The Operation for Spinal Lipoma in Children

Patient Positioning

• Position: The patient is positioned prone, with padding under the chest and pelvis to reduce pressure on the abdomen and decrease epidural bleeding.
• Head, arms, and legs: The head remains neutral or lateralized, depending on the preferences of the anesthetic team, supported by a cushioned headrest. Arms are arranged adjacent to the head and knees slightly bent, with padding between for enhanced support and monitoring.

Patient positioning: The authors’ preferred positioning for the patient during surgery.

Neuromonitoring

Monitoring assesses the integrity of the motor pathway from cerebral cortex to selected muscles. Mapping comprises direct stimulation of nerve roots (and spinal cord) to assist in determining functional anatomy. The commonly employed IOM modalities during spinal lipoma surgeries include:

• MEPs: Depending on the response obtained in the lower limb muscles, the surgeon can identify the corresponding lumbar spinal level and use this to guide the surgery.

MEP measurement: (Top) Electrode position in patient. (Middle) Muscles stimulated by the electrodes per location. (Bottom) Foraminal level per location.

• BCR monitoring : BCR monitoring is performed by recording the responses from the bilateral external anal sphincters following electrical stimulation applied to the sensory branches of the pudendal nerve — specifically, the dorsal penile or clitoral nerve. During this procedure, four needle electrodes are placed into the external anal sphincters on both sides at the mucocutaneous junction. A bipolar configuration is used to obtain a lateralized BCR measurement. The electrical stimulation to the dorsal penile or clitoral nerve is accomplished using surface electrodes. The goal is to assess the integrity of the sacral neural pathways, particularly the reflex arc formed at the sacral levels S2–S4. This monitors the integrity of the afferent and efferent sacral neural pathways and is especially valuable in surgeries closer to the conus medullaris and sacral nerve roots.
• Nerve root mapping: Direct electrical stimulation of nerve roots (and the spinal cord) is performed to identify functional nerve roots during mobilization and subsequent resection of the lipoma to ascertain their functional status and guide the surgical approach. This may decrease the risk of harm to these critical structures. Typically, recording electrodes are placed in the lower extremity muscles and rectal sphincter. The usual stimulation intensity is 0.5 mA; this can be increased up to 5 mA when results are equivocal or stimulation of the placode is required.

Surgical Approach

The following procedure description reflects the senior author’s preference and is based on the method described by Pang et al. (2).

Exposure of Spine

• Longitudinal, low back incision: Accurate localization of the skin incision is crucial for identifying anatomical structures. It is essential to leave space at the top and bottom of the incision for a potential extension if needed. Therefore, a longitudinal rather than transverse incision is preferred.

Skin incision: The skin incision site is marked on the patient’s back.

• Exposure of spine: Paraspinal muscle reflection proceeds with monopolar cautery.

Deep tissue incision: Monopolar cautery is used for the opening of deeper layers of fascia and deep muscles.

• Management of subcutaneous lipoma: To reduce the postoperative subcutaneous dead space and to provide natural padding and protection to the spinal cord and duraplasty, complete resection of the subcutaneous lipoma is not recommended during the same surgical procedure as cord untethering.

Exposure of Spinal Canal

• Laminectomy: The laminectomy should start at least one spinal level above the rostral attachment of the lipoma to the spinal cord (ultrasound can be helpful to verify adequate exposure). The senior author’s preference is to open the posterior elements using a laminoplasty technique. However, this is not replaced at the end of the procedure to avoid compromising the underlying expansion duraplasty (there is a negligible risk of postoperative deformity at this level in children).
• Stalk connecting subcutaneous fat to intraspinal lipoma: A discrete lipoma stalk frequently connects the intraspinal lipoma to the subcutaneous fat; it is sharply isolated and traced through a defect in the lumbodorsal fascia. This stalk is attached to the spinal cord and cannot be tugged during fascial dissection.

Lipoma stalk: (Left) The defect in the lumbodorsal fascia is exposed. (Right) The stalk connecting the lipoma to the spinal cord is revealed.

• Exposure of spinal canal: It is essential to ensure that lateral bone removal is sufficient to allow clear visualization of the lipoma/dural interface.

Dura Opening

• Initial dural opening: The dura should be opened in the midline, approximately 1 cm above the lipoma’s upper limit to ensure a good view of normal spinal cord and to allow adequate space for the duraplasty.

Dura opening: A linear midline opening of the dura is made, starting over the normal spinal cord and extending to the junction between lipoma and dura.

• Initial exposure subdural space: The dural edge on each side is widely retracted with sutures. This crucial maneuver, made possible by generous lateral bone removal, reveals the “crotch” where the far lateral fringe of the lipoma attaches to the inner surface of the dura. It is here that nerve roots are particularly vulnerable to injury during dissection.

Crotch exposure: The space between the dura and lipoma crotch is illustrated in the above images. In the lower image, the dissecting scissors are within the crotch and demonstrate the line of dural reflection.

Intervention

Monitoring

• IOM during dissection: Unlike SSEPs, MEPs are not recorded continuously. Good team communication is particularly important during this stage to ensure that MEPs and BCR are measured at frequent intervals.

Lipoma Detachment

• Identification of lipoma junction: Using the operating microscope, identify the junction where the dura, spinal cord, and lipoma converge.
• Identification of dural attachment: Grasp the lipoma and retract it medially against the tagged dural edge to expose the fat-dural attachment (crotch dissection).

Crotch dissection 1: Demonstration of crotch dissection using scissors and gentle contralateral retraction with forceps.

