The Diagnosis: Genetic Tests and Brain Scans

The diagnostic journey for Miller-Dieker Syndrome (MDS) often involves two distinct types of “pictures”: a picture of the brain’s structure and a picture of the body’s genetic instructions. Understanding how to read these results is a vital step in advocating for your child’s care.

Brain Imaging: Visualizing the “Smooth Brain”

The first piece of the puzzle is usually an MRI (Magnetic Resonance Imaging). Unlike an X-ray, an MRI uses magnets to create a detailed map of the brain’s surface.

In MDS, doctors are looking for signs of the lissencephaly spectrum ^[1]:

• Agyria: Areas where the brain surface is completely smooth, with no folds (gyri) ^[2].
• Pachygyria: Areas where the folds are unusually broad and few in number ^[2].
• Cortical Thickening: The outer layer of the brain (the cortex) often appears thicker than normal because the neurons didn’t migrate to their proper thin layers ^[1].

Radiologists may use a “grading” system from 1 (most severe, totally smooth) to 6 (mildest) to describe the extent of these findings ^[3].

Genetic Testing: Mapping the Deletion

Once lissencephaly is seen on an MRI, genetic testing is used to find the specific cause. There are two primary tools used for MDS:

1. Chromosomal Microarray (CMA): Think of this as a high-resolution scanner that looks for “missing” or “extra” pieces of genetic code ^[4]. It is the most common tool used to find the 17p13.3 microdeletion ^[5].
2. FISH (Fluorescence In Situ Hybridization): This is a targeted test where scientists use “glowing” markers to see if a specific gene—like PAFAH1B1—is present or missing on the chromosome ^[6].

Reading the Genetic Report

Genetic reports can look like a different language. Here is a guide to the common terms you may see:

• 17p13.3 microdeletion: This confirms that a small piece of the “p” arm (short arm) of chromosome 17 at position 13.3 is missing ^[7].
• arr [hg19] or [hg38]: “arr” stands for array (the test used). The numbers in brackets refer to the “version” of the human genome map the lab used for comparison ^[8].
• del: Short for “deletion,” meaning genetic material is missing.
• Copy Number Loss: Another way of saying a deletion has occurred (normally, we have two “copies” of every gene; “loss” means only one is left).

Your Genetic Report Checklist

Ensure your child’s report includes the following details:

• [ ] The exact chromosomal coordinates (the “start” and “stop” points of the deletion).
• [ ] A list of specific genes included in the deleted section (e.g., PAFAH1B1, YWHAE, CRK).
• [ ] Whether the finding is classified as “Pathogenic” (meaning it is known to cause the syndrome).

Testing for Parents: The “Balanced Translocation”

In about 80% of MDS cases, the deletion is de novo, meaning it happened by chance and isn’t something the parents carry.

However, in about 20% of cases, a parent may have a balanced translocation. This means all their genetic material is present, but two pieces of chromosomes have “swapped” places. The parent is healthy because they aren’t missing any information, but they have a higher risk of passing on an “unbalanced” set (where information is missing) to future children ^[9].

Because interpreting recurrence risk is incredibly complex and emotionally fraught, we strongly recommend asking your care team for a referral to a certified Genetic Counselor. They can help you navigate testing (often a karyotype or FISH) and explain the risks for future pregnancies in clear, supportive language ^[10][11].