Diagnostic Testing and Interpreting Results

Confirming a diagnosis of MICU1-related MPXPS typically requires a compatible clinical picture (phenotype) combined with definitive genetic testing. While laboratory markers and specialized imaging can be highly supportive, they are not strictly mandatory to establish the diagnosis if the genetic evidence is clear.

The Biochemical Signature

While no single blood test can diagnose the disease, certain patterns observed in small patient cohorts strongly suggest MICU1 deficiency.

• Elevated CK Levels: A very consistent finding is a creatine kinase (CK) level that is persistently elevated above normal. In reported cohorts, values vary widely (e.g., from 1,500 to over 4,000 U/L), and can fluctuate based on activity, age, or illness ^[1][2][3]. These numbers are observations, not strict diagnostic rules, and CK can be high even in siblings who do not yet show clear muscle weakness ^[4].
• Elevated Transaminases: You may see high levels of AST and ALT on lab reports ^[1][5]. While often called “liver enzymes,” they are also found in muscle. Clinicians will look at the overall clinical picture to determine that these are muscle-related rather than assuming a liver problem.
• Lactate Levels: Many children with MICU1 deficiency have normal lactate levels in their blood ^[1][3]. However, a normal lactate does not rule out a mitochondrial disorder, and an elevated lactate is nonspecific and can result from acute illness or simply the way the blood was drawn.

Supportive Tests: MRI and EMG

Doctors may use imaging or electrical tests to gather more clues, though a normal result on these tests does not exclude the disorder.

• Muscle MRI Patterns: In some reported cases, MRIs have shown a specific pattern of fatty infiltration in the muscles. The hamstrings, the inner calf (medial gastrocnemius), and the soleus may show involvement, while the anterolateral thigh (front/side of the upper leg) is “spared” ^[6][3]. This is a supportive finding, not a required one.
• Electromyography (EMG): This test measures the electrical activity of muscles. In MPXPS, the EMG typically shows a myopathic pattern, meaning the muscle fibers themselves are affected, but this finding is nonspecific ^[3].

The Definitive Proof: Genetic Testing

The diagnosis of MICU1-related MPXPS is established by identifying two disease-causing variants in the MICU1 gene. This is usually done through Whole Exome Sequencing (WES), Whole Genome Sequencing (WGS), or a comprehensive neuromuscular gene panel ^[7][4].

A conclusive genetic diagnosis usually involves checking several boxes:

1. Biallelic Variants: The report must identify two variants in the MICU1 gene ^[7].
2. Pathogenic Status: The variants should be classified as “Pathogenic” or “Likely Pathogenic.” If a result is a Variant of Uncertain Significance (VUS), it means the lab isn’t sure if that specific change causes disease. A VUS alone should not be used for reproductive planning or major treatment decisions ^[2].
3. Inheritance (Phase): Doctors often test the parents to ensure one variant came from each parent (meaning the variants are in trans).
4. Copy Number Variants: The testing method should be capable of detecting missing pieces of the gene (deletions) or extra pieces (duplications), as these occur in some cases ^[1][8].

In complex cases, researchers or clinicians may use specialized tests, like RNA analysis or studying skin cells (fibroblasts), to prove that a specific variant disrupts the protein’s function ^[9][8].