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Sensory Ataxia from Nitrous Oxide Poisoning: A Hidden Danger in PLAB 2 Neurology Stations



Summary:

This scenario revolves around a patient presenting with neurological symptoms due to Vitamin B12 deficiency secondary to nitrous oxide (N₂O) abuse. The candidate is expected to identify the cause, address acute and long-term management, and demonstrate safe, structured consultation within the GP clinic setting.



Key Points:


Neurology/Systemic Effects

  • Nitrous oxide (N₂O) inactivates Vitamin B12 → impairs DNA and myelin synthesis.

  • Leads to Subacute Combined Degeneration (SCD) of the spinal cord.

  • Symptoms:

    • Bilateral lower limb weakness

    • Sensory ataxia

    • Paresthesia (tingling)

    • Positive Romberg’s test

    • Brisk knee reflexes, possible absent ankle jerks


Data Gathering

  • Early identification of lifestyle factors is critical: vegetarianism and recreational drug use.

  • Signpost before asking sensitive questions (e.g., drugs, alcohol, sexual history).

  • Use patient's terms (e.g., "balloons," "hippy crack," or "whippets") to identify N₂O.

  • Explore psychosocial triggers (e.g., stress-induced substance use).

Investigations

  • GP setting:

    • CBC: macrocytic anemia, hypersegmented neutrophils

    • Vitamin B12 levels, methylmalonic acid, homocysteine

    • TFTs to rule out hypothyroidism

  • Secondary care:

    • MRI spine (posterior column T2 hyperintensities)

    • Nerve conduction studies


Important Considerations:

  • Avoid confusing Nitric Oxide (NO) with Nitrous Oxide (N₂O).

  • Use exact terminology in exams—contextual understanding is crucial.

  • Time management: aim for ~4 minutes each for data gathering and management.

  • Wrap up data gathering once diagnosis is clear to preserve time for management.


Diagnostic Approach:

  1. History: Neurological symptoms + dietary habits + drug use

  2. Examination: Focused neurological (lower limbs)

  3. Investigations: CBC, B12, MMA, homocysteine, TFTs

  4. Diagnosis: Clinical + biochemical + lifestyle correlation


Management:

Immediate

  • Stop N₂O use immediately—clear, firm warning of irreversible damage

  • Administer IM Hydroxocobalamin (1 mg)—available in GP setting

  • Refer urgently:

    • To A&E for same-day evaluation (if severe neurological symptoms)

    • To neurology for non-urgent but specialist evaluation

Supportive

  • Address psychosocial contributors (e.g., stress)

  • Counseling services: substance misuse clinics, mental health referrals

  • Leaflets and information: regarding B12 deficiency and recreational drug risks

Follow-Up

  • Arrange GP follow-up for response monitoring

  • Repeat blood work to assess B12 response

Safety Netting

  • Educate on signs of deterioration: bowel/bladder involvement, worsening gait

  • Clear guidance to seek emergency help if symptoms worsen


Communication Skills:

  • Signpost lifestyle and sensitive questions.

  • Use empathetic, non-judgmental tone especially in drug-related cases.

  • Use simple, understandable explanations avoiding jargon.

  • Provide written information to reinforce verbal advice.



Ethical Considerations:

  • Ensure confidentiality and trust, especially in drug use history.

  • Respect autonomy while firmly guiding management.

  • Non-discriminatory, sensitive handling of substance misuse.


Additional Resources:

  • GMC Good Medical Practice (2024): Professionalism, confidentiality, patient-centered care

  • NICE guidelines on anemia and neurological evaluation

  • Public Health England substance misuse support references

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