Decompression Sickness (Type II - Neurological)

Decompression Sickness (DCS) Type II: An Exhaustive Clinical Guide

Decompression Sickness (DCS), colloquially known as "the bends," remains one of the most critical hyperbaric emergencies encountered in diving medicine, aviation, and aerospace physiology. While Type I DCS typically involves localized musculoskeletal pain or cutaneous manifestations, Type II DCS—specifically the neurological variant—represents a life-threatening, systemic insult requiring immediate intervention.

This guide serves as a clinical reference for healthcare providers, hyperbaric medicine specialists, and emergency responders regarding the pathophysiology, diagnosis, and management of neurological Type II DCS.

1. Clinical Definition and Overview

Type II Decompression Sickness (Neurological) is defined as a systemic disease process occurring when inert gas (primarily nitrogen) dissolved in body tissues comes out of solution and forms bubbles during a reduction in ambient pressure. Unlike Type I, Type II DCS involves the central nervous system (CNS), peripheral nervous system (PNS), or vestibular system.

The Spectrum of Neurological DCS

• Cerebral DCS: Involving the brain, leading to altered mental status, seizures, or cognitive deficits.
• Spinal Cord DCS: The most common form of serious DCS, typically affecting the white matter of the thoracic or lumbar spinal cord.
• Vestibular DCS ("Staggers"): Direct involvement of the inner ear, causing profound vertigo and ataxia.

2. Pathophysiology and Mechanism of Injury

The development of bubbles following a decompression insult is the primary driver of disease. However, the pathology is not merely mechanical; it is a complex biochemical cascade.

The Mechanical-Biochemical Axis

1. Direct Mechanical Obstruction: Bubbles act as emboli, blocking microvascular circulation in the spinal cord or brain, leading to ischemia.
2. Endothelial Activation: Bubbles act as foreign surfaces, triggering an inflammatory response. The endothelium becomes "leaky," leading to plasma loss into the extravascular space, hemoconcentration, and secondary tissue edema.
3. Coagulation Cascade: The bubble-blood interface activates platelets and the coagulation cascade, promoting microthrombi formation and further vascular compromise.
4. Neuro-Inflammation: The breakdown of the blood-brain barrier (BBB) allows neurotoxic substances to penetrate the CNS, exacerbating neuronal damage.

3. Clinical Staging and Presentation

Clinical assessment is paramount. Symptoms often appear within 1–6 hours of surfacing, though delayed presentations (up to 24-48 hours) occur.

Clinical Staging Table

Key Clinical Signs

• Spinal cord: Paraparesis or paraplegia, sensory level changes, sphincter dysfunction (urinary retention).
• Cerebral: Confusion, "scotoma" (visual field defects), hemiparesis, seizures, loss of consciousness.
• Vestibular: Severe rotational vertigo, nystagmus, nausea, vomiting, and ringing in ears (tinnitus).

4. Differential Diagnosis

Distinguishing Neurological DCS from other diving-related pathologies is essential for appropriate treatment.

• Arterial Gas Embolism (AGE): Typically occurs within 10 minutes of surfacing. Often features rapid onset of unconsciousness or stroke-like symptoms.
• Inner Ear Barotrauma: Often presents with vertigo but is usually associated with difficulty equalizing during descent, not ascent.
• Nitrogen Narcosis: Occurs at depth; symptoms resolve immediately upon ascent (DCS symptoms persist).
• Hypothermia/Exhaustion: Can mimic the fatigue and cognitive slowing seen in early-stage DCS.
• Stroke/TIA: Always consider in older divers with cardiovascular risk factors.

5. Diagnostic Testing and Evaluation

While DCS is primarily a clinical diagnosis, ancillary testing helps rule out mimics and assess damage.

Primary Diagnostic Modalities

1. Neurological Examination: A standardized neurological exam (e.g., SOAP note format) is mandatory. Documenting the level of sensory loss and muscle strength (0-5 scale) is critical for monitoring progress.
2. MRI (Magnetic Resonance Imaging): The gold standard for identifying spinal cord lesions or cerebral infarction. Findings may include hyperintense lesions in T2-weighted images.
3. CT Scan: Primarily used to rule out intracranial hemorrhage or other acute pathology in the emergency setting.
4. Laboratory Analysis:
   • CBC: To assess for hemoconcentration.
   • Urinalysis: To check for myoglobinuria or bladder distention.

