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Tonsil of cerebellum

The tonsil of the cerebellum is a rounded, ovoid structure located on the inferior surface of each cerebellar hemisphere. It is a part of the posterior lobe of the cerebellum and lies adjacent to the medulla oblongata, forming the lower lateral boundary of the fourth ventricle. The tonsils are most clinically significant due to their potential for downward herniation through the foramen magnum (termed “tonsillar herniation” or “coning”), which can compress the medulla and be life-threatening. The tonsils play a role in coordination and balance as part of the cerebellar hemispheres.

Synonyms

  • Cerebellar tonsil

  • Tonsilla cerebelli

  • Tonsillar lobe (less commonly used)

Arterial Supply

  • Posterior inferior cerebellar artery (PICA) – primary arterial supply to the cerebellar tonsil.

  • Small branches from the vertebral artery may also contribute.

Venous Drainage

  • Drains via inferior cerebellar veins.

  • Ultimately drains into the superior petrosal sinus, transverse sinus, and partly the straight sinus.

Function

  • Part of the cerebellar hemisphere involved in:

    • Coordination of voluntary motor activity.

    • Fine motor control, especially for posture and balance.

    • Plays a minor role compared to other cerebellar lobules but is integral to overall cerebellar function.

    • No specific isolated function, but involved in integration and modulation of motor signals.

MRI Appearance

  • Appears as a rounded, hypointense (darker) structure on T1-weighted images and iso- to hyperintense on T2-weighted images.

  • Easily visualized on sagittal and axial sequences.

  • Normal tonsils are located at or above the foramen magnum; descent >5 mm below the foramen magnum is considered abnormal (Chiari I malformation).

  • No intrinsic contrast enhancement unless pathology is present.

CT Appearance

  • Appears as a soft-tissue density mass at the inferomedial aspect of each cerebellar hemisphere.

  • Has similar density to the rest of the cerebellum.

  • Visualized best on axial and coronal sections.

  • CT is less sensitive than MRI for tonsillar position, but downward herniation (coning) can sometimes be seen, particularly if there is compression of the fourth ventricle or brainstem.

MRI images

Tonsil of cerebellum MRI 3T axial image

MRI images

Tonsil of cerebellum MRI 3T coronal image

MRI images

Tonsil of cerebellum MRI 3T sagittal image

CT image

Tonsil of cerebellum of cerebellum CT axial anatomy image -img-00001-00001