Deafness-dystonia-optic neuronopathy syndrome

Key Points at a Glance

1. Deafness-Dystonia-Optic Neuronopathy Syndrome

Deafness-Dystonia-Optic Neuronopathy (DDON) syndrome is a rare neurodegenerative disorder caused by mutations in the TIMM8A gene. It follows an X-linked recessive inheritance pattern. It typically begins with sensorineural hearing loss in early childhood, followed by movement disorders (dystonia, ataxia) in adolescence, visual decline in young adulthood, and dementia in middle age.

This condition was previously known as Mohr-Tranebjaerg syndrome (MTS). Opticoacoustic nerve atrophy (Jensen syndrome) and deafness-dystonia syndrome are the same entity. The term “neuronopathy” rather than “neuropathy” is used to reflect the loss of neuronal cell bodies in the central nervous system rather than peripheral nerve involvement.

Epidemiology

First reported in a single Norwegian family in 1960. The exact prevalence is unknown. As of 2012, 91 patients from 37 families had been identified. Cases have been reported in various populations worldwide, including Japanese, Chinese, Filipino, Spanish, and African American individuals, not limited to Nordic populations. Due to X-linked recessive inheritance, it primarily affects males. Female carriers are usually mildly affected or asymptomatic.

Q What is the inheritance pattern of DDON?

Because it follows an X-linked recessive inheritance pattern, it primarily affects males. Females can be carriers but usually have mild or no clinical symptoms. For details, see the “Causes and Risk Factors” section.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

The symptoms of DDON appear in stages.

Early Childhood

Sensorineural hearing loss: Onset around 18 months on average. May be congenital. Progresses to severe within the first 10 years.

Intellectual disability/psychiatric symptoms: May appear intermittently.

Adolescence

Dystonia: Presents with repetitive movements or abnormal postures due to involuntary muscle contractions. May be focal, such as torticollis, writer’s cramp, or blepharospasm.

Ataxia: Impaired coordination of voluntary muscles. More common in the upper body and arms.

Adulthood

Decreased visual acuity: Often begins with photophobia around age 15. Corrected visual acuity may drop to 0.2 or less by the mid-30s.

Dementia: Onset around age 40.

Gait disturbance: Mobility becomes limited due to progression of motor impairment.

Early visual symptoms include photophobia, blurred vision, and color vision abnormalities. Decreased visual acuity always appears after sensorineural hearing loss. In some families, visual impairment may not be observed until the 30s, and there are reports of no visual impairment in men aged 33 and 35, indicating wide phenotypic variability. Blepharospasm occurs in both men and women as a form of focal dystonia.

Clinical Findings (Findings Confirmed by Physician Examination)

Fundus examination: Normal in childhood. As the disease progresses, optic disc pallor appears. Initially, temporal pallor is observed, which then progresses to the entire disc. The retina maintains a normal appearance throughout the disease.
Visual field: Advanced cases show concentric visual field constriction. Constriction to 30 degrees in both eyes at age 32 and to 5 degrees at age 49 has been reported.
Color vision: Abnormalities are detected by Ishihara test from the late 20s to 30s. Some cases progress to complete achromatopsia by age 32.
Visual evoked potentials (VEP): Prolongation of P100 latency is the earliest marker. Abnormal values can be detected even when subjective symptoms and fundus findings are normal. As the disease progresses, the response disappears.
Electroretinogram (ERG): Normal in most cases, suggesting a central etiology for vision loss.
Neurological findings: Hyperreflexia, positive Babinski sign, and ankle clonus may be present.
MRI: Cortical atrophy of the occipital and parietal lobes is observed, progressing to generalized cortical atrophy around age 40.
PET: Hypometabolism is seen in the occipital lobe, parietal lobe, and basal ganglia.

Postmortem histopathology shows marked atrophy of the retinal ganglion cell layer, vacuolation and loss of optic nerve fibers, and spongiosis and neuronal loss in the striate cortex along the calcarine sulcus. In the cochlea, hair cells and the organ of Corti are preserved, but cochlear nerve cells are almost completely lost (true auditory neuropathy).

