33.1.1 Case Summary

A 5-year-old Caucasian girl was referred by her pediatrician to the senior author [MHHN] for evaluation because of concerns about the development of scoliosis. She had been born at full term to non-consanguineous parents. Her Apgar scores were 8 and 10 at 1 and 5 minutes after birth. Her developmental milestones had been normal up to her first birthday. Subsequently, recurrent chest infections and emergency admissions to a pediatric intensive care unit for a couple of years warranted investigations for cystic fibrosis and other genetic / inherited conditions. Extensive work-up by pediatricians and geneticists revealed a diagnosis of type 2 SMA. By her fourth and fifth birthdays, she was lagging in motor milestones, with frequent falls, and had a Gowers sign (suggestive of proximal trunk and lower extremity muscle weakness).

At her first clinic visit, she was in the 70% percentile for weight and age and the 75% percentile for height. On clinical examination, she had unequal shoulders and sat leaning to the left side. She was capable of walking a few steps with support. Plain radiographs revealed a left sided thoracolumbar scoliosis of 65 degrees, which was largely flexible. She was prescribed a TLSO (thoracolumbosacral orthosis), to be worn full-time (at least 22 hours per day), and it was arranged for her to be seen at 4-month intervals. By her sixth birthday, despite her compliance with full-time brace wear, the scoliosis had worsened to 85 degrees (Fig. 33.1). Pulmonary function tests revealed the FEV₁ (forced expiratory volume in 1 second) to be 20% of the predicted value, and the FVC (forced vital capacity) was 23% of the predicted value. Surgery was offered, and the parents were counseled about the importance of arresting progression of the deformity. Discussion among anesthesiologists, pediatricians, and intensive care physicians at multidisciplinary team meetings yielded a consensus that her pulmonary function was too poor for her to withstand conventional growing rod surgeries and regular 6-monthly distractions under general anesthesia. She was discussed at the multi-disciplinary meeting and a collective decision was made for her to have the novel implant inserted. Thereafter, she was enrolled into the magnetic growing rods program to have dual growing rods inserted submuscularly from T2 to the pelvis.

33.1.2 Surgical Technique

The patient was placed in the prone position on a Montreal mattress with adequate padding of all pressure points. It was ensured that there was no compression on the eyes. General anesthesia was administered via endotracheal intubation. Special care was taken to avoid hyperextension of the upper extremities to avoid the risk of causing brachial plexopathy / traction neuropraxia. After preparation with 2% chlorhexidine and draping under strict asepsis, two incisions were made in the midline over the proximal thoracic spine to expose the T2-T5 vertebrae and over the distal lumbar spine to expose the L3-S1 vertebral segments by subperiosteal dissection. A freehand technique was used to insert 4.5 × 25-mm pedicle screws at the T3 and T4 vertebrae bilaterally. Two down-going transverse process hooks were inserted into T2. The transverse process hooks exert laterally directed forces, protecting the spinal cord from the screws by preventing medial migration if they happen to loosen with time. Two 5 × 30-mm pedicle screws were then inserted into the L4 and L5 pedicles. Two 7.5-mm-diameter iliac wing screws were then inserted into the pelvis under C-arm image intensifier guidance. A flexible rod template was used to measure the length of rod needed, and the magnetic growing rod was cut to the desired length. The actuator area containing the magnet measures 9 cm in length and 9 mm in diameter. Appropriate sagittal contouring was done proximal and distal to the actuator area to ensure that the actuator area remained straight and to facilitate ease of insertion. The magnetic coil was tested with the handheld device before it was inserted.

A 20-gauge chest tube was used to railroad the rod submuscularly in a caudocranial direction on the concave side. The rod was then attached to the pedicle screws and iliac bolts (with the use of lateral connectors). Cranially, the rod was attached to the hybrid construct (hooks and pedicle screws). The procedure was repeated on the convex side, and all anchors were tightened with gentle distraction on the concave side to level the pelvis and correct the scoliosis. Finally, a cross-link was attached at the proximal end to link the two rods as a single unit.

The posterior elements were then decorticated and mixed with bone marrow aspirate obtained at the time of pedicle screw insertions. Silicated calcium phosphate (Actifuse granules; Baxter BioSurgery, Deerfield, Illinois) was mixed with native / host bone and laid over both the proximal and distal fixation anchors. Hemostasis was secured, and the wound was closed in layers with absorbable sutures and no drains were used.

33.1.3 Postoperative Care and Rehabilitation

The child was allowed to sit up erect with effect from postoperative day 1 and to mobilize from day 3. No brace was needed at the time of discharge. She was seen at regular 3-monthly intervals in the office by the senior author, who also performed serial 3-monthly distractions. The initial 3 months of close observation was needed to ensure adequate arthrodesis at the anchor sites, to facilitate regular serial distractions. The patient underwent lengthening on an outpatient basis wherein a handheld magnetic wand was used to identify the precise area of the magnetic coil. Low-dose radiography was used after each distraction to confirm that the spine had indeed lengthened.

Profound improvement in her health was noted 1 year postoperatively, by which time she had undergone four lengthening procedures and gained 12 mm in height. There was a significant improvement in her quality of life, with markedly fewer visits to the accident and emergency department. Her occasional episodes of pneumonia were very few and far between. At the end of 2 years postoperatively, she had an FEV₁ of 50% predicted and an FVC of 55% predicted (statistically significant difference; p < 0.0002). Elongation of the right-sided convex rod was 23.2 mm, and elongation of the left-sided concave rod was 32.4 mm (Fig. 33.2). The parents were extremely satisfied with the clinical results.

33.1.4 Discussion

The natural history of type 2 SMA is progressive proximal muscle weakness with deterioration in pulmonary function over time. ⁵ Mortality is inevitable by the end of the second decade. The evolving parasol deformity of the chest (i.e., vertical positioning of ribs with development of a bell-shaped thorax due to weak intercostal muscles and a strong diaphragm) causes volume depletion deformity of thorax, resulting in thoracic insufficiency syndrome. ⁶ Conventional growing rods require regular 6-monthly distractions under general anesthesia, which is not without complications (i.e., risk for aspiration and psychological stress for the child, parents, and extended family). ⁷ The vertebral expandable prosthetic titanium rib (VEPTR) is an alternative option wherein the VEPTR gantry supports the collapsing chest cage (parasol deformity). However, it is also fraught with multiple complications (at least 25%) and an increased risk for wound dehiscence. ⁸

Although a dramatic improvement in pulmonary function was seen in this child, we do not believe that magnetic growing rods alter the natural history of SMA. They do, however, significantly delay the rate of pulmonary deterioration, which creates a spurious impression of improvement during pulmonary function testing. This is the first report of a case of type 2 SMA in the world with a 2-year follow-up that has reported an improvement in pulmonary function.