Objective: To investigate the value of the supraorbital keyhole approach in the resection of small saddle-nodal meningiomas, the surgical technique and its indications. Methods: We retrospectively analyzed the clinical data of 21 patients with saddle-nodal meningioma treated by supraorbital keyhole approach in our department. A limited supraorbital keyhole approach was used for craniotomy, with a bone foramen 25-30 mm wide and 15-20 mm high, and the surgical indications, surgical techniques, surgical outcomes and complications are discussed. RESULTS: Although the craniotomy hole was small, it provided sufficient space for intracranial manipulation to remove the tumor and protect the brain and other vital structures. 21 patients were discharged with good recovery, 20 (93.3%) with complete tumor resection, 19 (86.7%) with improved postoperative vision, and no serious postoperative complications related to the surgical approach. Conclusion: For experienced neurosurgeons, most saddle-nodal meningiomas smaller than 3 cm can be resected through the supraorbital keyhole approach with a high rate of total resection, few surgical complications, and good surgical outcomes. Saddle-nodal meningiomas account for 5% to 10% of intracranial meningiomas and are a challenging problem in neurosurgery. Because these tumors originate from the saddle node area, cross sulcus, and grow in the suboptic area, surgery is difficult. In the past, they were mostly removed by pterygoid approach or unilateral inferior frontal approach. In recent years, with the development of microsurgical techniques and neuroimaging, more precise and minimally invasive neurosurgery is required to achieve more perfect treatment results [1]. The supraorbital “keyhole” approach is a minimally invasive neurosurgical technique that has been developed in the last decade or so [2]. From January 2003 to December 2007, 21 cases of saddle-nodular meningioma were treated by supraorbital keyhole approach through the brow arch incision with satisfactory results. Clinical data and methods 1. General data: 9 male cases and 12 female cases, age 36-66 years, average 45.4 years. The shortest duration of the disease was 2 months, the longest was 3 years, and the average was 1 year and 3 months. 2. Clinical manifestations: Visual disturbance was the most common first symptom, with 20 cases, and one case was found incidentally. Every patient in the clinical manifestations had visual loss, including 13 cases of unilateral visual loss, 8 cases of bilateral visual loss, and 1 case with only light perception. Visual field defects were present in 17 cases, 15 of which were unilateral temporal blindness, 1 case was quadrant blindness, and only 1 case was typical bilateral temporal hemianopia. Headache was uncommon and was seen in only 3 patients. 11 patients had hormone measurements, 3 had hypopituitarism, 1 had mild hyperprolactinemia, and the rest had normal endocrine function. Funduscopic examination revealed different degrees of optic nerve papillary atrophy on one or both sides. Imaging: All patients underwent cranial CT examination and/or MR examination. All the tumors were located in the dura of the saddle nodes and septum, and the enhancement scans showed obvious uniform enhancement of the tumors. The tumors were classified according to the maximum diameter of the tumor measured by CT and MR. Tumors <2cm were classified as small, with 3 cases. 2-3cm were classified as medium-sized, with 18 cases, and no large tumors >3cm. 4. Surgical methods: The surgeries were performed by experienced chief surgeons in microscopic neurosurgery. After general anesthesia, a lumbar puncture tube was placed first. The patient was placed in supine position, fixed with Mayfield head frame, and generally used the right keyhole approach, with the head rotated 20~40° to the opposite side and posteriorly extended 10~15°. Important anatomical landmarks such as the anterior temporal line, the zygomatic arch, the outer edge of the frontal process, the supraorbital foramen and the location of the supraorbital nerve are marked with a marker. The skin incision is usually made on the lateral half of the eyebrow, beginning at the supraorbital nerve from the supraorbital notch and ending outward at the frontal zygomatic process lateral to the eyebrow. A small hole is drilled behind the temporal line in the temporal sulcus and a free bone flap is made with a milling cutter, usually 25-30 mm wide and 15-20 mm high. the inner edge of the bone on the orbital rim is ground away, and the anterior skull base crest is also ground away if present. The dura is opened in an arc and the base is suspended toward the orbital rim. The base of the frontal lobe is lifted with an automatic retractor and the subarachnoid space is opened to drain the cerebrospinal fluid. If the intraoperative cerebral pressure drop is unsatisfactory, some cerebrospinal fluid may also be released from the lumbar pool drainage as appropriate. After the brain tissue is retracted, the structure of the suprasellar region and tumor tissue will be revealed under the microscope, and