproheptadine [1 mark]:

• Serotonin 5-HT2A receptor antagonist
• Loading dose: 12 mg PO/NG
• Then 2 mg every 2 hours until response
• Maximum 32 mg in 24 hours
• Only available orally (not IV)

Specific considerations:

• Avoid antipsychotics (may worsen hyperthermia)
• Avoid tramadol/opioid antagonism with naloxone (does not treat serotonin syndrome)
• ICU admission for monitoring

(6 marks)

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Examiner: The patient stabilises. Two days later, his back pain is severe and he needs analgesia. What opioid would you choose?

Candidate: Given the recent serotonin syndrome precipitated by tramadol + fluoxetine, I need to select an opioid with minimal serotonergic activity.

Recommended: Oxycodone or hydromorphone

Rationale [2 marks]:

I would choose oxycodone:

• No significant serotonergic activity
• No active metabolites that accumulate
• Good oral bioavailability
• Effective for chronic pain

Important considerations [2 marks]:

• Fluoxetine has a long half-life (4-6 days) and its active metabolite norfluoxetine persists for 2+ weeks
• Avoid tramadol for at least 5 half-lives of fluoxetine (3-4 weeks) after discontinuation
• Long-term: Review need for both SSRI and opioid; consider non-serotonergic antidepressant
• Discuss case with psychiatry regarding antidepressant options (mirtazapine, bupropion have lower serotonin syndrome risk)

(4 marks)

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Examiner: What is the mechanism by which tramadol causes serotonin syndrome?

Candidate:

Tramadol has a unique dual mechanism of action that includes serotonergic effects:

Serotonin reuptake inhibition [2 marks]:

• The (+)-tramadol enantiomer inhibits the serotonin transporter (SERT)
• This prevents reuptake of serotonin from the synaptic cleft
• Increased synaptic serotonin concentration results
• This is similar to the mechanism of SSRIs like fluoxetine

When combined with an SSRI:

• Both drugs inhibit serotonin reuptake
• Additive/synergistic increase in synaptic serotonin
• Excessive 5-HT1A and 5-HT2A receptor activation
• Clinical serotonin syndrome results

Additional considerations:

• Tramadol may also have weak direct serotonin-releasing activity
• The opioid component (via M1 metabolite) does NOT contribute to serotonin syndrome
• This is why naloxone does not treat serotonin syndrome
• The (-)-tramadol enantiomer inhibits norepinephrine reuptake, which may contribute to autonomic features (tachycardia, hypertension)

(3 marks)

Total: 20 marks

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References

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