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PubMed This is a summary of 16 peer-reviewed journal articles Updated
Neurology · Machado-Joseph Disease

What is the Difference Between MJD and SCA3?

At a Glance

There is no difference between Machado-Joseph Disease (MJD) and Spinocerebellar Ataxia type 3 (SCA3)—they are the exact same condition. MJD is the historical name, while SCA3 is the modern genetic classification for this neurological disorder caused by an ATXN3 gene mutation.

In this answer

3 sections

01

Why Are There Two Names?

02

Understanding the Genetics: CAG Repeats

03

Hallmark Symptoms of SCA3/MJD

The short answer is that there is no difference: Machado-Joseph Disease (MJD) and Spinocerebellar Ataxia type 3 (SCA3) are two different names for the exact same condition [1][2][3].

If you were diagnosed with MJD or SCA3, you have the most common genetic subtype of Autosomal Dominant Cerebellar Ataxia (ADCA) worldwide [4][5][6].

Why Are There Two Names?

The confusion comes from how diseases are historically named versus how they are classified by modern genetics.

  • Machado-Joseph Disease: This is the older, historical name [7]. In the 1970s, doctors first identified this specific pattern of symptoms in families of Portuguese-Azorean descent (such as the Machado and Joseph families). For years, it was simply known by this name.
  • Spinocerebellar Ataxia type 3 (SCA3): As genetic testing advanced, scientists began mapping out the exact genetic causes of different ataxias [8]. They started naming them “SCA1,” “SCA2,” “SCA3,” and so on, based on the order in which the gene mutations were discovered. When they found the gene mutation responsible for MJD, it was the third SCA gene discovered, so it was officially named SCA3 [9][2].

Today, the medical community uses MJD, SCA3, or the combined term “SCA3/MJD” interchangeably [1].

Understanding the Genetics: CAG Repeats

Both names refer to a progressive, neurodegenerative disorder caused by a specific mutation in the ATXN3 gene [8][9]. This mutation is called a CAG repeat expansion [2]. This means a specific sequence of DNA (the letters C-A-G) repeats too many times. If you look at your genetic test results, you will likely see a “CAG repeat length” or number. This number is important because the length of the expansion often influences the age your symptoms begin and how quickly they progress [2][10].

Additionally, SCA3/MJD is an autosomal dominant condition [8]. This means that it only takes one copy of the mutated gene to cause the disease. If you have the condition, each of your biological children has a 50% chance of inheriting the mutation.

Hallmark Symptoms of SCA3/MJD

Because it is an ataxia, the primary feature of SCA3 is difficulty with coordination and balance. However, the condition presents with a multifaceted clinical picture that includes both motor (movement) and non-motor symptoms that progress over time [11][12].

Some of the signature features that help distinguish SCA3/MJD from other types of ataxia include:

  • Ocular Movement Abnormalities: Issues with eye movements, such as a bulging appearance of the eyes, double vision, or sluggish eye tracking, are characteristic features of SCA3 and can sometimes appear even before balance issues begin [13][14].
  • Dystonia and Muscle Symptoms: Dystonia refers to involuntary, sustained muscle contractions that cause repetitive twisting movements or abnormal postures [15]. It is a frequent and sometimes disabling feature in MJD, sometimes taking the form of task-specific difficulties like writer’s cramp [15][16]. Patients may also experience muscle stiffness (spasticity) or slowness of movement (parkinsonism).
  • Speech and Swallowing Difficulties: As the condition progresses, issues with muscle coordination often lead to dysarthria (slurred speech) and dysphagia (difficulty swallowing) [11][12].
  • Sleep Disturbances: Non-motor symptoms are also very common, notably sleep disorders like restless legs syndrome (an uncontrollable urge to move the legs) [11][12].