the tumor will be separated and resected by the microscopic neurosurgery technique. Special attention will be paid to the anatomical relationship between the important neurovascular structures such as optic nerve, optic cross, anterior cerebral artery, anterior communicating artery and pituitary stalk and the tumor to avoid injury. After perfect hemostasis, the intradural cavity was filled with Ringer’s solution, and the dura was tightly sutured without leakage. The bone flap was fixed with 1-2 titanium connecting pieces, and the subcutaneous and skin were sutured separately. RESULTS 1. Extent of tumor resection: Despite the reduction of the cranial bone window, this access reveal still provides enough space for operation, resection of tumor, and protection of brain and other important structures. Among the 21 cases in this group, complete resection of the tumor was obtained in 20 cases (95.2%) and subtotal resection in 1 case (Figure 1 and 2). The dural attachments of the saddle nodes were not resected and treated with bipolar electrocoagulation. 2.Surgical results: All cases in this group recovered well after surgery, with an average hospitalization of 8.5 days (7-12 days) and no surgical death. Among the 21 cases with preoperative vision loss, 19 cases (92.3%) had different degrees of visual acuity improvement, 2 cases had no change in visual acuity and no deterioration in visual acuity. 16 cases were followed up for 1-4 years (average 2.3 years), no tumor regrowth was found, and all of them returned to normal work or independent life. 3.Surgical complications: 1 case of transient uropathy occurred after surgery, which recovered completely within 3 months. One case of small infarction in the head of caudate nucleus was found on early postoperative CT review, with no obvious clinical symptoms. Early postoperative symptoms such as periorbital swelling and local numbness in patients recovered in about one week after surgery, and no other surgical complications related to access occurred. Discussion The saddle nodes are slight bony elevations that separate the anterior parietal and crossed anterior sulci of the pituitary fossa. Meningioma growth in the saddle node is governed by adjacent anatomic structures [1]: the internal K artery laterally; the optic nerve anteriorly; the pituitary stalk, funnel, and Liliequist’s membrane posteriorly; and the optic cross, end plate, and anterior communicating artery superiorly. The mean age at the time of discovery of saddle node meningioma is 30-39 years, with a female prevalence and a slightly higher mean age in this group. It is detected earlier because it can cause early visual impairment, and most patients have tumors smaller than 4 cm [2]. As it extends from the saddle node area to the superior border of the posterior bed protuberance, the average length is 8 mm (5-13 mm) and the width is 11 mm (6-15 mm). This explains that a tumor as long as it is larger than 1.5 cm can cause visual impairment [3]. Smaller tumors can also cause visual loss if they originate medial to the optic foramen. Visual impairment in saddle node meningiomas is different from pituitary tumors and bilateral temporal hemianopia is rare. Perneczky and Fries [4,5,6] first introduced the concept of “keyhole” microneurosurgery, which is based on a limited skin incision through the foramen ovale. The main point is to choose a direct and precise approach to reach deep intracranial lesions through a limited skin incision and craniotomy, without exposing and interfering too much with the brain, blood vessels and neural structures surrounding the lesion, in order to minimize surgical trauma and achieve a perfect treatment result. The “keyhole” approach does not mean that the size of the craniotomy is the same as the keyhole, but rather that the individualized craniotomy is keyed to a specific intracranial space and can be performed with minimal trauma [7]. The supraorbital keyhole approach theoretically provides excellent access and exposure of the following structures: medial portion of the lateral fissure, anterior bed process, pterygoid crest, optic canal and falciform ligament, bilateral optic nerves, ipsilateral optic tract, pituitary stalk, saddle septum, saddle dorsum, posterior bed process, and bilateral actinic nerves. Therefore, for smaller saddle node meningiomas, usually less than 3 cm, the supraorbital keyhole can be used to achieve good results. Compared with the previous craniotomy approach, we experience that the supraorbital keyhole approach has the following advantages: (1) the surgical incision is concealed, the cranial window is reduced, and the nerve and blood supply of the temporalis and frontalis muscles are fully protected, so the patient not only recovers quickly after surgery but also does not affect the patient’s cosmetic appearance. (2) Because the approach releases cerebrospinal fluid by fully dissecting the subarachnoid space, the brain tissue is naturally retracted to reveal the lesion area, which causes less damage to normal brain tissue, thus reducing the occurrence of some complications associated