The Role of Ketamine in Acute and Chronic Pain – A Review of Therapeutic Benefits

Abstract

Ketamine, a phencyclidine derivative, has emerged as a potent analgesic for both acute and chronic pain conditions. Acting primarily as an N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine reduces central sensitisation, opioid tolerance, and hyperalgesia—major contributors to persistent pain. Its utility spans perioperative pain, trauma management, burn care, cancer pain, and neuropathic pain syndromes. Low-dose ketamine provides opioid-sparing benefits with minimal respiratory depression, making it an attractive option in multimodal analgesic regimens. Despite concerns related to psychotomimetic effects and cardiovascular stimulation, ketamine maintains a strong safety profile when used in controlled settings. This review summarises ketamine’s mechanisms, therapeutic applications, dosing considerations, safety issues, and future directions in pain management.

Keywords

Introduction

Pain is a major global health burden affecting individuals physically, psychologically, and socioeconomically. While opioids have been central to managing moderate-to-severe pain, rising concerns over adverse effects—including respiratory depression, dependence, tolerance, and opioid-induced hyperalgesia—have intensified the search for safer alternatives.

Ketamine, synthesised in 1962 and approved for clinical use in 1970, is traditionally recognised for its dissociative anaesthesia. Over recent decades, however, its role at sub-anaesthetic doses as an analgesic, particularly in refractory and neuropathic pain, has gained significant attention. Ketamine’s unique pharmacological properties position it as a bridge between conventional analgesics and advanced interventional therapies.

MECHANISM OF ACTION

Ketamine exerts analgesia through a multifaceted pharmacodynamic profile:

1. NMDA Receptor Antagonism:

Ketamine’s primary mechanism involves noncompetitive blockade of NMDA receptors in the central nervous system, thereby inhibiting the excitatory neurotransmitter glutamate. This reduces central sensitisation, wind-up phenomena, and long-term potentiation—key drivers of chronic and neuropathic pain.

2. Opioidergic Modulation:

Ketamine interacts with μ and κ opioid receptors, enhancing endogenous analgesic pathways and potentially reversing opioid-induced tolerance.

3. Monoaminergic Pathway Effects:

It increases synaptic concentrations of serotonin and norepinephrine, contributing to analgesic and antidepressant effects.

4. Anti-Inflammatory Actions:

Ketamine reduces pro-inflammatory cytokines such as IL-6 and TNF-α, which play a role in chronic pain states.

5. Voltage-Gated Channel Modulation:

By affecting calcium and sodium channels, ketamine dampens neuronal excitability and transmission of pain signals.

6. Neuroplasticity and Synaptogenesis:

Evidence suggests that ketamine enhances brain-derived neurotrophic factor (BDNF), leading to structural and functional changes supporting long-term pain relief.

ROLE IN ACUTE PAIN MANAGEMENT

Ketamine has proven beneficial across various acute pain settings:

1. Perioperative Pain:

Low-dose ketamine (0.1–0.3 mg/kg IV bolus followed by infusion) reduces intraoperative anaesthetic needs and postoperative pain. It also diminishes opioid requirements and prevents postoperative hyperalgesia.

2. Trauma and Emergency Pain:

In emergency departments, ketamine provides rapid analgesia without compromising airway reflexes or hemodynamics, making it suitable for trauma, fractures, and acute severe pain.

3. Burns and Procedural Pain:

Ketamine remains one of the safest agents for burn dressing changes and painful procedures, particularly in children and hemodynamically unstable patients.

4. Acute Sickle Cell Crisis:

Sub-anaesthetic ketamine infusions have shown benefit in reducing pain scores and opioid usage during vaso-occlusive episodes.

ROLE IN CHRONIC PAIN MANAGEMENT

Chronic pain often involves neuroplastic changes and central sensitisation—conditions responsive to NMDA receptor modulation.