Common questions in this guide

Are Machado-Joseph Disease and SCA3 the same thing?
No, there is no difference. Machado-Joseph Disease (MJD) and Spinocerebellar Ataxia type 3 (SCA3) are two different names for the exact same neurological condition. MJD is the older historical name, while SCA3 is the modern genetic classification.
What causes SCA3 or MJD?
The condition is caused by a genetic mutation in the ATXN3 gene, specifically a CAG repeat expansion. It is an autosomal dominant disorder, which means it only takes one mutated copy of the gene from either parent to inherit the disease.
What does the CAG repeat number mean in my MJD genetic test?
The CAG repeat number indicates how many times a specific DNA sequence repeats in your ATXN3 gene. This number is highly important because the length of the repeat expansion often influences what age your symptoms will begin and how quickly the disease will progress.
What are the hallmark symptoms of SCA3?
While the primary symptom is ataxia (difficulty with balance and coordination), early signs often include abnormal eye movements, such as bulging eyes or double vision. As the disease progresses, patients may experience muscle twisting (dystonia), slurred speech, difficulty swallowing, and sleep disorders.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my exact CAG repeat number on the ATXN3 gene, and how might that influence my specific timeline and symptoms?
  2. 2.Should my immediate family members undergo genetic counseling to understand their risk of inheriting this condition?
  3. 3.Are there specific therapies or medications we can explore to help manage symptoms like dystonia and restless legs?
  4. 4.When should I start seeing specialists, such as a physical therapist or a speech and swallowing pathologist, to stay ahead of my symptoms?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

Does Higher CAG Repeat Mean Worse Spinocerebellar Ataxia?Does Cerebellar Ataxia Affect Memory and Emotions?Does Spinocerebellar Ataxia Cause Sleep Problems?Does Weight Loss Make Spinocerebellar Ataxia Progress Faster?When Will I Need a Wheelchair with Spinocerebellar Ataxia?Is Ataxia Commonly Misdiagnosed as Parkinson's Disease?What Is the Chance of Passing ADCA to Children?What Is the SARA Score for Ataxia Progression?What Is the Treatment for SCA27B Spinocerebellar Ataxia?What Medications Treat Ataxia Muscle Stiffness & Bladder?Why Does SCA7 Cause Vision Loss? | SCA7 ExplainedWhy Do Standard Genetic Tests Miss Spinocerebellar Ataxia?Why is Swallowing Dangerous in Late-Stage Ataxia?

References

References (16)
  1. 1

    Rural Environment as a Risk Factor for the Age at Onset of Machado-Joseph Disease.

    Martins AC, Furtado GV, Pinheiro JDS, et al.

    Movement disorders clinical practice 2025; (12(4)):520-526 doi:10.1002/mdc3.14338.

    PMID: 39831726
  2. 2

    CAG Repeat Size Influences the Progression Rate of Spinocerebellar Ataxia Type 3.

    Leotti VB, de Vries JJ, Oliveira CM, et al.

    Annals of neurology 2021; (89(1)):66-73 doi:10.1002/ana.25919.

    PMID: 32978817
  3. 3

    Pre-ataxic Changes of Clinical Scales and Eye Movement in Machado-Joseph Disease: BIGPRO Study.

    de Oliveira CM, Leotti VB, Bolzan G, et al.

    Movement disorders : official journal of the Movement Disorder Society 2021; (36(4)):985-994 doi:10.1002/mds.28466.

    PMID: 33438269
  4. 4

    Brain structural abnormalities in the preclinical stage of Machado-Joseph disease/spinocerebellar ataxia type 3 (MJD/SCA3): evaluation by MRI morphometry, diffusion tensor imaging and neurite orientation dispersion and density imaging.

    Li M, Chen X, Xu HL, et al.

    Journal of neurology 2022; (269(6)):2989-2998 doi:10.1007/s00415-021-10890-2.

    PMID: 34783886
  5. 5

    Machado Joseph-Disease Is Rare in the Peruvian Population.

    Cornejo-Olivas M, Solis-Ponce L, Araujo-Aliaga I, et al.