6. Treatment Protocols: Hyperbaric Oxygen Therapy (HBOT)

The definitive treatment for Type II DCS is recompression in a hyperbaric chamber, typically following U.S. Navy Treatment Table 6 (TT6).

Mechanisms of HBOT

• Mechanical Reduction: Boyle’s Law dictates that increasing ambient pressure reduces bubble volume.
• Hyperoxygenation: Increasing the partial pressure of oxygen (PO2) improves oxygen delivery to hypoxic tissues despite compromised circulation.
• Edema Reduction: High-pressure oxygen helps reduce CNS swelling.

Adjunctive Therapies

• Fluid Resuscitation: Isotonic crystalloids to maintain circulating volume (avoid over-hydration, which may increase spinal cord edema).
• Anti-platelet therapy: Aspirin (325mg) is frequently administered in the field to prevent further thrombosis.
• Positioning: Keep the patient supine to maintain cerebral perfusion.

7. Prognosis and Long-Term Outlook

Prognosis is heavily dependent on the time to recompression.

• Excellent: Rapid recompression (within 2–4 hours of symptom onset) often results in full neurological recovery.
• Guarded: Delayed treatment (beyond 12–24 hours) correlates with permanent sequelae, such as chronic paresthesia, gait abnormalities, or persistent bladder dysfunction.
• Rehabilitation: Post-acute care often involves intensive physical and occupational therapy.

8. Frequently Asked Questions (FAQ)

1. Does "Type II" mean the condition is always permanent?

No. With immediate hyperbaric intervention, many patients recover completely. Permanent injury occurs when bubble-induced ischemia causes irreversible neuronal death.

2. Can I fly after a suspected DCS incident?

Absolutely not. Exposure to altitude causes further expansion of residual bubbles. Evacuation should be via ground transport or a pressurized aircraft at sea-level cabin pressure.

3. Are there long-term risks after a DCS incident?

Some individuals may develop dysbaric osteonecrosis (bone death) or chronic neurological symptoms, though these are more common in commercial deep-sea divers with repetitive exposure.

4. What is the role of Aspirin?

Aspirin is used for its anti-platelet properties to mitigate the inflammatory and thrombotic response triggered by bubbles in the vascular system.

5. Can a diver be "cured" by just breathing oxygen on the surface?

Surface oxygen is a vital first-aid measure that can improve symptoms by accelerating nitrogen washout, but it is rarely curative for Type II DCS. Recompression is mandatory.

6. Is spinal cord DCS common?

It is the most frequent presentation of serious DCS. The thoracic region of the spinal cord is particularly vulnerable due to its vascular supply.

7. What if symptoms resolve before I get to the chamber?

Seek medical evaluation regardless. "Spontaneous resolution" can be transient, and symptoms often recur or worsen as the inflammatory cascade progresses.

8. How does vestibular DCS differ from motion sickness?

Vestibular DCS presents with true vertigo (the world spinning), often accompanied by hearing loss and tinnitus, whereas motion sickness is typically nausea-dominant and related to physical motion.

9. Are there contraindications to Hyperbaric Oxygen?

The only absolute contraindication is an untreated pneumothorax. However, patients with high fever or severe seizure disorders require careful monitoring during the chamber dive.

10. Can I return to diving after Type II DCS?

Return to diving is a medical decision that should only be made after a full neurological recovery and clearance by a board-certified diving medicine physician. This process often takes 3–6 months.

9. Conclusion and Clinical Summary

Type II Neurological Decompression Sickness is a medical emergency requiring a high index of suspicion, rapid clinical assessment, and immediate hyperbaric referral. As clinical specialists, our goal is to minimize the "time-to-treatment" interval, as this remains the single most significant factor in preventing long-term morbidity. By understanding the mechanical and biochemical pathways of bubble injury, we can provide precise, life-saving care to the diving and aviation communities.

Disclaimer: This guide is for educational and informational purposes only and does not constitute medical advice. In the event of a suspected diving accident, contact your local emergency services or the Divers Alert Network (DAN) immediately.