Neurology · Moderate
This case is fictional and exists for study. It describes no real patient.
62 years · male
A 62-year-old male presents to the outpatient clinic complaining of a 4-month history of progressive numbness, tingling, and a sensation of walking on cotton wool in both feet. He also reports unsteady gait, particularly in the dark. His medical history includes type 2 diabetes mellitus managed with metformin for 8 years. Physical examination demonstrates symmetrical reduction of vibration and joint position sense up to the ankles bilaterally. Pain and temperature sensation are mildly decreased in a stocking distribution. Ankle reflexes are absent, but knee reflexes are preserved. Romberg's test is positive.
Vital signs
- HR
- 76 bpm
- BP
- 130/82 mmHg
- RR
- 15/min
- Temp
- 36.7 °C
- SpO₂
- 98%
Examination findings
- Symmetrical sensory loss of proprioception and vibration up to the ankles
- Absent ankle reflexes bilaterally with preserved knee jerks
- Positive Romberg sign
Investigations
| Haemoglobin | 11.0 g/dL | reference: 13.0-17.0 |
| MCV | 105 fL | reference: 80-100 |
| Serum Vitamin B12 | 115 ng/L | reference: 190-900 |
| HbA1c | 54 mmol/mol | reference: 20-42 |
Question 1 of 3
Which of the following is the most likely primary cause of this patient's neurological symptoms?
Long-term metformin therapy impairs vitamin B12 absorption in the terminal ileum, leading to vitamin B12 deficiency neuropathy and subacute combined degeneration, characteristically accompanied by macrocytic anaemia (elevated MCV and low serum B12). While diabetic distal polyneuropathy is a common differential in diabetic patients, it does not explain the macrocytic anaemia or low vitamin B12 levels. Chronic inflammatory demyelinating polyneuropathy (CIDP) typically causes prominent motor weakness and hyporeflexia throughout. Lumbar spinal stenosis presents with neurogenic claudication worsened by walking and relieved by sitting.
Teaching point: Long-term metformin use is a key risk factor for vitamin B12 deficiency, which presents with macrocytic anaemia and peripheral sensory neuropathy.
Question 2 of 3
Which of the following is the most appropriate next step in management for this patient's condition?
Patients presenting with neurological impairment due to vitamin B12 deficiency require prompt parenteral replacement (e.g., intramuscular hydroxocobalamin injections given on alternate days until clinical improvement occurs, followed by maintenance therapy). Oral pregabalin treats neuropathic pain symptoms but does not arrest or reverse the underlying metabolic nerve damage. Metformin therapy should be reviewed, but stopping it abruptly without adjusting glycaemic control is unsafe. Urgent spinal MRI is unnecessary given the clear metabolic aetiology confirmed by laboratory findings.
Teaching point: Vitamin B12 deficiency presenting with neurological signs requires immediate parenteral cobalamin replacement to prevent irreversible damage.
Question 3 of 3
Which neurological pathway impairment best accounts for the reduced vibration sense and positive Romberg sign observed in this patient?
Vibration sense, proprioception, and fine touch are conveyed via large myelinated sensory fibers through the posterior (dorsal) columns of the spinal cord. Impairment in this pathway leads to loss of joint position sense and sensory ataxia, manifested as a positive Romberg sign (loss of balance when visual input is removed). Spinothalamic tracts transmit pain and temperature sensations. Anterior horn cell degeneration leads to lower motor neuron motor weakness without sensory loss. Cerebellar ataxia causes unsteadiness that persists even with eyes open (Romberg negative).
Teaching point: A positive Romberg sign with loss of vibration and joint position sense indicates dorsal column or large sensory nerve fibre dysfunction.
Key learning points
- Metformin can cause malabsorption of vitamin B12, necessitating periodic screening in patients on long-term therapy.
- Vitamin B12 deficiency causes subacute combined degeneration of the spinal cord and peripheral neuropathy, presenting with macrocytic anaemia and sensory ataxia.
- Neurological manifestations of vitamin B12 deficiency should be treated urgently with parenteral hydroxocobalamin to prevent permanent deficits.
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