The issuer credit ratings for Springfield-based Massachusetts Mutual Life Insurance Co. (MassMutual) have been downgraded to "aa+" from "aaa," according to rating company A.M. Best Co.

The Oldwick, N.J.-based firm said it also affirmed the financial strength rating (FSR) of A++ (superior) of MassMutual and its life/health subsidiaries.

The downgrading of the group’s ICRs and debt ratings reflects A.M. Best’s view that the volatility inherent in MassMutual’s business profile and investment mix within the current economic environment exceeds A.M. Best’s expectations for the "aaa" rating category, the firm said.

For the 18 months ending March 31, impairment write-downs totaled more than $2.5 billion, which surpassed the weighted average for much of the industry. MassMutual also is facing challenges related to some of its asset management subsidiaries that include addressing lawsuits and improving investment performance.