    Cerebellum (London, England) 2023; (22(6)):1192-1199 doi:10.1007/s12311-022-01491-4.

    PMID: 36323979
  6. 6

    Itajaí, Santa Catarina - Azorean ancestry and spinocerebellar ataxia type 3.

    Teive HA, Moro A, Arruda WO, et al.

    Arquivos de neuro-psiquiatria 2016; (74(10)):858-860 doi:10.1590/0004-282X20160125.

    PMID: 27759814
  7. 7

    Reconstructing the History of Machado-Joseph Disease.

    Meira AT, Pedroso JL, Boller F, et al.

    European neurology 2020; (83(1)):99-104 doi:10.1159/000507191.

    PMID: 32344416
  8. 8

    The Ratio of Expanded to Normal Ataxin 3 in Peripheral Blood Mononuclear Cells Correlates with the Age at Onset in Spinocerebellar Ataxia Type 3.

    Breuer P, Rasche T, Han X, et al.

    Movement disorders : official journal of the Movement Disorder Society 2022; (37(5)):1098-1099 doi:10.1002/mds.28962.

    PMID: 35278005
  9. 9

    Split hand and minipolymyoclonus in spinocerebellar ataxia type 3: a case report.

    Eki A, Sugiyama A, Shibuya K, et al.

    BMC neurology 2024; (24(1)):434 doi:10.1186/s12883-024-03948-x.

    PMID: 39516744
  10. 10

    Genetic risk factors for modulation of age at onset in Machado-Joseph disease/spinocerebellar ataxia type 3: a systematic review and meta-analysis.

    de Mattos EP, Kolbe Musskopf M, Bielefeldt Leotti V, et al.

    Journal of neurology, neurosurgery, and psychiatry 2019; (90(2)):203-210 doi:10.1136/jnnp-2018-319200.

    PMID: 30337442
  11. 11

    Cognitive-affective manifestations since premanifest phases of Spinocerebellar Ataxia Type 3/Machado-Joseph Disease.

    Bolzan G, Müller Eyng ME, Leotti VB, et al.

    Cortex; a journal devoted to the study of the nervous system and behavior 2024; (171()):370-382 doi:10.1016/j.cortex.2023.09.021.

    PMID: 38091940
  12. 12

    Sleep disorders in Machado-Joseph disease.

    Pedroso JL, Braga-Neto P, Martinez AR, et al.

    Current opinion in psychiatry 2016; (29(6)):402-8 doi:10.1097/YCO.0000000000000287.

    PMID: 27584711
  13. 13

    Oculomotor deficits in spinocerebellar ataxia type 3: Potential biomarkers of preclinical detection and disease progression.

    Wu C, Chen DB, Feng L, et al.

    CNS neuroscience & therapeutics 2017; (23(4)):321-328 doi:10.1111/cns.12676.

    PMID: 28195427
  14. 14

    Altered retinal structure and function in Spinocerebellar ataxia type 3.

    Toulis V, Casaroli-Marano R, Camós-Carreras A, et al.

    Neurobiology of disease 2022; (170()):105774 doi:10.1016/j.nbd.2022.105774.

    PMID: 35605759
  15. 15

    Dystonia in Machado-Joseph disease: Clinical profile, therapy and anatomical basis.

    Nunes MB, Martinez AR, Rezende TJ, et al.

    Parkinsonism & related disorders 2015; (21(12)):1441-7.

    PMID: 26552869
  16. 16

    Writer's cramp: a new dystonic feature in spinocerebellar ataxia type 3.

    Vasconcellos LFR, Novis LE, Nassif DV, Spitz M

    Acta neurologica Belgica 2019; (119(2)):291-292 doi:10.1007/s13760-018-1022-9.

    PMID: 30238435

This page provides educational information about the terminology and genetics of Machado-Joseph Disease and SCA3. It is not intended as medical advice. Always consult your neurologist or genetic counselor for guidance on diagnosis, testing, and symptom management.

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