The Operation Selective Dorsal Rhizotomy in Children

Patient Positioning

• Prone: Position prone on bolsters.
• Avoid abdominal compression: Allow space for abdominal relaxation between bolsters to reduce epidural bleeding at time of laminotomy.

 

Patient positioning. Patient positioned prone for SDR. The incision is marked in the midline, and subdermal needles and urinary catheter are positioned.

Confirmation of the Level of Opening

• Clinical examination: Confirmation is usually done by clinical examination; the iliac crest line is at the L4/5 spinous process interval and the sacral-iliac joint line is at the L5-S1 interspace.
• X-rays can be used to confirm: Radiographical screening remains the gold standard. Preoperative imaging with a fiducial marker (such as an ECG sticker) placed over a spinal landmark or taking an x-ray on the table can be used to confirm a spinal level.

Surgical Approach

There are two categories of approaches for the modern SDR:

Multilevel vs. single level exposures. On the left is the multilevel laminectomy approach where the spinal canal is exposed for four or more levels of the lumbosacral spine.. On the right is the single-level exposure at the level of the caudal end of the conus (32).

Equina Approach (“Peacock” (25))

The benefit of the cauda equina approach is that the level of the root can be confirmed anatomically at the corresponding exit foramen. The potential drawback is a longer-segment laminotomy and the need for separation of the dorsal root from ventral root at their exit foramen.

• T12-sacral incision: A linear midline incision from T12-L1 to sacrum.
• Frequent spina bifida occulta or cartilaginous L5 and S1 lamina: Beware of the high rate of spina bifida occulta at L5 and S1.
• Hinged S1-L1 laminotomy: Enter canal at L5-S1 or S1-S2 interspasces and perform a  narrow laminotomy for 5 or 6 levels using a high speed craniotome or drill (see image below).  The freed lamina and spinous processes are then retracted rostrally at T12-L1 (the rostral end of the ligamentum flavum at the interspace just rostral to the uppermost freed lamina is left intact, establishing a hinge so the freed lamina can be reflected out of the wound).

 

Laminotomy technique used to expose the thecal sac: A high-speed craniotome is inserted into the epidural space at the S1-2 or L5-S1 interspace and used to cut the lamina medial to the facet joints on either side of the spinous process up to the L1-L2 or T12-L1 levels (32).

• Prepare lamina for replacement: Three perforator holes per level are created for reattachment of the laminotomy  (i.e., a laminoplasty) at the time of closure; one in spinous process and two in laminae (56).  Two more holes are created in the lamina lateral to the laminotomy to match corresponding holes made in the laminae medial to the laminotomy.   Care is taken to avoid entry into the facet joints.

Modification of Cauda Equina Approach (Sindou (31))

This approach takes a little longer than the multievel or single level laminotomy approaches but has the benefit of leaving the laminae and much of the interspinous and interlaminar ligaments in place.

• KIDr approach of Sindou and Georgoulis: The KIDr requires no laminectomy, using instead bilateral interlaminar keyhole openings at every second laminar level from L1 to S2. This approach, in essence, uses the same technique and principles as the cauda equina approach discussed above, but the exposure technique is different. Through these interlaminar keyholes, a midline durotomy is performed, and an operative microscope is used for the magnification and light source.

 

KIDr approach from Lyon group (31). Shown is x-ray image of patient after undergoing a KIDr.   Seen are the shadows of laminotomies performed at L1-L2, L3-L4, and L5-S1 interspaces. Arrows point to authors’ drawn positions of nerve roots found at surgery on which the SDR was performed.

Conus Approach (“Park” Approach, also Used by Foerster and Gros(20))

The main benefit of this approach is that it allows easier separation between the dorsal and ventral roots as the ventral and dorsal root have not merged at this level yet; a second benefit is the potentially smaller laminar opening required. The drawback is the risk of wrong-level surgery and including the pudendal plexus in the surgery with potential urogenital fall-out.

• Determine level of laminotomy: The dorsal spine at the T12-L2 levels is exposed and the ultrasound used via the interlaminar spaces to image the spinal canal .  The caudal tip of the conus is identified.
• Short-segment laminotomy to expose conus: A 1- to 3-level laminectomy is performed (different units open differing numbers of laminae).

Intervention

Montreal Protocol- Multilevel Exposure

• Open dura: The dural opening is down to “cul de sac” to allow identification of lower sacral roots as they exit dural sac.
• Nerve roots exposed: The arachnoid sleeve forming around each of the targeted root as they exit the canal is cut to free the roots from the investing arachnoid of the cauda equina.
• Dorsal roots separated from ventral: Dissectors are used to separate dorsal from ventral root just rostral to the foramen where they are visibly separate. At this point, gravity can be used in most cases to accomplish the separation by simply suspending the sensory root and allowing the ventral root to fall ventrally at the natural cleft.
• Not everyone performs neurophysiological testing: As previously mentioned, some authors (16,30) have made strong cases that fascicle dissection and electrophysiological mapping of evoked muscle contraction in response to sensory root/rootlet stimulation is not needed when performing sensory rhizotomy for treating spasticity. But most units will still perform neurophysiological monitoring routinely for all SDR cases.

