Where is the Glabella Located and What are its Borders?
The glabella is located on the external surface of the cranium (facies externa) in the midline, directly superior to the root of the nose.
- Inferior boundary: The lower limit of the external surface in this zone is the supraorbital margin (margo supraorbitalis).
- Adjacent structures: Near the glabella, in the medial half of the supraorbital margin, the frontal notch or frontal foramen (incisura frontalis / foramen frontale) may be present. The supratrochlear vessels and nerve (a., v. et n. supratrochlearis) pass through them.
- Venous system: A single or paired frontal vein runs along the midline superior to the glabella. Arteries typically do not course through this area, and the veins of the cranial vault lack valves.
Additionally, the glabella, together with the supraorbital margin of the frontal bone, serves as the anterior boundary of the frontoparietooccipital region, encompassing the first three unpaired areas of the cranial vault.
What is its Role in Craniofacial Topography?
The glabella is used as a landmark when constructing craniocerebral topography diagrams (such as Kronlein's scheme), which allow for the projection of major cerebral sulci and the path of the middle meningeal artery onto the skull.
- Median sagittal line: Constructed by drawing a line from the glabella to the external occipital protuberance (protuberantia occipitalis externa).
- Projection of the dura mater: The upper margin of the falx cerebri—the dural fold separating the cerebral hemispheres—projects onto this sagittal line. Dural folds act as shock absorbers protecting the brain from concussions and trauma.
What is its Clinical Significance in Obstetrics?
In obstetrics, the glabella is the leading point in brow presentation of the fetal head. Deflexed attitudes occur with a frequency of 0.5–1%.
- Leading point: The glabella is the guiding point in brow presentation.
- Biomechanism: In this presentation, the fetal head passes through the pelvic planes via its largest dimension — the mentooedipital (or rather, maximum transverse/oblique size, specifically the vertical/occipitomental or maxillo-occipital depending on exact definitions, but keeping text faithful: largest oblique size). Its diameter is 13.5 cm, and its circumference is 41–42 cm. Extension of the head occurs at the pelvic inlet, followed by internal rotation in the wide part of the pelvic cavity, resulting in a posterior position.
- Prognosis: Brow presentation is considered the most unfavorable variant. It causes substantial labor dystocia, and vaginal delivery is only possible with a very small fetus.
- Dynamics: The presentation is unstable. With further extension during labor, a brow presentation may convert to a face presentation, where the chin becomes the leading point. Persistent deflexed presentations are rare and associated with pathologies such as a large goiter or extensive cystic hygroma of the fetal neck, or large uterine submucosal myomas in the mother.