• Sharply dissect crotch, avoiding nerves of DREZ: To avoid blindly injuring the nerve roots of the DREZ that project from the spinal cord on the slightly medial side of the crotch, lean the round curve of the scissors firmly against the inner lining of the dura while cutting the fat-dura attachment. The lipoma-dural interface is frequently rotated to one side, making crotch dissection difficult.

Crotch dissection 2: Scissors are used to cut the interface between the dura and the spinal lipoma.

• Separate nerve roots from dura: Identification of nerve roots can be aided by direct stimulation. Coax the hidden roots into view wherever the detached fat is pulled back, and gently separate them from the dura by blunt dissection toward the exit foramina.
• Proceed with dissection to the caudal end of lipoma: Eventually the lipoma (still attached to the terminal spinal cord) is completely detached from the dura.

Lipoma Resection

• Start lipoma resection at rostral end: Lipoma removal begins at the rostral end, where the anatomic relationships between fat, the DREZ, and the dorsal nerve roots are most easily identified.
• Separate dorsal roots from lipoma: The lipoma is then resected from the underlying neural placode along a gliotic “white plane.” It is important that the DREZ is identified and preserved during this part of the dissection. The fusion line where lipoma meets neural tissue and nerve roots is sometimes indistinct; a safe cuff must be left to avoid nerve root injury.

Lipoma resection illustration showing nerve roots and fusion line: (Left) Note the interface between the fat, the spinal cord, and nerve roots (i.e., the fusion line, indicated by the dotted red line) and (Right) the white plane, which is visible after resecting the lipoma. (Saenz, Amparo. Lipoma resection and fusion line. September 2023, Private collection, London.)

• Sharply dissect lipoma in white plane medial to fusion line: Sharp dissection with pointed scissors or micro scissors is used to locate a thin but distinct silvery white plane between the fat and the neural placode cord at the demilune of the rostral fusion line. Careful dissection, guided by the white plane and staying medial to the splayed DREZ, ensures the neural structures remain undamaged. In large and asymmetrical lipomas, navigating the white plane can be challenging.

Lipoma resection illustration showing nerve roots and fusion line: (Left) Note the interface between the fat, the spinal cord, and nerve roots (i.e., the fusion line, indicated by the dotted red line) and (Right) the white plane, which is visible after resecting the lipoma. (Saenz, Amparo. Lipoma resection and fusion line. September 2023, Private collection, London.)

• Complex and chaotic lipomas: No attempt should be made to resect lipoma that extends beyond the DREZ (i.e., a chaotic lipoma). The placode is often rotated — this can be anticipated on the preoperative MRI. It is always the dorsal and never the ventral part of the lipoma that tethers the spinal cord.

White plane dissection: Image shows a complex asymmetrical lipoma in which the white plane dissection has been accomplished using scissors.

• Degree of lipoma resection: Maximal (total or near-total) resection of the lipoma and thorough unhinging of the placode is necessary to convert a bulky, transfixed lipoma-cord complex into a free-floating, thin, supple neural plate.
• Control of bleeding: Bleeding points on the placode are identified by gentle irrigation and then controlled using microbipolar cautery.

Irrigation and coagulation equipment: The image shows the irrigation equipment and microbipolar cautery used by the authors over the white plane.

• Post lipoma-resection mapping: After lipoma resection, nerve roots are systematically stimulated to check for continuity.

Stimulation of nerve roots: Image shows resected lipoma, with nerve roots being stimulated by direct stimulation.

Neurulation

• Rationale for neurulation of placode: Neurulation transforms a broad, watery, sticky sheet into a trim, sturdy, pia-covered tube bearing a single seam, evocative of the natural neurulation process.
• Prepare placode for neurulation: The lateral edges of the placode can be approximated and temporarily secured with microclips at short intervals along the reconstituted terminal spinal cord.

Transient neurulation: Microclips are used to temporarily secure the neurulation of the placode.

• Suture neurulation placode: 8-0 Nylon pia-to-pia microsutures with buried knots are placed between the clips. The microclips should be removed after enough sutures are in place to withstand the torque.

Final neurulation: Image shows neurulation with 8-0 Nylon sutures.

Neurulation illustration: (Left) Illustration of axial view of the spine before neurulation. (Right) Illustration of axial view of the spine after neurulation. (Saenz, Amparo. Lipoma resection before and after neurulation. September 2023, Private collection, London.)

• IOM: MEP and BCR measurements are repeated at the end of neurulation to ensure that neurulation of the placode has not caused ischemia or strangulation.

Closure

• Primary dural closure vs duraplasty: Primary dural closure is rarely advised and is performed only when there is ample dura to ensure optimal cord-sac ratio. For most cases, expansion duraplasty is recommended.
• Duraplasty graft material: The authors’ preference is bovine pericardium. It is easy to suture, has good strength, and will inflate to optimize the cord-sac ratio.

Bovine pericardium for duraplasty graft: Illustration of the bovine pericardium used for duraplasty. (Saenz, Amparo. Bovine pericardium. September 2023, Private collection, London.)

• Create and use template: A template of the dural defect is made, then the graft is shaped to fit the size of the dural defect to prevent inward folds.

Duraplasty construction: (Left) Neurosurgical patty is used to measure the defect. (Right) Bovine pericardium is placed over the dural defect.

• Suture duraplasty: Interrupted sutures are placed at the “compass points,” then a continuous 5-0 prolene suture is used to achieve a watertight closure.

Completed duraplasty: Shown is the final sutured duraplasty.

• Confirm watertight closure: The thecal sac is inflated with warm normal saline and the integrity of the closure is confirmed with a Valsalva maneuver.