 

Montreal Protocol. After exposure of the nerve roots, the dorsal (sensory) roots are separated from the ventral (motor) roots at the chosen level of opening (75).

• Confirm responsiveness of muscles to evoked action potentials: Under optimal anesthetic conditions and steady-state conditions, motor roots S2 and S1 are stimulated to confirm integrity of neuromuscular junction and level based on response to 0.2-mA square waves.
• Determine threshold of sensory roots: A sensory root threshold run for the S2 through L2 roots bilaterally is done using 1-Hz stimuli in increments starting at 0.2 mA/root and advancing. Usually, a muscle response to a 1-ms square pulse at 0.2–3.0 mA threshold is witnessed both by the physiotherapy examiner (muscle palpation) and by EMG recording.
• Examine roots with stimulus train: A second round of stimulation using a 50-Hz train over 1-second train is done for the S2 through L2 sensory roots bilaterally. We use a supra-threshold amplitude that is 2–4 times the threshold value (usually 6-8 mA) for each root.
• Pattern of responses reviewed: The responses for both the physiotherapy and EMG examinations are then reviewed and abnormally responding nerve roots identified.
• Abnormal roots separated into rootlets, which are then tested: Dissection of abnormally responding dorsal roots into rootlets. Usually 12 dorsal roots produce 48–50 rootlets (i.e., on average 4 rootlets/root). They are stimulated with same current intensity as used on the stimulation train run for the whole roots.
• Abnormal rootlets cut: The worst 1, 2, or 3 rootlets of a given root are sectioned as determined by grade 4  responses to alleviate the spasticity. Lesioning is limited to 50% of total of rootlets for both sides. At the L4 level, only 1 or 2 rootlets out of the 4-5 found will be cut on each side. In the Cape Town and Lyon groups, anywhere between 30% and 80% of the dorsal root is cut, depending on the preoperative clinical evaluation and management plan. (The more abnormal spasticity in an area that impairs function, the higher the percentage of rootlets that are sectioned. L4 will only be sectioned above 30% if there is clear abnormal distal or contralateral spread found clinically and on IOM.)

Grade	Muscle response to 1 sec 50Hz stimulus train
0	Unsustained CMAP in muscles innervated by nerve being stimulated
+1	Sustained CMAP in muscles innervated by nerve being stimulated
+2	Same as Grade +1 plus CMAP in muscle innervated by nerve root at an adjacent level
+3	Same as Grade +1 plus CMAP in multiple ipsilateral leg muscles innervated by nerves outside segmental level of stimulated nerve
+4	Same as Grade +1 plus CMAP spread to muscles in upper extremities

Park Grading of abnormal response to nerve root stimulation. Grading of the motor responses during IOM for SDR, based on the original Fasano principles (4, 19, 38).

KIDr Protocol

Intervention protocol similar to Montreal Protocol

 

Park Protocol – Short-Segment Exposure

• The dura is opened
• Identification L2 Roots:  The L2 roots are identified as they exit their foramina bilaterally.  The dorsal L2 roots are separated from the ventral roots as they near the conus and this separation used to identify the cleft between the lumbosacral dorsal and ventral roots/rootlets on the lateral surface of the spinal cord. This separation is then maintained during performance of the SDR.
• Identification of Sacral Roots:  The S2 dorsal rootlets can then be differentiated from dorsal sacral rootlets caudal to it by fact the latter will be more tightly grouped than rootlets rostral to them.  Once this has been defined, a Silastic sheet can be placed so that the L2 through S2 rootlets lay on the sheet, separated from the rest of the cauda equina bilaterally.
• SDR Lesioning:  Identification and lesioning for the SDR then proceeds using criteria as discussed in the Montreal Protocol.

Closure

• Dura closed “watertight”: A routine watertight dural closure is done. We prefer to use a dural sealant placed epidurally to reinforce dural closure (this is not required, and many units do not use sealants). We routinely perform a Valsalva test to confirm watertight seal.
• Epidural catheter for analgesic administration: Epidural catheter for pain through separate stab wound advanced to T10. (This is not done in all units.)
• Laminar roof replaced: The lamina are then repositioned and sutured through the previously drilled holes to obtain a 3-point fixation at every level of laminotomy flap. The spinous process hole is used to pass suture through reflected periosteum to bring the latter into contact with the spinous processes to encourage bony healing. Replacing the lamina is elective. We prefer to do this to improve the appearance of the surgical site after healing.
• Watertight closure of muscles tested: A test dose of morphine with bupivacaine is given under mild pressure after the fascial plane closure to test the integrity of the closure (not done routinely but good tip).
  Routine skin closure: Subcuticular skin closure with steri-strips can be used for skin closure. This permits earlier pool therapy in recovery phase.