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 M 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 pubococcygeal line (PCL) system, and the M line.