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Excessive nitrous oxide inhalation oxidises the cobalt ion of vitamin B12, interfering with its bioavailability as a co-enzyme. Patients can then develop clinical symptoms of B12 deficiency, particularly subacute combined degeneration of the cord. This affects the posterior and lateral columns of the spinal cord, presenting with gradual upper and lower limb weakness, peripheral paraesthesia, and upper lower motor neurone signs, particularly with loss of vibration and touch sense
33088
a
Subacute combined degeneration of the cord
QuestionChoice
Syringomyelia presents with a bilateral loss of sensation to pain and temperature, but with sparing of light touch, vibration, and proprioception. It typically affects the neck, shoulders, and arms when a syrinx is present in the cervical spine
33090
c
Syringomyelia
QuestionChoice
Hypoxic brain injury initially affects areas of high metabolic activity, predominantly the hippocampi, thalami, cerebral cortex, corpus striatum, and cerebellar vermi. The consequences of such can be predicted by the areas affected, such as memory impairment, ataxia, poor motor control, and difficulties with complex learning tasks. It would not present with a normal cognitive state and progressive peripheral paraesthesia
33089
b
Hypoxic brain injury
QuestionChoice
While oxidative stress is known to result in neuroinflammation and subsequent neurodegeneration, it not a known complication from excessive nitrous oxide use
33092
e
Oxidative toxic neuropathy
QuestionChoice
Guillain-Barré Syndrome (GBS) presents with progressive ascending weakness of all limbs. Reflexes are reduced or absent and, if paraesthesia is present, it is often mild
33091
d
Guillain-Barré Syndrome
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TIL. Good question
26368
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Anterior Gallbladder
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ahh its the bristol special 
26435
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Kidneystones808
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can see these sort of niche questions coming up 

28972
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Serpiginous Yeast
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# Summary

Sub-acute combined degeneration of the cord (SCDC) is a neurological disorder stemming from vitamin B12 deficiency, characterised by the degeneration of the dorsal columns and corticospinal tracts. Key signs and symptoms include symmetrical distal sensory symptoms, diminished vibration sense and proprioception in the legs, and mixed upper and lower motor neuron signs. Investigations primarily involve B12, folate levels, and homocysteine levels, in addition to MRI of the spine and nerve conduction studies. The primary management strategy involves B12 replacement therapy.

# Definition

Sub-acute combined degeneration of the cord (SCDC) is a neurological complication associated with vitamin B12 deficiency. It is characterised by degeneration of both the dorsal columns and corticospinal tracts, whilst preserving pain and temperature sensations. It typically manifests as symmetrical sensory symptoms starting distally and progressing proximally, along with varying degrees of ataxia.

# Aetiology

The underlying cause of SCDC is vitamin B12 deficiency, which can result from various factors including pernicious anemia, malabsorption syndromes, dietary deficiencies, and misuse of nitrous oxide (this causes a **functional** rather than a true B12 deficiency) .

# Signs and Symptoms

- Symmetrical distal sensory symptoms, typically beginning in the feet and progressing to the hands
- Varying degrees of ataxia
- Diminished vibration sense and proprioception in the legs
- Mixed upper and lower motor neuron signs, which may present as exaggerated, diminished, or absent limb reflexes
- Autonomic bladder or bowel symptoms
- SCDC may manifest in both the presence and absence of haematological manifestations.

# Investigations

Diagnostic workup for SCDC should include:

- Assessment of B12 and folate levels
- Homocysteine level analysis; a raised level despite normal B12 could indicate a functional B12 deficiency
- MRI of the spine to exclude cervical myelopathy
- Nerve conduction studies, which will primarily show axonal neuropathy

# Management

The mainstay of treatment for SCDC is:

- Urgent Vitamin B12 replacement, typically using hydroxocobalamin 1 mg IM on alternate days until there is no further improvement, followed by maintenance treatment with hydroxocobalamin 1 mg IM every two months, as per [NICE guidelines](https://cks.nice.org.uk/topics/anaemia-b12-folate-deficiency/).
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1976
Subacute combined Degeneration of the Cord
Topic
34
Neurology
6618
Excessive nitrous oxide inhalation can lead to subacute combined degeneration of the cord due to vitamin B12 deficiency.
A 21-year-old woman presents to the emergency department having tripped and fallen onto her right hand. X-ray shows a minimally-displaced clavicular fracture.

On neurological exam, she has bilateral loss of vibration sense in her hands and feet with peripheral sensory loss. She has brisk knee reflexes and upgoing plantars bilaterally.

On further questioning, she states she has been tripping and falling more frequently recently. She reveals that she has been inhaling nitrous oxide recreationally every day for the last 6 months.

Which of the following is the most likely diagnosis?
a