with the surgical approach and has the advantage of microinvasive. (iii) The angle and extent of exposure of the approach are similar to those of the inferior frontal approach. Despite the reduced cranial opening, the keyhole approach technique expands the surgical field of view with increasing depth and even reveals the contralateral vascular-neural structures according to the “portal-mirror” effect principle proposed by Lindert [8], the importance of which has been deeply appreciated from the surgery. However, the supraorbital keyhole approach has its own intrinsic shortcomings [9]. The main shortcomings are: (1) the scope of surgical exposure is limited and there is little room for intraoperative changes in the surgical plan. It is technically demanding and should not be used for surgery of giant tumors. ②Fine microsurgery-specific instruments and certain special instruments are required, otherwise the operation is difficult. ③It is difficult to release cerebrospinal fluid at the beginning, and sometimes lumbar puncture is required to place a tube. It is difficult to open the optic nerve canal and should not be used for those with tumor invasion in the canal. It is inconvenient to deal with the ipsilateral medial optic nerve tumor and to separate the anterior communicating artery, especially when the tumor is hard. To make up for these shortcomings, we believe that the following points should be noted: ① In order to make microsurgical operations safe and effective, preoperative surgical planning is very important. Of course, accurate preoperative planning can only be made after complete diagnostic imaging is obtained. If the tumor is huge, invades the optic nerve canal or grows invasively, other approaches should be used instead. The technique should be recommended for well-trained neurosurgeons with experience in microsurgery. Although the supraorbital keyhole approach provides good visualization, surgical technique is the key to total tumor resection [2]. Therefore, during the supraorbital keyhole approach, the following points should be noted: (1) Before opening the dura, the inner layer of bone at the superior orbital rim should be ground down and the anterior skull base bone crest should be smoothed to avoid obstructing the surgical view and operation. The lateral fissure and carotid pool should be opened from distal to proximal direction under the microscope to release the cerebrospinal fluid to make the brain tissue collapse automatically and reduce the cerebral traction, and if necessary, the lumbar pool drainage can be reserved in advance for the release of cerebrospinal fluid during the operation. ③When surgically removing the tumor, the midline crest of the pterygoid platform must be used as the anatomical marker of spatial direction, and the tumor can be digested by low intensity bipolar electrocoagulation in the midline of the saddle node or pterygoid platform, so that the contralateral and ipsilateral optic nerves can be identified after the tumor shrinks to avoid optic nerve injury. ④ The tumor is first removed from under the lateral nerve and the medial aspect of the contralateral internal carotid artery. Then the tumor is excised from the contralateral to the ipsilateral direction under the crossed pool, which helps to identify the optic nerve covered by the tumor and important vascular nerve structures such as the internal carotid artery and the anterior cerebral artery. The tumor is then removed from under the ipsilateral optic nerve and internal carotid artery, and finally the tumor is separated from the pituitary stalk and crossed pools. ⑤ Surgical separation should be performed in the subarachnoid space as much as possible, i.e., between the tumor envelope and the arachnoid membrane, leaving a layer of arachnoid membrane over the optic nerve, optic cross and pituitary stalk, which can reduce direct damage to the optic nerve, and the small vessels seen in the arachnoid layer should not be electrocoagulated as much as possible, and by protecting these vessels, there is a better chance of improving visual function [2,10]. Applying this technique, visual acuity remains stable or improves in 90% of patients with relatively good preoperative visual acuity and good postoperative visual recovery. In addition, the pituitary stalk can be clearly separated from the tumor without damage by a well identified arachnoid plane. Despite preoperative compression of the pituitary stalk and intraoperative electrocoagulation of the superior pituitary artery, postoperative pituitary insufficiency is rare. This may be due to the fact that the superior pituitary artery is not the main blood supply to the pituitary gland and the pituitary stalk. In conclusion, the supraorbital keyhole approach is a safe, effective, and ideal minimally invasive surgical approach for saddle-nodular meningiomas, and is particularly suitable for resection of tumors with a maximum diameter of <3 cm. Experienced neurosurgeons, appropriate case selection and meticulous micro-neurosurgical techniques are necessary for successful completion of the procedure.