Objective: To introduce an alternative surgical ap- proach for the treatment of Chiari I Malformation with mild-to-moderate syringomyelia following a decompres- sive suboccipital craniectomy. This approach involves incising only the exterior layer of the dura mater and subsequently dissecting it from the interior layer without opening the latter.

Methods and patients: We employed this technique in a brief series of cases that were admitted to our de- partment for moderate symptoms, including intermittent headache and dissociated sensory loss in the upper ex- tremities, which were caused by a Chiari Malformation Type I. The patients were placed in the sitting position. We conducted a reduced median suboccipital craniecto- my and resection of the posterior arch of C1 that was tai- lored to the level of tonsil descent, ranging from a limited superior half to a complete resection. Subsequently, we excised the outer dural layer while preserving the inner one by either a liniar or a “V” shaped incision. We then separated the outer and inner layers to the craniectomy margin using a delicate dissector. The cerebellum and the medulla were visible and pulsating due to the transparen- cy of the inner layer and the arachnoid. Duraplasty with autologous periosteum was used in the initial four cases. Results: The short-term postoperative course was fa- vorable in all cases, with the patients being discharged with complete or almost complete symptom resolution and no deficits. The brainstem compression and syringo- myelia were significantly reduced during the follow-up at 3, 6, and 12 months following the surgery.

Conclusions: The interlayer dural split technique is a viable treatment option for symptomatic cases of type I Chiari malformation in adults with mild-to-moderate syrin- gomyelia. It is potentially more effective than decompres- sive craniectomy and C1 laminectomy alone, and it is less invasive than exposing the arachnoid. A larger case-control series is still required to validate this technique.