MKSAP quiz: New unsteadiness while walking in type 2 diabetes
This month's quiz asks readers to evaluate a 66-year-old man with type 2 diabetes, diagnosed 6 years ago, who has had fatigue and unsteadiness while walking for the past 4 months.
A 66-year-old man is evaluated for management of type 2 diabetes mellitus, diagnosed 6 years ago. For the past 4 months, he has had fatigue and unsteadiness while walking. Medications are metformin and glipizide, which he has been taking since his diagnosis.
On physical examination, vital signs are normal. BMI is 28. He has decreased vibratory sense in the great toes and loss of patellar and Achilles reflexes. The remainder of the examination is normal.
Laboratory studies show hemoglobin A1c of 7.4%, mean corpuscular volume of 115 fL, hematocrit of 35%, and creatinine of 1.3 mg/dL (115 μmol/L).
Which of the following is the most appropriate diagnostic test?
A. Electromyelography and nerve conduction studies
B. MRI of the spine
C. Serum vitamin B6 measurement
D. Serum vitamin B12 measurement
MKSAP Answer and Critique
The correct answer is D. Serum vitamin B12 measurement. This item is available to MKSAP 19 subscribers as item 13 in the Endocrinology and Metabolism section. More information about MKSAP is online.
The most appropriate diagnostic test to perform next is serum vitamin B12 measurement (Option D). This patient with type 2 diabetes mellitus has new neuropathic symptoms that have developed during the past 4 months. His examination findings of decreased vibratory sense and areflexia suggest posterior column disease. Laboratory findings reveal a macrocytic anemia. The combination of neurologic findings and anemia in a patient taking metformin for several years is consistent with vitamin B12 deficiency. Although the mechanism is not entirely understood, it is believed to be related to interference with the absorption of food-derived B12 at the level of the ileum. A high level of suspicion for vitamin B12 deficiency is needed in a patient with type 2 diabetes taking long-term metformin who develops peripheral polyneuropathy. Anemia and macrocytosis may not be present, and neurologic symptoms and findings may be the only manifestation of vitamin B12 deficiency. A serum vitamin B12 level test is most often used for the initial assessment. Treatment is oral or parenteral vitamin B12 replacement; metformin may be continued. The prevalence of vitamin B12 deficiency approaches 20% in patients taking metformin for 5 years. Therefore, vitamin B12 levels should be checked annually in patients receiving long-term metformin therapy.
Although electromyelography and nerve conduction studies (Option A) would likely be abnormal in this patient, they would not help determine the cause of peripheral polyneuropathy because these tests are not specific for subacute combined degeneration.
This patient's presentation is not consistent with a compressive spinal or nerve root process such as a herniated disc or an epidural mass and cannot account for the patient's macrocytic anemia. The depressed reflexes are also inconsistent with a process limited to the spinal cord; therefore, an MRI of the spine (Option B) is unnecessary.
Vitamin B6 (pyridoxine) deficiency presents as nonspecific stomatitis, glossitis, cheilosis, confusion, and bilateral distal limb numbness and burning paresthesia. Distal limb weakness is rare. This patient's presentation is not consistent with this deficiency, so vitamin B6 measurement (Option C) is unnecessary.
Key Points
- Long-term use of metformin is associated with an increased risk for developing vitamin B12 deficiency.
- Vitamin B12 levels should be measured annually in patients receiving long-term metformin therapy.