MOFFITT AM REPORT PEARLS 7/20/16: Low Anion Gap and Spinal Nerve Roots!

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)
  • Hypercalcemia/magnesemia
  • Intoxications: bromide, lithium, iodide


Nerve root anatomy:

Spinal nerve.jpg

*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.


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