Mamillopontine distance

Last revised by Bjoern Picker on 8 May 2023

The mamillopontine distance or pontomamillary distance is defined as the distance between mamillary bodies to the superior aspect of the pons. It can be used to help diagnose intracranial hypotension 1,2 or classify hydrocephalus 3,4

The mamillopontine distance can be measured on midsagittal T1 images, but the exact landmarks used have been variably described with impact on the normal and abnormal values.

The easiest and probably most frequently used method is to measure the shortest distance between the inferior margin of the mammillary bodies and the upper surface of the pons 1,3,4. Even so, the values obtained are fairly variable (see below). 

Alternative measures reported include from the anterior root of the mammillary body to the top of the pons parallel to the anterior mesencephalon or the shortest distance between the centre of the mammillary bodies and the upper surface of the pons 2​​

In normal individuals, the pontomamillary distance has been variably reported as 7 mm (SD, ± 1.3; CI, 6.5–7.5) 1, or >9 mm 3.

It is decreased in conditions that either depress the floor of the third ventricle or change the position of the brain within the intracranial cavity. Such conditions include: 

  • intracranial hypotension: it is one of the helpful quantitative signs helpful in improving the accuracy of MRI diagnosis, the cutoff value is under 5.5 mm 1

  • hydrocephalus: as one of the morphological changes at mid-sagittal MRI 3

Note, mamillary can be spelt with either one or two ms. The former (mamillary) tends to be more common in UK spelling, whereas the latter (mammillary) is more common in the US 5

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