To describe the clinical presentation and pathologic findings of a unique case of amyloid-like IgM deposition neuropathy with secondary microvasculitis.

Amyloid-like IgM deposition neuropathy is a distinct entity in which endoneurial perivascular entire IgM-particle accumulation and microvasculitis leads to a painful sensory followed by motor peripheral neuropathy. It has been described in the setting of IgM monoclonal gammopathy with IgM lambda deposition and it is important to distinguish it from AL-amyloid neuropathy characterized by congophilic light chain deposits.

A 77-year-old man presented with 2.5 years of progressive multiple mononeuropathies starting with painless right foot drop followed by pain and numbness spreading proximally up to lower abdomen and hands with asymmetric hand weakness. Electrodiagnostic findings showed severe axonal sensory-motor neuropathy superimposed by multiple mononeuropathies. Autonomic testing showed severe sudomotor and mild cardiovagal impairment with preserved adrenergic function. Laboratory investigations showed biclonal gammopathy of IgM kappa and IgA lambda and were otherwise unrevealing of alternative metabolic, autoimmune, or paraneoplastic etiologies. Abdominal fat aspirate and skin biopsy was negative for amyloid. Skeletal survey showed 2 discrete sclerotic foci within the bony pelvis. A right sural nerve biopsy was performed and showed multifocal axonal neuropathy with large endoneurial deposits of amorphous material with negative Congo red staining in addition to prominent microvasculitis. Mass spectrometry identified the deposits as kappa light chain and mu heavy chain revealing the diagnosis. This case offers multiple distinctive features including phenotypically with motor preceding sensory involvement and pathologically with strikingly large IgM kappa deposits, rather than lambda, replacing most of the endoneurium of involved fascicles.

Amyloid-like IgM deposition disease is a rare and important to recognize type of peripheral neuropathy associated with IgM monoclonal gammopathy and can present in varying clinical and pathological ways.