Objective To report the process of diagnosis, treatment of a patient with Exogenous insulin autoimmune syndrome (EIAS) combined with mitochondrial diabetes mellitus (MDM).

Methods The pathogenesis, medical history and relevant auxiliary examinations of a case admitted to the General Hospital of the Southern Theater of Operations of the People's Liberation Army (PLA) in December 2018 with EIAS combined with MDM were retrospectively analyzed, and the process of its definitive diagnosis, treatment, and special characteristics were summarized.

Results The patient was 30 years old, with a history of diabetes mellitus for more than 7 years, tinnitus and hearing loss appeared at the beginning of the disease, and various insulin dosage forms had been applied in external hospitals. During the course of treatment, hypoglycemia appeared, blisters appeared on the skin at the injection site, accompanied by itching of the skin, there was a significant increase in the level of insulin, which was "detached" from the result of C-peptide. The test showed that the insulin autoantibody (IAA) was positive, then the diagnosis of EIAS was considered. The insulin injection was discontinued, and oral hypoglycemic drugs such as metformin, DPP-Ⅳ inhibitors, and acarbose were given, then the insulin was gradually lowered to normal, and the blood glucose control was acceptable. However, neurological symptoms gradually appeared during the treatment, and the head MRI suggested that the brainstem and bilateral cerebellar hemispheres were atrophied. After reviewing the relevant literature and combining with the patient's medical history, the genetic test finally clarified that the patient's MDM was caused by the mutation of A3243G in MT-TL1 gene of mitochondrial DNA, and at the same time, the patient was detected to have high lactic acid, and metformin might aggravate the accumulation of lactic acid and even induce lactic acidosis, therefore, the patient stopped using metformin to lower glucose, and at the same time, the patient strengthened the control of diet and exercise. The patient was followed up regularly in outpatient clinic, and his blood glucose control was fair.

Conclusion In this case, EIAS is induced by exogenous insulin preparation after the initial diagnosis of diabetes mellitus, but due to the continuous progression of the disease, unexplained neurological symptoms appear, and MDM is clearly identified by genetic testing.