Maternally Inherited Diabetes and Deafness

Maternally Inherited Diabetes and Deafness (MIDD) is a rare genetic condition that primarily causes diabetes and hearing loss. It is caused by changes (mutations) in the DNA of mitochondria, which are the energy-producing powerhouses inside our cells. Because mitochondrial DNA is inherited only from mothers, this condition is passed down from a mother to her children. The symptoms usually start when a person is a young adult, often between the ages of 20 and 40. People with MIDD are typically not overweight, which makes their diabetes different from the more common type 2 diabetes. The hearing loss in MIDD is usually progressive, meaning it gets worse over time, and it often affects the ability to hear high-pitched sounds first. The diabetes happens because the pancreas cannot produce enough insulin, the hormone that controls blood sugar. While some people can manage their diabetes with diet or pills at first, most will eventually need insulin injections. MIDD can also affect the eyes, specifically the macula (the center of the retina), causing changes in pigmentation. However, this eye condition usually does not cause severe vision loss. Patients with MIDD should know that the condition can affect other parts of the body that require a lot of energy, such as the heart, kidneys, and muscles. Regular check-ups with specialists, including eye doctors, heart doctors, and hearing specialists, are important to monitor for any complications. It is also important to let your doctor know about your diagnosis, as certain diabetes medications might need to be avoided or used with caution. Genetic counseling can be very helpful for understanding how the condition might affect other family members.

Condition category: Mitochondrial Disorder

Prevalence: Accounts for 0.5% to 2.8% of patients with diabetes mellitus

Inheritance patterns: Mitochondrial

Age of onset: Second to fourth decade of life

Clinical overview: Maternally Inherited Diabetes and Deafness (MIDD) is a rare, multisystem mitochondrial disorder characterized primarily by the development of diabetes mellitus and sensorineural hearing loss. It is classified as a mitochondrial disorder and is caused by pathogenic variants in mitochondrial DNA (mtDNA), most commonly the m.3243A>G mutation in the MT-TL1 gene. The condition accounts for up to 2.8% of all diabetes cases but is frequently misdiagnosed as type 1 or type 2 diabetes due to overlapping clinical features. MIDD typically presents in young to middle-aged adults who are non-obese, with a strong maternal family history of diabetes and/or deafness. The clinical presentation of MIDD is highly variable due to heteroplasmy, the coexistence of mutated and wild-type mtDNA within cells. Diabetes in MIDD results from progressive pancreatic beta-cell failure due to impaired mitochondrial ATP production, which is essential for glucose-stimulated insulin secretion. While initially manageable with oral agents, the diabetes often progresses to require insulin therapy. Hearing loss is typically bilateral, sensorineural, and progressive, predominantly affecting high frequencies. In addition to diabetes and deafness, patients with MIDD may exhibit a range of other clinical features associated with mitochondrial dysfunction. A specific retinal lesion known as macular pattern dystrophy is common, though it rarely causes significant visual loss. Other potential manifestations include myopathy, cardiomyopathy (often hypertrophic), renal disease (frequently presenting as proteinuria), and central nervous system involvement such as stroke-like episodes or cognitive decline. Early recognition of MIDD is crucial for appropriate management, avoiding certain medications like high-dose metformin due to the theoretical risk of lactic acidosis, and screening for associated multisystem complications.

Patient and family guide: Maternally Inherited Diabetes and Deafness (MIDD) is a rare genetic condition that primarily causes diabetes and hearing loss. It is caused by changes (mutations) in the DNA of mitochondria, which are the energy-producing powerhouses inside our cells. Because mitochondrial DNA is inherited only from mothers, this condition is passed down from a mother to her children. The symptoms usually start when a person is a young adult, often between the ages of 20 and 40. People with MIDD are typically not overweight, which makes their diabetes different from the more common type 2 diabetes. The hearing loss in MIDD is usually progressive, meaning it gets worse over time, and it often affects the ability to hear high-pitched sounds first. The diabetes happens because the pancreas cannot produce enough insulin, the hormone that controls blood sugar. While some people can manage their diabetes with diet or pills at first, most will eventually need insulin injections. MIDD can also affect the eyes, specifically the macula (the center of the retina), causing changes in pigmentation. However, this eye condition usually does not cause severe vision loss. Patients with MIDD should know that the condition can affect other parts of the body that require a lot of energy, such as the heart, kidneys, and muscles. Regular check-ups with specialists, including eye doctors, heart doctors, and hearing specialists, are important to monitor for any complications. It is also important to let your doctor know about your diagnosis, as certain diabetes medications might need to be avoided or used with caution. Genetic counseling can be very helpful for understanding how the condition might affect other family members.

