Peripheral Neuropathy in MGUS and Progression to Amyloid Light-Chain Amyloidosis: A Population-Based Study Including 15,351 MGUS Cases
Authors: Rögnvaldsson Sæmundur , Turesson Ingemar , Björkholm Magnus , Landgren Ola , Kristinsson Sigurður Yngvi 
Published: November 2019

Introduction
Peripheral neuropathy (PN) is a common disorder that can be caused by amyloid light-chain amyloidosis (AL). AL is a rare disorder caused by the deposition of amyloid fibers, originating from malignant plasma cells. Amyloid deposition in peripheral nerves causes PN and is present in 35% of patients with newly diagnosed AL. Diagnosis of AL can be difficult, leading to under-recognition, diagnostic delay, and delayed treatment. Virtually all instances of AL are preceded by monoclonal gammopathy of undetermined significance (MGUS). MGUS is relatively common with a reported prevalence of 4.2% in the general Caucasian population over the age of 50 years. Although MGUS is usually considered asymptomatic, a significant proportion of affected individuals develop PN. However, we are not aware of any studies assessing how PN affects risk of MGUS progression to AL. We were therefore motivated to conduct a large population-based study including 15,351 Swedish individuals with MGUS diagnosed 1986-2013.
Methods
Participants diagnosed with MGUS between 1986-2013 were recruited from a registry of a nationwide network of hematology- and oncology centers and the Swedish Patient Registry. We then cross-linked data on recorded diagnoses of AL and PN from the Swedish Patient Registry, diagnoses of lymphoproliferative disorders form the Swedish Cancer Registry, and dates of death from the Cause of Death Registry to our study cohort. Individuals with a previous history of other lymphoproliferative disorders were excluded from the study. A multi-state survival model was created. At inclusion, participants started providing person time into the PN or the non-PN states depending on whether they had a previous diagnosis of PN. Those with MGUS who developed PN after inclusion were included into the PN state at the time of PN diagnosis and provided person time in the PN state after that. We then created a Cox proportional hazard regression model with AL as the endpoint. Participants were censored at diagnosis of other lymphoproliferative disorders. We adjusted for sex, age, and year of MGUS diagnosis.
Results
We included 15,351 participants with MGUS. Of those, 996 participants provided person-time with PN (6.5%). About half of those had PN at MGUS diagnosis (55%). A total of 174 cases of AL were recorded, with AL being more common among those who had PN (2.1% vs 1.0% p=0.002). Those who had PN had a 2.3-fold increased risk of AL as compared to those who did not have PN (hazard ratio (HR): 2.3; 95% confidence interval (95% CI): 1.5-3.7; p<0.001). The results were similar for those who had PN at MGUS diagnosis and those who did not. More than half of AL cases (53%) were diagnosed within one year after MGUS diagnosis. The rate was even higher among those with PN, with 82% of AL cases among those who presented with PN being diagnosed within one year after MGUS diagnosis. In the first year after inclusion, the incidence of AL was 15.2 and 6.1 per 1000 person-years for participants with and without PN respectively (HR: 1.8; 95% CI:1.0-3.4; p=0.04). Participants with PN continued to have an increased risk of progression to AL after the first year with an incidence of AL of 2.6 per 1000 person-years as compared to 1.1 per 1000 person-years among participants who did not have PN (HR:2.4; 95% CI: 1.1-5.0; p=0.02) (Figure).
Discussion
In this large population-based study, including 15,351 individuals with MGUS, we found that individuals with MGUS who develop PN have an increased risk of progression to AL. In fact, individuals with MGUS who have PN at MGUS diagnosis might already have AL. This risk of AL was highest during the first year after MGUS diagnosis with participants with PN having a higher risk than those who did not have PN. PN continued to be associated with a higher risk of MGUS progression to AL throughout the study period. This is the largest study that we are aware of assessing the association of PN and MGUS progression to AL. Since this is a registry-based study based on recorded diagnoses, some clinical data, including MGUS isotype, is not available. These findings suggest that increased awareness of PN as a feature of MGUS might decrease diagnostic delay and improve outcomes for patients with AL.
Figure

Disclosures
Landgren: Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Theradex: Other: IDMC; Adaptive: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Other: IDMC; Abbvie: Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.