Thanks to James for presenting the case of an elderly woman with back pain and lower extremity weakness with weight loss who was found to have a new diagnosis of multiple myeloma. We’ve done myeloma cases before, but a few interesting new issues jumped out at me, see pearls below!
Low anion gap:
Normal range: Varies, traditionally 8-16
Taken from Kraut 2007 (Serum Anion Gap: Its Uses and Limitations in Clinical Medicine- Thanks Harry!):
Causes of low anion gap:
- Lab error
- Hypoalbuminemia (correct the gap for albumin!)
- Monoclonal or polyclonal gammopathies (including multiple myeloma)
- Intoxications: bromide, lithium, iodide
Nerve root anatomy:
*Pearl: The dorsal (sensory) and ventral (motor) nerve roots combine to form spinal nerves (aka spinal nerve roots) BEFORE they exit through the neural foramina.
*Pearl: The two most common causes of lumbosacral radiculopathy are disc herniation or neural foraminal stenosis due to degenerative arthritis (both causing spinal nerve root compression).
*Pearl: Isolated motor radiculopathy is atypical. Usually, mild radiculopathy is sensory only, and motor + sensory symptoms arise with more severe radiculopathies since motor fibers are relatively resilient.
MGUS risk factors for progression to multiple myeloma:
- M-spike > 1.5 g/dL on SPEP
- Non-IgG MGUS (ie. IgA, IgM, IgD)
- Abnormal serum free light chain ratio (high or low!)
Multiple myeloma is risk-stratified by cytogenetic features (translocations/deletions). Prognosis can further be delineated by beta-2-microglobulin, albumin, and LDH (low B2M, high albumin, and low LDH are better!). Performance status, age, and other laboratory factors play a role as well.