Symptoms and clinical features: Symptoms of MIDD typically begin in early adulthood (20s to 40s). The hallmark symptoms are the onset of diabetes mellitus and bilateral sensorineural hearing loss. The hearing loss is progressive, often starts with high frequencies, and may precede the onset of diabetes. Diabetes symptoms include polyuria, polydipsia, and weight loss, though patients are usually non-obese. Other symptoms can include visual disturbances or mild vision changes due to macular pattern dystrophy, though severe vision loss is rare. Patients may also experience muscle weakness, exercise intolerance, fatigue, cardiac palpitations or shortness of breath (if cardiomyopathy is present), and symptoms related to renal dysfunction or neurological issues like migraines or cognitive changes.

Molecular pathology: Maternally Inherited Diabetes and Deafness (MIDD) is primarily caused by mutations in mitochondrial DNA (mtDNA). The most common genetic alteration is an A-to-G transition at nucleotide position 3243 (m.3243A>G) in the MT-TL1 gene, which encodes the mitochondrial transfer RNA for leucine (tRNALeu(UUR)). Less frequently, mutations in other mitochondrial genes, such as MT-TE and MT-TK, have been implicated. These mutations disrupt the normal synthesis of mitochondrial proteins, leading to a deficiency in the mitochondrial respiratory chain complexes and a subsequent reduction in cellular ATP production. The pathogenesis of MIDD is closely linked to the high energy demands of specific tissues. In the pancreas, beta cells rely on ATP generated by mitochondria to couple glucose sensing with insulin secretion. Reduced ATP production impairs the closure of ATP-sensitive potassium channels, preventing membrane depolarization and the subsequent calcium influx required for insulin exocytosis. Over time, the energy deficit and potential oxidative stress lead to progressive beta-cell dysfunction and apoptosis, resulting in insulin deficiency. Similarly, the high metabolic requirements of the stria vascularis and hair cells in the inner ear make them highly susceptible to mitochondrial dysfunction, leading to sensorineural hearing loss. The retinal pigment epithelium is also affected, causing macular dystrophy.

Genetics: Maternally Inherited Diabetes and Deafness (MIDD) is caused by pathogenic variants in mitochondrial DNA (mtDNA). Because mitochondria are inherited exclusively from the mother, the condition follows a maternal inheritance pattern. An affected mother will transmit the mtDNA mutation to all of her children, but an affected father will not transmit the mutation to any of his children. The clinical expression of the disease is highly variable, even among family members, due to heteroplasmy—the presence of a mixture of mutated and normal mtDNA within a single cell. The proportion of mutated mtDNA must exceed a certain threshold for cellular dysfunction and clinical symptoms to occur. This threshold effect, combined with the random distribution of mutated mitochondria during cell division, leads to variable penetrance and expressivity. Some individuals carrying the mutation may remain asymptomatic if their heteroplasmy levels are low, while others may develop severe multisystem disease. Genetic counseling is crucial for affected families to explain the maternal inheritance pattern and the unpredictable nature of disease severity in offspring. Prenatal testing is technically possible but challenging to interpret due to the inability to accurately predict the heteroplasmy level and resulting phenotype in the child.

Diagnostic evaluation: Diagnosis of MIDD is based on clinical features including maternal inheritance of diabetes or impaired glucose tolerance, sensorineural hearing loss, and maculopathy. The diagnosis is confirmed by molecular genetic testing identifying a pathogenic variant in mitochondrial DNA, most commonly the m.3243A>G mutation in the MT-TL1 gene. The mutation is often present in a heteroplasmic state, meaning a mixture of mutant and wild-type mtDNA exists within cells. Testing can be performed on peripheral blood, but due to declining heteroplasmy levels in blood with age, testing of other tissues like buccal mucosa, hair follicles, or urinary sediment may be necessary if blood testing is negative but clinical suspicion remains high. Additional evaluations include audiometry to assess hearing loss, fundoscopy for macular dystrophy, and cardiac evaluation (ECG, echocardiography) to check for cardiomyopathy or conduction defects.

Differential diagnosis: Type 1 Diabetes Mellitus, Type 2 Diabetes Mellitus, Latent Autoimmune Diabetes in Adults (LADA), Wolfram Syndrome, Rogers Syndrome, Herrmann Syndrome, MELAS Syndrome, Kearns-Sayre Syndrome

Natural history: The natural history of MIDD typically begins with the onset of sensorineural hearing loss or diabetes in early adulthood, usually between the second and fourth decades of life. Hearing loss often precedes the onset of diabetes and is progressive, initially affecting high frequencies. Diabetes usually presents insidiously, resembling type 2 diabetes, and can initially be managed with diet or oral hypoglycemic agents. However, due to the progressive decline in pancreatic beta-cell mass and function, approximately half of the patients require insulin therapy within 10 years of diagnosis. Macular dystrophy, characterized by pigmentary changes, may develop but rarely leads to severe visual impairment. Over time, patients may develop other mitochondrial disease manifestations, including myopathy, cardiomyopathy, renal disease (such as focal segmental glomerulosclerosis), and neurological symptoms. The progression and severity of the disease are highly variable and depend on the tissue-specific heteroplasmy levels of the mtDNA mutation.

