Cryo Analgesia (Cryoanalgesia / Cryoneurolysis)

Definition

Mechanism of Action

• Uses the Joule-Thomson effect: a gas under high pressure (CO2 or N2O) is allowed to expand rapidly at the probe tip, causing a sudden and dramatic drop in temperature.
• The cryoprobe tip reaches temperatures of -50°C to -70°C (Morgan & Mikhail) or -40°C (AtriCure CryoaSphere probe in clinical use).
• Freezing causes axonotmesis (Wallerian degeneration of axons) while leaving the endoneurium, perineurium, and epineurium intact.
• Because the nerve sheath is preserved, there is guided axonal regeneration - nerve function returns over weeks to months, unlike neurotmesis.
• The intact sheath also means there is minimal risk of deafferentation pain or neuroma formation (in contrast to surgical neurectomy or radiofrequency ablation).

Equipment

• Cryoprobe: introduced via a 12- to 16-gauge catheter
• Gas used: carbon dioxide (CO2) or nitrous oxide (N2O) at high pressure
• Probe tip temperature: -50°C to -70°C
• Electrical stimulation is used to confirm correct positioning:
  • 2-5 Hz for motor responses
  • 50-100 Hz for sensory responses
• Freeze-thaw cycles: typically two or more 2-minute cycles

Technique

1. Position the cryoprobe at the target nerve using fluoroscopic guidance or thoracoscopic visualization.
2. Confirm correct placement with electrical stimulation.
3. Apply 2 or more freeze-thaw cycles of 2 minutes each.
4. Allow the probe tip to warm before repositioning.
5. For intercostal application (e.g., Nuss procedure): each nerve is identified below the associated rib posteriorly but lateral to the sympathetic chain, and the probe is held in contact for 120 seconds per nerve. Nerves T3/T4 through T7 are typically targeted bilaterally.

Clinical Applications

Duration of Effect

• Analgesia lasts weeks to months - significantly longer than local anaesthetic blocks.
• Nerve regeneration is guided by the intact nerve sheath, so function eventually returns.
• Duration is longer than radiofrequency ablation (RFA typically 3-12 months with risk of deafferentation pain), but cryoanalgesia carries lower risk of neuroma and deafferentation pain.

Advantages over Other Techniques

Comparison with Radiofrequency Ablation (RFA)

• RFA uses heat (60-90°C for 1-3 min) via a special needle electrode under fluoroscopic guidance.
• RFA produces a thermally ablated lesion; the lesion shape depends on needle gauge, tip temperature, and duration.
• Pulsed RFA at 42°C is a gentler variant being evaluated.
• Cryoneurolysis is preferred when reversibility and minimal deafferentation risk are priorities.

Precautions / Risks

• Probe must be kept away from the lung parenchyma and pericardial sac (thoracoscopy helps).
• Skin and overlying muscle must be retracted during the freeze to avoid unwanted soft tissue injury.
• When used for intercostal nerves via the Nuss procedure: the final freeze is allowed to thaw for 3 minutes before reinflating the lung on that side.
• Pneumothorax risk if used in conjunction with intercostal nerve blocks.

Key Points for Exam

1. Gas used: CO2 or N2O (Joule-Thomson expansion)
2. Temperature achieved: -50°C to -70°C
3. Catheter size: 12-16 gauge
4. Cycles: 2 or more 2-minute freeze-thaw cycles
5. Type of nerve injury: axonotmesis (Wallerian degeneration) with intact nerve sheath
6. Duration: weeks to months (reversible)
7. Primary indication: post-thoracotomy pain / intercostal neuralgia
8. Advantage over RFA: lower risk of deafferentation pain and neuroma
9. Motor stimulation frequency: 2-5 Hz | Sensory: 50-100 Hz

• Morgan & Mikhail's Clinical Anesthesiology, 7th ed., Ch. 47 - Radiofrequency Ablation & Cryoneurolysis
• Miller's Anesthesia, 10th ed., Vol. 2 - Truncal Blocks
• Fischer's Mastery of Surgery, 8th ed. - Nuss Procedure / Cryoablation