At this point, the endoscope is introduced through the central scalp incision and using elevator #5 through the temporal incision, the lowest portion of the forehead is dissected at the subperiosteal plane until the lateral orbital rim and the zygomaticofrontal suture line are identified. Then dissection to identify the supraorbital nerve is proceeded from lateral to medial about the superior orbital rim. When the supraorbital nerve is identified, the elevator #6 is introduced to split the periosteum at or just a few millimeters above the arcus marginalis. This is initially done lateral to the supraorbital nerve. Then the elevator #7 is introduced to spread the periosteum in the lateral half of the orbital rim, this spreading of the periosteum is continued in front of the supraorbital nerve (in its deep plane), and the dissection is proceeded medially to elevate the periosteum, looking from inside out, in front of the supratrochlear nerves and the corrugator muscles. At this point, the areas are also packed with neurosurgical pledgets and the dissection is done in the same fashion on the contralateral side.

Because the supratrochlear nerve is in a plane slightly inferior to the supraorbital nerve, it is not readily accessible at this point.

The next steps of the operation are critical to avoid frustrating events such as excessive bleeding, damage of the supraorbital nerve, and incomplete resection of the corrugator muscles. The following steps are done in a sequential fashion.

Step one: The subperiosteal dissection at the glabellar area with extension up to the frontal nasal suture line is done with elevator #5. This assures identification of the midline which will be helpful for orientation. The surgeon should avoid the temptation of spreading the periosteum in a vertical direction at this point, because the edges of the periosteum are constantly hanging in front of your endoscope like a small flap obscuring your vision. With this maneuver, on many occasions, the most medial portions of the origin of the corrugator muscles can be detached and you may start seeing the supratrochlear nerve.

Step two: Introduce the nerve protector through the temporal incision and with a vertical motion separate the fibers of the corrugator muscle off the supraorbital nerve and place the tip of the nerve protector hooking the supraorbital nerve. The weight of this instrument is very light, the portion hooking the nerve is very short, and there is no risk of injuring the supraorbital nerve. While the supraorbital nerve is being protected and the tip of the nerve protector is being used as a guiding element, we proceed with the endoscopic grasper to spread the fibers of the corrugator muscle to identify the supratrochlear nerve.

Step three: I prefer to do a careful identification and fascicular dissection of the supratrochlear nerve rather than trying to do a semi-blind grasping of the corrugator muscle. In that way, I assure that all the fascicles of the supratrochlear nerve are left intact (Fig 3). This fascicular dissection is done using the endoscopic grasper with vertical maneuvers separating each one of the fascicles of the supratrochlear and the fibers of the corrugator muscles surrounding these fascicles. Usually there is an average of four fascicles. Only when the fascicles of the supratrochlear nerve are identified should we proceed with excision of the corrugator muscle fibers. When the most medial fascicles of the supratrochlear nerve are identified, then the nerve protector is moved to gently retract this structure and proceed with excision of the corrugator muscle medial to it using the endoscopic corrugator muscle biter. To continue the resection of the corrugator, the nerve protector is moved again to retract the supraorbital nerve and the remains of the corrugator muscle medial to the supraorbital nerve and in between, fascicles of the supratrochlear nerves are excised using either the grasper or the bitter. In this way, you assure a very clean and complete resection of all the fibers of the corrugator medial to the supraorbital nerve. This represents about 70% of the muscle bulk. If needed, you could chase the fibers of the corrugator, which at this point are located in front of the supraorbital nerve up to its decussation with the frontalis muscle. However, this last maneuver is not routinely recommended. If there is any bleeding of small veins, these are electrocoagulated either using the insulated graspers or the suction coagulator. Bleeding happens only if you bite one of the branches of the supraorbital or supratrochlear veins. If at any point you find there is too much bleeding and you cannot see very well, it is better to irrigate well the cavity, apply neurosurgical pledgets moistened in epinephrine solution, apply gentle pressure in the area, do the surgery in some other area, and come back 5 or 10 minutes later to do the dissection in the area of bleeding. Usually, at this time you will notice the bleeding has slowed down and you can see better which vessel is bleeding and do a very accurate electrocoagulation. The resection of the corrugator muscles is done according to the clinical indications and given the situation in which both are equal in hyperactivity, then equal amounts of corrugator muscle should be removed. If the patient presents significant lowering of the medial head of the brow, then the dissection is proceeded inferiorly toward the lateral nasal bone to identify the depressor supercilii muscle and resection of this muscle in the same fashion as the corrugator muscle is performed. Although, initially I recommended to go through the nose because it was easier to approach the depressor supercilii, with the new instrumentation the endonasal approach has become less frequent.