Ketamine has demonstrated efficacy in:

1. Complex Regional Pain Syndrome (CRPS):

Ketamine infusions, ranging from short single-day treatments to multi-day protocols, provide significant and sometimes prolonged relief in CRPS patients.

2. Neuropathic Pain:

Disorders such as postherpetic neuralgia, diabetic neuropathy, and phantom limb pain exhibit improvement with ketamine therapy.

3. Cancer Pain and Palliative Care:

Ketamine is increasingly used as an adjuvant in refractory cancer pain, especially when opioids fail to provide relief.

4. Fibromyalgia and Central Sensitivity Syndromes:

Emerging evidence supports ketamine’s role in reducing widespread pain and fatigue.

5. Spinal Cord Injury Pain:

Long-standing neuropathic pain after spinal cord trauma may respond to ketamine's central-modulating actions.

OPIOID-SPARING EFFECTS

Opioids can paradoxically worsen chronic pain through tolerance and hyperalgesia, mediated in part by NMDA receptor activation. Ketamine interrupts this cycle:

• Reduces postoperative and chronic opioid consumption
• Reverses opioid tolerance
• Enhances analgesic synergy when combined with opioids
• Minimises the need for high-dose opioids in palliative and perioperative settings

These properties make ketamine a cornerstone of modern multimodal analgesia.

DOSING AND ADMINISTRATION

Common dosing strategies include:

• Bolus: 0.1–0.3 mg/kg IV for acute pain
• Infusion: 0.1–1 mg/kg/hr for perioperative or chronic pain
• Subcutaneous/Intramuscular routes: Used in resource-limited settings
• Oral and intranasal forms: Emerging for chronic neuropathic pain and home-based care

Careful titration is essential to balance analgesia with side effects.

SAFETY PROFILE AND ADVERSE EFFECTS

Ketamine is generally safe when administered at analgesic doses.

Common adverse effects:

• Nausea, dizziness
• Mild dysphoria or perceptual disturbances
• Increased secretions

Less common but significant effects:

• Psychotomimetic reactions (hallucinations, vivid dreams)
• Cardiovascular stimulation (tachycardia, hypertension)
• Rare hepatotoxicity with repeated high-dose infusions
• Bladder dysfunction with long-term recreational use (rare in therapeutic contexts)

Importantly, ketamine preserves airway reflexes and does not cause major respiratory depression, making it suitable for high-risk patients.

CONTRAINDICATIONS

Absolute and relative contraindications include:

• Uncontrolled hypertension
• Active psychosis or schizophrenia
• History of aneurysms
• Elevated intracranial or intraocular pressure
• Severe hepatic dysfunction
• Caution in patients with substance use disorders

Appropriate screening and monitoring reduce risks significantly.

FUTURE DIRECTIONS AND RESEARCH OPPORTUNITIES

Research continues to expand ketamine’s therapeutic roles:

1. Adjunct in Opioid Withdrawal and Detoxification:

Ketamine shows promise in reducing withdrawal symptoms and craving.

2. Use in Chronic Migraine and Cluster Headaches:

Intranasal and IV ketamine are being explored as alternatives for refractory headache syndromes.

3. Combination Therapies:

Co-administration with gabapentinoids, regional blocks, and antidepressants may enhance analgesic outcomes.

4. Role in Mental Health-Comorbid Pain:

Ketamine’s antidepressant effects may benefit chronic pain patients with depression or anxiety.

5. Development of S-Ketamine and Novel Analogues:

Newer formulations aim to improve efficacy and reduce adverse effects.

CONCLUSION

Ketamine remains a vital analgesic tool in both acute and chronic pain management, offering unique advantages through NMDA antagonism, neuroplastic modulation, and opioid-sparing effects. At sub-anaesthetic doses, it provides effective analgesia with a favourable safety profile. Continued research, protocol standardisation, and careful patient selection will further integrate ketamine into advanced multimodal pain strategies and enhance clinical outcomes.

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