Management and treatment research: ### Current management and monitoring Maternally Inherited Diabetes and Deafness (MIDD) is most often caused by a change in mitochondrial DNA. Because mitochondria help cells produce energy, MIDD can affect multiple organs. Care is individualized and may involve endocrinology, audiology, ophthalmology, cardiology, nephrology, and genetics specialists. - **Diabetes management:** Healthy eating, physical activity, glucose monitoring, and diabetes medicines are tailored to the individual. The insulin-producing beta cells of the pancreas may gradually lose function, so many people with MIDD eventually need insulin. Treatment needs should be reviewed regularly. - **Diabetes medicines:** Metformin may be used cautiously in selected individuals. Clinicians consider kidney function, other medical conditions, and the risk of lactic acidosis—an uncommon but serious buildup of lactic acid that can occur in certain settings of mitochondrial dysfunction. Other glucose-lowering medicines may be appropriate based on a person’s diabetes and overall health. - **Hearing loss:** Regular hearing evaluations are important. Hearing aids may help, and cochlear implants may be considered for some people with severe hearing loss. - **Eye care:** Regular dilated eye examinations and retinal imaging can monitor macular dystrophy and other retinal changes. There is no established treatment that reverses retinal changes caused by MIDD. - **Kidney and heart monitoring:** Monitoring may include blood pressure checks, urine testing for protein, kidney-function blood tests, electrocardiograms, and heart imaging when indicated. Kidney disease, high blood pressure, and cardiomyopathy are treated using standard approaches. ACE inhibitors or ARBs may be used when appropriate for blood pressure control and kidney protection. - **Genetic counseling:** Mitochondrial DNA is generally inherited through the mother. Genetic counseling can help affected people and relatives understand inheritance, genetic testing, and family-planning options. Some clinicians recommend supplements such as coenzyme Q10. However, strong clinical-trial evidence that supplements improve MIDD symptoms or slow progression is limited. ### Approved therapies There are no therapies approved specifically to treat the underlying mitochondrial genetic cause of MIDD or to reverse MIDD-associated retinal disease. Current care focuses on management of diabetes, hearing loss, retinal monitoring, and other possible complications. ### Investigational therapies There are no MIDD-specific investigational treatment programs in the current treatment pipeline data. Research into mitochondrial diseases more broadly is ongoing. Potential treatments must be studied for safety and benefit in people with specific mitochondrial conditions, including MIDD, before they can be considered established treatments. ### Clinical trial participation Current recruiting studies that may include people with mitochondrial disease are registries and natural-history studies. These studies do not test a new treatment, but they can help researchers understand disease progression, collect samples, and support future research. - **NCT01694940** — North American Mitochondrial Disease Consortium Patient Registry and Biorepository (NAMDC); recruiting. - **NCT01793168** — Rare Disease Patient Registry & Natural History Study – Coordination of Rare Diseases at Sanford; recruiting. Eligibility varies by study. Participation may include sharing medical information, providing blood or other samples, and completing follow-up visits.

Outlook: The prognosis for patients with MIDD is variable and depends heavily on the degree of heteroplasmy and the specific organs involved. The diabetes is typically progressive, with most patients eventually requiring insulin therapy due to beta-cell failure. Hearing loss is also progressive and often requires hearing aids. While macular dystrophy is common, it generally does not lead to severe visual impairment or blindness. However, the presence of other systemic complications, such as cardiomyopathy, renal failure, or severe neurological involvement, can significantly impact life expectancy and quality of life. Regular multidisciplinary monitoring is essential to manage complications early and optimize outcomes.

Epidemiology: MIDD is estimated to account for 0.5% to 2.8% of all patients with diabetes mellitus. It is a rare disorder but is considered underdiagnosed due to misclassification as type 1 or type 2 diabetes. The condition has been reported worldwide, with the highest number of cases identified in Asian populations, followed by European populations. The mean age of onset for diabetes is typically in the late 30s, though it can present at any age. Patients often present with a normal or low body mass index (BMI) and are rarely obese. Maternal family history of diabetes and/or deafness is present in the majority of cases.

Selected references: 1. Ballinger SW, et al. Maternally transmitted diabetes and deafness associated with a 10.4 kb mitochondrial DNA deletion. Nat Genet. 1992. 2. van den Ouweland JM, et al. Mutation in mitochondrial tRNA(Leu)(UUR) gene in a large pedigree with maternally transmitted type II diabetes mellitus and deafness. Nat Genet. 1992. 3. Guillausseau PJ, et al. Maternally inherited diabetes and deafness: a multicenter study. Ann Intern Med. 2001. 4. Murphy R, et al. Clinical features, diagnosis and management of maternally inherited diabetes and deafness (MIDD) associated with the 3243A>G mitochondrial point mutation. Diabet Med. 2008. 5. Yang M, et al. The Mutations and Clinical Variability in Maternally Inherited Diabetes and Deafness: An Analysis of 161 Patients. Front Endocrinol (Lausanne). 2021.