Diagnostic Tests and Evaluations

Because PRDM8-related disorders are rare and can look like other conditions, doctors use a specific set of tests to “see” what is happening in the brain and nerves. These tests help them distinguish between different types of movement issues and confirm the genetic cause.

Specialized EEG: More Than Just a Brain Scan

A standard EEG (electroencephalogram) measures the brain’s electrical activity. However, for conditions involving muscle jerks (myoclonus), doctors often use simultaneous EEG-EMG polygraphy ^[1].

In this test, the EEG (brain waves) and EMG (electromyogram, which measures muscle activity) are recorded at the exact same time. This allows doctors to:

• Evaluate the Correlate: It helps an experienced clinician assess whether a movement has a cortical (brain surface) correlate ^[1]. However, a missing EEG correlate does not unequivocally prove the jerk originated outside the cortex.
• Catch Discharges: Some individuals with PRDM8 mutations have generalized epileptiform discharges—abnormal electrical patterns in the brain—even if they are not having a physical seizure ^[2].
• Map Muscle Patterns: It records how muscles (like the “push” and “pull” muscles in an arm) work together during a jerk ^[3].

Brain MRI: Looking for Patterns

A Brain MRI creates detailed pictures of the brain’s structure. In the early stages of progressive myoclonus epilepsies, the MRI can often be unremarkable or “normal” ^[4][5]. This can be frustrating, but it is a common finding in the beginning.

As some related PME conditions progress (though not exclusively documented as universal in PRDM8), an MRI might eventually show changes such as:

• Atrophy: A “shrinking” or thinning of certain areas, such as the cerebellum (responsible for balance) or the brainstem ^[4][6].
• White Matter Changes: Alterations in the tissue that connects different parts of the brain ^[6].

Repeat imaging is not done on a strict schedule but is ordered based on the clinical course and the treating team’s assessment.

Skin Biopsy: An Optional Clue

When an individual has symptoms that look like Lafora disease, doctors may perform a skin biopsy, usually from the underarm area (axilla) ^[7].

The goal is to look for Lafora bodies—abnormal clumps of sugar inside the cells. In classic Lafora disease, these are often present. However, in both families documented with PRDM8-related disorders, skin biopsies were negative for these clumps ^[2]. This is an optional, historical test that does not mandate a PRDM8 diagnosis.

Genetic Testing: The Definitive Answer

While other tests provide clues, molecular genetic testing (such as targeted panels, WES, or WGS) is the definitive way to confirm a PRDM8-related disorder ^[2].

For the highest chance of a clear answer, doctors often prefer trio-based testing. This means testing the patient and both biological parents at the same time ^[8]. Trio testing is more powerful because:

• Inheritance Patterns: It helps doctors see if a genetic change was inherited from parents or if it is a new (de novo) change ^[8][9].
• Filtering: It makes it easier for scientists to filter out harmless genetic variations to find the change causing symptoms ^[10].

Your Completeness Checklist

When navigating a rare diagnosis, patients and families often become the central keepers of medical history. Ensure you have the following reports in a dedicated file:

• [ ] Genetic Report: The full report from the targeted panel, WES, or WGS, including variant classification and parental testing information.
• [ ] EEG Reports: Both the “summary” and detailed findings from any video-EEG or polygraphy sessions.
• [ ] MRI Report: The written radiologist’s report, and ideally, copies of the actual images.
• [ ] Skin Biopsy Report (Optional): If performed, the pathology report confirming whether or not Lafora bodies (PAS-positive inclusions) were found.
• [ ] Symptom Diaries: A personal record or videos of concerning movements, seizure logs, or baseline mobility measures.