H and M lines

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The H and M lines are reference lines for the pelvic floor on imaging studies and help detect and grade pelvic floor prolapse on defecography studies.

The H line is drawn from the inferior margin of the pubic symphysis to the posterior aspect of the anorectal junction, and represents the diameter of the levator hiatus, and a normal value is < 6 cm. Grading of hiatal enlargement during evacuation using H line dimensions is as follows:

< 6 cm: normal
6-8 cm: mild
8-10 cm: moderate
> 10 cm: severe

The M line is drawn perpendicularly from the posterior end of the H line to the pubococcygeal line, and represents the descent of the hiatus, and a normal value is < 2 cm. Grading of hiatal descent during evacuation using H line dimensions is as follows:

< 2 cm: normal
2-4 cm: mild
4-6 cm: moderate
> 6 cm: severe

There is a discrepancy in the literature about the grading of pelvic floor descent with variation between the measurements used for the posterior compartment in the PCL system, and the M line.