Objective:

To report a case of acquired Chiari-like malformation and upper cervical syringomyelia secondary to a large posterior tentorial tumor and to discuss the pathophysiological mechanisms involved.

Approach:

• Imaging and Diagnosis: Imaging demonstrated a 4.9 × 7.1 × 7.0-cm well-defined, lobulated extra-axial mass arising from the superior surface of the posterior tentorium, with marked mass effect, inferior tentorial deformation, and a small C1 syrinx.

Key Findings:

• Tonsillar descent and syrinx formation can occur as secondary effects of supratentorial lesions.
• Post-treatment imaging showed complete resolution of the syrinx and restoration of normal anatomy.

Interpretation:

The findings suggest that the observed Chiari-like state and syringomyelia were secondary to altered intracranial hydrodynamics.

Limitations:

• The case study is based on a single patient, limiting generalizability.
• The literature on acquired Chiari-like states due to supratentorial lesions is limited.

Conclusion:

This case highlights the need for careful cranial imaging before considering posterior fossa decompression in adults with newly identified tonsillar descent and syringomyelia.