There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the Research
Cerebellar Tonsil Extension and Chiari Malformation
- A cerebellar tonsil extension of 4 mm below the foramen magnum is generally considered to be below the normal range, but not necessarily indicative of a Chiari malformation 1, 2.
- Studies have shown that extension of the tonsils below the foramen magnum is considered normal up to 3 mm, borderline between 3 and 5 mm, and clearly pathologic when it exceeds 5 mm 1.
- However, some studies suggest that a low degree of ectopia of the cerebellar tonsils may be of questionable significance, and that MR demonstration of less than 2 mm of tonsillar ectopia is probably of no clinical significance in the absence of syringomyelia 2.
- In the context of Chiari malformation, a tonsillar herniation of more than 5 mm below the foramen magnum is typically considered diagnostic 3, 4.
Clinical Significance and Recommendations
- The clinical significance of a 4 mm tonsillar extension is not clearly established, and would likely depend on the presence of other symptoms or radiographic findings 3, 5.
- In the absence of symptoms or other abnormal findings, a tonsillar extension of 4 mm may not require immediate attention 1, 2.
- However, if symptoms are present, or if there are other radiographic abnormalities, further evaluation and potential treatment may be necessary 3, 4.