Neurology · Moderate
This case is fictional and exists for study. It describes no real patient.
58 years · male
A 58-year-old male presents to the general practice clinic with a 4-month history of progressive numbness and tingling in both feet, extending up to his lower calves. He reports feeling unsteady when walking, particularly in the dark or when closing his eyes in the shower. He has a 10-year history of type 2 diabetes mellitus, which is well-controlled on metformin 1000 mg twice daily. Physical examination reveals reduced vibration and proprioception in both lower limbs up to the ankles, with impaired light touch sensation in a stocking distribution. Ankle reflexes are bilaterally absent, while knee reflexes are intact. Romberg's test is positive.
Vital signs
- HR
- 74 bpm
- BP
- 128/82 mmHg
- RR
- 14/min
- Temp
- 36.7 °C
- SpO₂
- 98%
Examination findings
- Bilateral loss of vibration sense and proprioception up to the ankles
- Impaired light touch sensation in a distal stocking distribution
- Absent ankle jerks bilaterally with normal knee jerks
- Positive Romberg sign
Investigations
| Haemoglobin | 11.2 g/dL | reference: 13.5-17.5 |
| Mean Corpuscular Volume (MCV) | 104 fL | reference: 80-100 |
| HbA1c | 48 mmol/mol | reference: <48 |
| Serum Creatinine | 82 µmol/L | reference: 60-110 |
Question 1 of 3
Which of the following blood tests is the most appropriate next step to identify the primary cause of this patient's neurological symptoms?
Long-term use of metformin (especially >4-5 years) is a well-established cause of vitamin B12 malabsorption, leading to B12 deficiency neuropathy and macrocytic anaemia. Given his prominent loss of proprioception/vibration (dorsal column involvement) and elevated MCV, checking serum vitamin B12 levels is the essential next step. Fasting blood glucose is unnecessary as his HbA1c shows good diabetic control. ESR and TSH can cause peripheral neuropathies, but macrocytic anaemia and long-term metformin use point directly to B12 deficiency.
Teaching point: Long-term metformin therapy is associated with vitamin B12 deficiency, which should be investigated in patients presenting with macrocytic anaemia or peripheral neuropathy.
Question 2 of 3
Laboratory investigation reveals a serum vitamin B12 level of 110 ng/L (reference range 180–900 ng/L). Which of the following is the most likely underlying diagnosis?
The combination of long-term metformin therapy, documented low serum B12, macrocytic anaemia, subacute sensory ataxia (impaired posterior column modalities such as proprioception and vibration), and absent ankle reflexes strongly points to metformin-induced B12 deficiency neuropathy. Diabetic distal symmetric polyneuropathy typically affects small fibers first (pain/temperature) before large fibers, and HbA1c here is well-controlled. CIDP usually causes prominent motor weakness, and there is no history of heavy alcohol intake.
Teaching point: Vitamin B12 deficiency preferentially affects large sensory nerve fibers and posterior columns, presenting with sensory ataxia and lost vibration/position sense.
Question 3 of 3
Which of the following is the most appropriate initial management step for this patient's neurological presentation?
According to UK (NICE) guidelines, patients presenting with vitamin B12 deficiency who have neurological involvement should receive parenteral vitamin B12 replacement with intramuscular hydroxycobalamin (1 mg on alternate days until there is no further improvement, followed by maintenance). Oral cyanocobalamin is inadequate for rapid repletion in patients with neurological deficits or malabsorption. Duloxetine is indicated for painful diabetic neuropathy, not sensory ataxia secondary to deficiency. Metformin should not be abruptly stopped without medical review and glycaemic management planning.
Teaching point: Vitamin B12 deficiency with neurological symptoms requires immediate treatment with intramuscular hydroxycobalamin rather than oral replacement.
Key learning points
- Long-term metformin use reduces intestinal absorption of vitamin B12 and can cause neurological impairment.
- B12 deficiency peripheral neuropathy and subacute combined degeneration preferentially affect large sensory fibers and posterior columns, producing sensory ataxia and a positive Romberg sign.
- Vitamin B12 deficiency presenting with neurological signs requires loading treatment with intramuscular hydroxycobalamin.
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