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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Medical Genetics

Understanding Your Tuberous Sclerosis Complex Diagnosis

At a Glance

Tuberous sclerosis complex can be diagnosed from specific clinical features or a pathogenic or likely pathogenic TSC1 or TSC2 gene change. A negative blood test does not rule it out because some changes are missed, so doctors may rely on clinical criteria or test another tissue.

Receiving a diagnosis of Tuberous Sclerosis Complex (TSC) can feel overwhelming because the condition is so variable. No two people with TSC have the exact same experience [1]. This page will help you understand what is happening in the body, how doctors use the 2021 guidelines to confirm a diagnosis, and why a genetic test result is only one part of the puzzle.

What is Tuberous Sclerosis Complex?

In the simplest terms, TSC is a genetic condition that causes the body to grow non-cancerous tumors, called hamartomas, and developmental malformations in different organs [1]. These growths are not like typical cancer; they are made of cells that simply grew in a disorganized way or in the wrong place [1][2]. Because these features can appear in the brain, skin, heart, kidneys, and lungs, TSC is called a “multisystem” disorder [3].

The Biological “Brake” System

To understand why these growths happen, it helps to think of your cells as having an engine and a brake.

  • The mTOR Pathway: This is the “engine” of the cell. It tells cells when to grow and divide [4][5].
  • TSC1 and TSC2 Genes: These genes produce two proteins, hamartin and tuberin, which work together like a “brake” on the mTOR engine [6][7].

In a person with TSC, one of these “brake” genes is mutated and cannot function correctly [8]. Without a working brake, the mTOR engine stays “on” all the time, leading to the overgrowth of cells that form hamartomas [5].

TSC1 vs. TSC2 Mutations

While both genes cause TSC, there are some general differences:

  • TSC2 Mutations: These are more common and, on average, tend to be associated with more severe symptoms, such as earlier-onset seizures or a higher number of kidney and brain growths [9][10].
  • TSC1 Mutations: These are often associated with milder symptoms, though this is not true for everyone [11].
    It is important to remember that these are group averages. A person with a TSC2 mutation can still have a mild case, and a person with a TSC1 mutation can have more significant challenges [12].

How TSC is Diagnosed

A diagnosis can be made in two ways: through clinical criteria (what a doctor sees) or through genetic criteria (what a lab test finds).

The 2021 Clinical Criteria

In 2021, international experts updated the rules for diagnosing TSC [13]. Doctors look for “Major” and “Minor” features across different parts of the body. You should not self-count these features; a clinician must evaluate them.

Major Features Minor Features
3+ light-colored skin spots (hypomelanotic macules), at least 5 mm in size “Confetti” skin lesions (1 to 2 mm white spots)
3+ facial bumps (angiofibromas) OR 1 fibrous cephalic plaque 3+ pits in tooth enamel
2+ growths under nails (ungual fibromas) 2+ growths in the gums or mouth
A thick, leathery patch of skin (shagreen patch) Retinal achromic patch (pale spot in the eye)
Multiple growths in the eye (retinal hamartomas) Multiple kidney cysts
Multiple cortical tubers and/or radial migration lines Non-kidney hamartomas (growths in other organs)
2+ brain nodules (subependymal nodules) Sclerotic bone lesions (dense areas in bone)
A specific brain tumor (SEGA)
A heart tumor (cardiac rhabdomyoma)
Lung disease (LAM)
2+ kidney tumors (angiomyolipomas)

The Diagnostic Rules [14][13]:

  • Definite TSC: You have 2 Major features, OR 1 Major and 2 Minor features. (Note: Having LAM and kidney angiomyolipomas alone does not establish definite TSC).
  • Possible TSC: You have 1 Major feature, OR 2+ Minor features.

The Genetic Criterion

If a genetic test finds a “pathogenic” or “likely pathogenic” mutation in the TSC1 or TSC2 gene, that alone is enough for a definite diagnosis, even if the person doesn’t have any physical symptoms yet [15]. A “variant of uncertain significance” (VUS) does not confirm a diagnosis.

Understanding Your Genetic Test

About one-third of people with TSC inherited the gene from an affected parent (autosomal dominant) [1]. However, roughly two-thirds of cases are de novo (sporadic), meaning the mutation happened for the first time in that individual and was not passed down from their parents [16]. If an affected parent has the variant, they have a 50% chance of passing it to each child; genetic counseling is highly recommended.

Why a Negative Test Doesn’t Rule Out TSC

About 10% to 15% of people who clearly have TSC will have a “negative” genetic test [17]. This happens for a few reasons:

  • Mosaicism: This means the mutation is only in some of the body’s cells. If the mutation isn’t in the blood cells used for the test, the test will come back negative [18][19].
  • Deep Intronic Variants: Sometimes the mutation is hidden deep inside the gene in areas that standard tests don’t always look at [20][21].
  • Technical Limits: Some mutations involve large chunks of DNA being missing or rearranged, which requires specialized testing to find [22].

If your clinical features meet the 2021 criteria, you have TSC regardless of what the blood test shows [13]. Your care team may suggest testing a different tissue, such as a sample from a skin growth, to try to find the mutation [19].

Common questions in this guide

How is tuberous sclerosis complex diagnosed under the 2021 criteria?
Clinicians diagnose definite TSC when a person has two major features or one major feature plus two minor features. One major feature or at least two minor features indicates possible TSC. A clinician should evaluate and count these features rather than having patients count them on their own.
Can a genetic test confirm a TSC diagnosis?
A pathogenic or likely pathogenic change in the TSC1 or TSC2 gene is enough to establish a definite TSC diagnosis, even if physical signs have not appeared. A variant of uncertain significance does not confirm TSC and must be interpreted with the clinical findings.
Does a negative blood test rule out tuberous sclerosis complex?
No. Some people with clear clinical features of TSC have a negative blood test because the change is present only in some cells, lies in a region standard testing may miss, or requires specialized testing to detect. A care team may consider testing another tissue or rely on the clinical criteria.
What is the difference between TSC1 and TSC2 mutations?
TSC2 changes are more often associated with earlier seizures and a greater number of kidney or brain growths, while TSC1 changes are often linked with milder features. These are average patterns across groups and cannot predict exactly how TSC will affect one person.
Can tuberous sclerosis complex be inherited?
Yes. When a parent has the TSC-causing variant, each child has a 50% chance of inheriting it. Many cases arise as a new change in the affected person rather than being inherited, so genetic counseling can help clarify family risk.
Why do hamartomas develop in tuberous sclerosis complex?
The TSC1 and TSC2 genes normally make proteins that help restrain mTOR, a pathway that signals cells to grow. When one of these genes is altered, this cellular brake may not work properly, allowing disorganized cell growth and hamartomas to form.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Given my (or my child's) current symptoms, do we meet the 2021 criteria for a 'definite' or 'possible' diagnosis?
  2. 2.Since our genetic test was negative, does this rule out TSC, or should we consider testing other tissues like a skin lesion?
  3. 3.Which specific major and minor clinical features were used to make this diagnosis?
  4. 4.Are my (or my child's) symptoms more consistent with a TSC1 or TSC2 mutation, and does that change our monitoring plan?
  5. 5.Based on the 2021 guidelines, what screening tests (like brain MRI or kidney ultrasound) do we need to schedule next?

Questions For You

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

References

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This page is for informational purposes only and does not constitute medical advice. Your clinician or genetic counselor should interpret your TSC findings and recommend next steps.

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