Q Is VEP testing useful for early detection of DDON?

Prolongation of P100 latency on visual evoked potentials (VEP) can be detected even before subjective symptoms or fundus abnormalities appear. In one case, VEP abnormalities were recorded at age 10, but fundus examination was normal at age 21. When DDON is suspected, VEP is a useful test for detecting early visual pathway impairment.

3. Causes and Risk Factors

The causative gene for DDON is TIMM8A, located at Xq22.1. There are three mechanisms of onset.

X-linked recessive inheritance: Pathogenic variants are inherited from the mother.
De novo mutation: Onset occurs without a family history.
Contiguous gene deletion: Caused by a large deletion including TIMM8A.

Approximately 20 pathogenic variants of the TIMM8A gene have been reported. No correlation between genotype and clinical severity has been established.

The TIMM8A gene is adjacent to the Bruton tyrosine kinase (BTK) gene. In contiguous gene deletions involving both genes, DDON is complicated by X-linked agammaglobulinemia (XLA).

Q What happens with contiguous gene deletion of the TIMM8A gene?

The TIMM8A gene is adjacent to the BTK gene, so a large deletion involving both genes can cause DDON combined with X-linked agammaglobulinemia (XLA). XLA is an immunodeficiency syndrome that increases the risk of infections.

4. Diagnosis and Testing Methods

A definitive diagnosis of DDON is based on the identification of a pathogenic variant in the TIMM8A gene.

Genetic testing: Multi-gene panel testing evaluates genes associated with auditory neuropathy. Whole exome sequencing (WES) is also useful ¹⁾.
Chromosomal microarray: Used to detect and determine the extent of deletions when X-linked agammaglobulinemia (XLA) is suspected.
VEP: Prolonged P100 latency can be an early marker.
Electroretinogram: Evaluates the presence of retinal pathology. A normal result supports a central etiology.
MRI: Used to assess cortical atrophy. Iron deposition in the basal ganglia has also been reported ¹⁾.
PET: Evaluates decreased metabolism in the occipital lobe and basal ganglia.

Differential Diagnosis

The main diseases that require differentiation from DDON are listed below.

Disease name	Main differences
Wolfram syndrome	Associated with type 1 diabetes mellitus and diabetes insipidus
Usher syndrome	Associated with retinitis pigmentosa
ADOA (autosomal dominant optic atrophy)	OPA1 mutation. No sex difference
MELAS	Associated with stroke-like episodes

Autosomal dominant optic atrophy (ADOA) is a mitochondrial disease caused by OPA1 gene mutations, typically detected as bilateral visual impairment during school age. It shows no gender difference and presents with acquired tritanopia. OCT reveals thinning of the papillomacular bundle. It is differentiated from DDON by inheritance pattern, presence of hearing loss, and presence of dystonia.

5. Standard Treatment

There is no specific treatment for DDON. Management focuses on symptomatic treatment and multidisciplinary collaboration.

Visual management: Regular ophthalmologic examinations (visual acuity, color vision, visual field, VEP, fundus) are recommended. Refractive correction for best corrected visual acuity should be performed.
Low vision care: Use of visual aids and early intervention are recommended. For hereditary optic neuropathies in general, low vision care and patient counseling are the mainstays in the absence of effective treatment.
Auditory management: Consider the indication for hearing aids or cochlear implants.
Motor impairment management: Maintain gross motor skills through physical therapy.
Dysphagia: Perform swallowing function evaluation and training through speech therapy¹⁾.
Genetic counseling: Provide options for carrier screening and prenatal diagnosis.
Social support: Facilitate strong social support and collaboration with patient and family services to promote positive adaptation from an early age.

Q Is there a fundamental treatment for DDON?

Currently, there is no specific treatment to stop disease progression. Management focuses on symptomatic therapy for visual, auditory, and motor impairments. However, since it is a monogenic disease, there is hope for future targeted therapies. For details, see the section on “Latest Research”.

