MIdline shift

Midline shift

Midline shift describes the displacement of midline structures, secondary to an underlying pathology exterting mass effect.

Relevant anatomy

The midline structures include:
- Falx cerebri: the curved fold of dura mater located between the two cerebral hemispheres of the brain.
- Septum pellucidum: thin, semi-transparent membrane that separates the left and right halves of the brain.
- Ventricles: third and fourth.
- Interpeduncular cistern: CSF space surrounding the brainstem.

Pathophysiology

Mass effect from an extra-axial collection (e.g. extradural haematoma), space occupying lesion (e.g. primary brain tumour) , or large ischaemic or haemorrhagic stroke displaces the midline structures

How to use imaging to assess for midline shift:

Identify the midline and measure the amount of shift.
If midline shift is present - compare with previous imaging (if available). Is the shift better or worse? Has the underlying causative pathology been treated?
Identify the underlying cause: does this need immediate management?

CT Findings

How to identify the midline and shift

Draw an anterior-posterior line connecting the attachment of the falx cerebri.
Identify whether there is any midline shift - if present, quantify the maximal amount of shift.

Extradural Haematoma

Both CTs demonstrate a large right sided hyperdense extra-axial collection consistent with an extradural haematoma causing local sulcal effacement and mass effect causing midline shift.

Middle cerebral artery

Both CTs demonstrate a large left sided area of ischaemia. CT(right) shows marked midline shift.

Space occupying lesion

Contrast CT-Head shows a right sided meningioma exterting significant mass effect, midline shift and partial effacement of the right lateral ventricles.

REferences

Midline identification: Case courtesy of David Cuete, Radiopaedia.org, rID: 23768Extradural haematoma: Case courtesy of Frank Gaillard, Radiopaedia.org, rID: 3428R MCA Infarct (left): Case courtesy of David Cuete, Radiopaedia.org, rID: 35732R MCA Infarct (right): Case courtesy of David Cuete, Radiopaedia.org, rID: 27908Space occupying lesion: Case courtesy of Dr Ruslan Asadov, Radiopaedia.org, rID: 8837

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