Transtemporal Isthmus Approach

The Transtemporal Isthmus Approach is a specialized surgical technique used to access deep-seated brain tumors, particularly in the insular region, mesial temporal lobe, and areas around the Sylvian fissure. This approach is designed to navigate through temporal lobe structures with minimal disruption to critical areas, preserving functional outcomes as much as possible. Here’s an overview:

### Key Features of the Transtemporal Isthmus Approach: 1. Access Route:

  1. The approach targets the temporal isthmus, a narrow passage in the middle temporal gyrus, allowing access to deep brain structures.
  2. By following this route, the surgeon avoids extensive retraction or manipulation of superficial brain areas that could impair function.

2. Primary Goals:

  1. This approach is especially useful for insuloopercular gliomas and deep-seated tumors in the temporal region.
  2. It offers direct access to the insular cortex, parts of the operculum, and structures surrounding the Sylvian fissure, enabling effective resection of tumors in these areas.
  3. It can be adjusted for lesions that extend medially to the basal ganglia or other subcortical structures.

3. Advantages:

  1. Reduced Risk to Functional Areas: The temporal isthmus is a relatively “silent” zone, meaning it has fewer critical functions, which reduces the likelihood of damaging areas essential for motor or sensory functions.
  2. Controlled Access to the Insula: This approach provides a path to the insular region without significant disruption to overlying opercular cortex, minimizing damage to pathways associated with language and motor function.
  3. Improved Surgical Visibility: The trajectory through the temporal isthmus offers a straight path to deeper structures, providing a clearer view of the target area with less manipulation of brain tissue.

4. Surgical Challenges:

  1. This approach requires precise anatomical knowledge and planning, as the temporal lobe contains critical networks for memory and language that must be preserved.
  2. Depending on the size and extent of the tumor, the surgeon may need to make decisions about how much of the temporal isthmus can be traversed without affecting adjacent structures.

5. Potential Risks:

  1. Though the temporal isthmus is relatively safe, there is still a risk of complications, such as damage to language or memory areas, particularly if the tumor is large or extends beyond the insula.
  2. The approach may also put delicate vessels, such as those in the MCA (Middle Cerebral Artery) branches, at risk of injury, which could lead to stroke or other vascular issues.

### Clinical Applications The transtemporal isthmus approach is frequently considered in low-grade gliomas and high-grade tumors in the insuloopercular region, where maximal safe resection is crucial for patient outcomes.

Sun et al. retrospectively examined 53 patients with insular glioma who underwent resection through the temporal isthmus approach using magnetic resonance imaging and functional neuronavigation guidance and intraoperative electrophysiological monitoring. Extent of resection was determined using intraoperative magnetic resonance imaging.

Fifty-three patients were included for analysis, 30 men and 23 women. The median (range) age was 45 (26-70) years. Tumor laterality was left in 22 patients and right in 31. All tumors involved at least zone III or IV (Berger-Sanai classification system), including zones I-IV were involved in 29 (54.7%) and zones III and IV in 17 (32.0%). Among the 37 low-grade gliomas, preoperative median (IQR) volume was 45.7 (31.8, 60.3) cm 3 , and gross total resection was achieved in 24 (64.9%). Among the 16 high-grade gliomas, preoperative median (IQR) volume was 45.3 (40.1, 54.0) cm 3 , and gross total resection was achieved in 14 (87.5%). The median (IQR) extent of resection of the whole group was 100% (89%-100%). The median (IQR) postoperative Karnofsky performance score 3 months after surgery was 90 (80-90). Mean temporal isthmus width was significantly higher in the affected side (involving tumor) than the contralateral one (21.6 vs 11.3 mm; 95% CI: 9.3 to 11.3, P < .01). Muscle strength was grade 4 or higher, and speech was nearly normal in all patients 3 months after surgery.

Insular glioma surgery using the transtemporal isthmus approach can achieve safe and maximum resection. A widened temporal isthmus provides a surgical pathway for transisthmic resection of insular tumor 1)

1)
Sun G, Shu X, Wu D, Zhao K, Xue Z, Cheng G, Chen L, Zhang J. The Transtemporal Isthmus Approach for Insular Glioma Surgery. Oper Neurosurg (Hagerstown). 2024 Aug 20. doi: 10.1227/ons.0000000000001308. Epub ahead of print. PMID: 39162411.