6. Pathophysiology and Detailed Mechanisms

The TIMM8A gene encodes a protein of the same name localized in the mitochondrial intermembrane space. The TIMM8A protein forms a complex with other small TIMM proteins and functions as a chaperone that transports nuclear-encoded proteins across the intermembrane space to the inner mitochondrial membrane.

Loss of TIMM8A function causes neuronal degeneration through the following mechanisms.

Impaired assembly of the DDP1/TIMM8a-TIMM13 complex: Tim23 levels in the inner mitochondrial membrane decrease, impairing protein transport¹⁾.
Mitochondrial morphological abnormalities: In DDON cell lines, elongated abnormal mitochondria have been observed. This reflects dysregulation of fusion and fission.
Oxidative stress: Inhibition of electron transport chain complex IV produces reactive oxygen species (ROS). Pro-apoptotic factors such as cytochrome c release are induced.
Iron metabolism abnormalities: In some cases, iron deposition in the basal ganglia has been confirmed, suggesting involvement of disrupted iron metabolism regulation due to mitochondrial dysfunction and ferroptosis (cell death caused by iron-dependent lipid peroxidation)¹⁾.

The TIMM8A gene product shows increased expression in large neurons of the central nervous system, brainstem, cerebellum, and basal ganglia. Therefore, these sites are selectively affected.

Regarding the visual pathway, in addition to loss of retinal ganglion cells and degeneration of optic nerve fibers, neuronal loss occurs in the striate cortex, particularly around the calcarine sulcus. The pattern of hypometabolism in the occipital lobe preceding diffuse cortical atrophy cannot be explained by peripheral damage alone, and extensive central nervous system degeneration is a feature of DDON.

In autosomal dominant optic atrophy (ADOA), mutations in the OPA1 protein produce short, spherical mitochondria, which is the opposite of the elongated morphology in DDON. However, both diseases are characterized by sensorineural hearing loss and optic atrophy, with dysregulation of mitochondrial fusion and fission as a common pathological basis.

7. Latest Research and Future Prospects (Research-stage Reports)

Since DDON is caused by a single gene abnormality, the development of targeted therapy is expected.

Mutation-specific readthrough therapy: An approach that induces “readthrough” of premature stop codons to produce full-length TIMM8A protein has been proposed. It is based on a mechanism similar to drugs used for cystic fibrosis.

Iron chelation therapy:

Ventura et al. (2025) reported a 16-year-old male with a novel TIMM8A mutation (c.98_101dupAGCA; p.Leu35fs) presenting with dystonia and basal ganglia iron deposition without hearing loss ¹⁾. The authors suggested that abnormal iron metabolism due to mitochondrial dysfunction may promote neurodegeneration via ferroptosis, and proposed that iron chelators such as deferoxamine could be therapeutic candidates. Experimental administration of pantothenic acid and pantethine was also initiated in this patient, but the effects are under evaluation.

Gene therapy: Approaches such as allotopic gene expression, gene editing, and stem cell therapy are being studied for hereditary optic neuropathies in general. Although no clinical reports of gene therapy specific to DDON have been published, it is theoretically a promising target because it is a monogenic disease.

8. References

Ventura I, Revert-Ros F, Revert F, Prieto-Ruiz JA, Hernández-Andreu JM. A novel TIMM8A mutation in Mohr-Tranebjaerg syndrome without hearing loss and with basal ganglia iron deposition. Orphanet J Rare Dis. 2025;20:327.
Adam MP, Bick S, Mirzaa GM, Pagon RA, Wallace SE, Amemiya A, et al. Deafness-Dystonia-Optic Neuronopathy Syndrome. . 1993. PMID: 20301395.
Neighbors A, Moss T, Holloway L, Yu SH, Annese F, Skinner S, et al. Functional analysis of a novel mutation in the TIMM8A gene that causes deafness-dystonia-optic neuronopathy syndrome. Mol Genet Genomic Med. 2020;8(3):e1121. PMID: 31903733.

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