The Mini-Craniotomy for cSDH Revisited: New Perspectives

Abstract

Background: Chronic subdural hematomas (cSDH) are increasingly prevalent worldwide with the increased aging population and anticoagulant use. Different surgical, medical, and endovascular treatments have had varying success rates. Primary neurosurgical interventions include burr hole drainage of the cSDH and mini-craniotomies/craniotomies with or without fenestration of the inner membrane. A key assessment of the success or failure of cSDH treatments has been symptomatic recurrence rates which have historically ranged from 5 to 30%. Pre-operative prediction of the inner subdural membrane by CT scan was used to guide our decision to perform mini-craniotomies. Release of the inner membrane facilitates the expansion of the brain and likely improves glymphatic flow. Methods: Consecutive mini-craniotomies (N = 34) for cSDH evacuation performed by a single neurosurgeon at a quaternary academic medical center/Level I trauma center from July 2018-September 2020 were retrospectively reviewed. Patient characteristics [age, gender, presenting GCS, GOS, initial CTs noting the inner subdural membrane, midline shift (MLS), cSDH width, inner membrane fenestration, cSDH recurrence, post-operative seizures, infections, length of stay] were extracted from the EMR. Results: Twenty nine patients had mini-craniotomies as primary treatment of the cSDH. Mean age = 68.9 ± 19.7 years (range 22-102), mean pre-operative GCS = 14.5 ± 1.1, mean MLS = 6.75 ± 4.2 mm, and mean maximum thickness of cSDH = 17.7 ± 6.0 mm. Twenty four were unilateral, five bilateral, 34 total craniotomies were performed. Thirty three had inner membrane signs on pre-operative head CTs and an inner subdural membrane was fenestrated in all cases except for the one craniotomy that didn't show these characteristic CT findings. Mean operating time = 79.5 ± 26.0 min. Radiographic and clinical improvement occurred in all patients. Mean improvement in MLS = 3.85 ± 2.69. There were no symptomatic recurrences, re-operations, surgical site infections, or deaths during the 6 months of follow-up. One patient was treated for post-operative seizures with AEDs for 6 months. Conclusion: Pre-operative CT scans demonstrating inner subdural membranes may guide one to target the treatment to allow release of this tension band. Mini-craniotomy with careful fenestration of the inner membrane is very effective for this. Brain re-expansion and re-establishment of normal brain interstitial flow may be important in long term outcomes with cSDH and may be related to the recent interests in brain glymphatics and dural lymphatics.

Keywords: chronic subdural hematoma; dural lymphatics; fenestration; glymphatics; inner membrane; mini-craniotomy; neurosurgery.

Figure 1

Histopathology of the dura and adjacent outer and inner subdural membrane. (A). H and E stain of the dura with adjacent outer membrane (**). (B). Higher magnification view of the outer subdural membrane demonstrating the high number of inflammatory cells. (C). H and E stain of the inner membrane. Note the thin and avascular and rather acellular appearance.

Figure 2

Intraoperative view of the mini-craniotomy demonstrating the inner and outer membrane. (A). Mini-craniotomy demonstrating the retraction of the dural leaflets and the intact outer membrane. (B). The outer membrane has been opened and the cSDH has been removed. Note the gentle lifting up of the opaque yet translucent inner membrane with the microhook that enables the fenestration of the inner membrane.

Figure 3

Diagram of the intraoperative view of the mini-craniotomy. The dura is retracted and the outer membrane is depicted with the underlying chronic subdural hematoma. A fenestration of the inner membrane is portrayed. The underlying pia-arachnoid membrane on gross examination is normal appearing.

Figure 4

Diagram showing a coronal view through the cSDH. Note the cross-sectional appearance of the layers with the outer membrane adjacent to the dura and the thinner inner membrane pressing upon the pia-arachnoid. The compression upon the underlying cortex affects the flow of interstitial fluid through the brain and the glymphatic system.

Figure 5

CT and MRI examples of cSDH. 88 yo F with largely chronic SDH with small acute component. (A). coronal CT, (B). Axial CT, (C). MPR coronal MRI, (D). T2-FLAIR Axial MRI. The arrows point to the inner subdural membrane which can be seen delineating a boundary between the cSDH and the pia -arachnoid. (E,F): 79 yoM with subacute cSDH. Once again, the arrows point out the location of the inner subdural membrane.

Figure 6

Diagram demonstrating the fenestration of the inner cSDH membrane. (a). The inner subdural membrane is elevated off of the pia-arachnoid using a microhook or by applying suction with a Rhoton #5 sucker. (b). Once this is lifted, a plane is readily developed between the pia-arachnoid and the membrane. A series of cuts may be made extending out radially to complete the fenestration. (c). The fenestration of the inner membrane allows the decompression of the cortical surface and the pia arachnoid. Note that the pia-arachnoid is not opened and rarely is scared to the inner membrane. Occasionally we have noted the accumulation of xanthochromic fluid that has accumulated between the inner membrane and pia arachnoid. This is irrigated out.

Figure 7

A Higher magnification diagram demonstrating the relationship of the inner cSDH membrane to the pia-arachnoid and the underlying cortex. Adhesions between the pia-arachnoid and the inner subdural membrane were very rare. When encountered these were easily released by cutting them with a microscissors. It is also possible to plan the fenestrations around these areas of adhesion, thus still affecting the release of the tension band of the inner membrane upon the underlying cortex.

Figure 8

Eighty three yo F who suffered a ground level fall who was on aspirin. She had mild headaches and speech difficulties with a right upper extremity pronator drift. (A): Preoperative coronal CT scan demonstrating a left sided cSDH with midline shift. Note the lentiform shape afforded by the inner subdural membrane that is displacing the underlying cortex. There is a smaller right sided acute on chronic SDH. (B): Post-operative coronal CT scan done within 1–2 h of reaching the recovery room. Note the decrease in the mass effect and midline shift with near complete removal of the cSDH and re-expansion of the brain on the left. There is very little intracranial air. On the right sided there is increased prominence of the subdural space and the acute on chronic subdural hematoma.