Achondroplasia & Thoracolumbar Kyphosis: Pediatric Spinal Stenosis & Myelopathy Case Study

Patient Presentation and History

A 5-year-old male with a genetically confirmed diagnosis of achondroplasia presented to the pediatric orthopedic trauma and spine clinic with a 6-month history of progressive gait disturbance, increasing difficulty with independent ambulation, and new-onset nocturnal enuresis. His parents reported that he had recently started falling more frequently, specifically when attempting to run or navigate uneven surfaces. They noted increased stiffness in his lower extremities, difficulty with stair climbing, and a noticeable change in his posture, with his upper back appearing increasingly rounded. There was no specific acute traumatic event reported; the neurological deterioration was insidious and progressive.

His medical history is significant for achondroplasia, diagnosed prenatally via ultrasound and confirmed postnatally via molecular genetic testing demonstrating the classic FGFR3 (fibroblast growth factor receptor 3) c.1138G>A activating mutation. This mutation results in a gain-of-function that constitutively inhibits endochondral ossification, leading to the characteristic skeletal dysplasia. He had a history of mild hydrocephalus that was managed conservatively with serial head circumference monitoring and had remained stable without the need for ventricular shunting.

Notably, the patient had undergone a suboccipital craniectomy and C1 laminectomy (foramen magnum decompression) at 18 months of age due to MRI evidence of severe craniocervical junction stenosis and mild cord compression, though he exhibited no significant neurological deficits at that time. He also had a history of recurrent otitis media, managed with bilateral tympanostomy tubes at age 2, and mild obstructive sleep apnea, managed with continuous positive airway pressure at night since age 3.

Developmental milestones were achieved with anticipated delays inherent to his skeletal dysplasia; he walked independently at 20 months, later than his unaffected peers but within acceptable limits for achondroplasia. Over the past six months, however, his ambulatory capacity had regressed from independent community ambulation to requiring a walker for distances greater than 50 feet. There were no other significant medical comorbidities. Family history was non-contributory for other skeletal dysplasias or heritable neurological conditions.

Clinical Examination Findings

Upon inspection, the patient exhibited classic achondroplastic phenotypic features including short stature (falling below the 3rd percentile for age- and sex-matched achondroplastic normative growth charts), disproportionate rhizomelic limb shortening, macrocephaly, frontal bossing, and midface hypoplasia. His hands demonstrated a characteristic trident hand configuration with short, broad phalanges and a widened space between the third and fourth digits.

A significant, rigid thoracolumbar kyphosis was evident, with its apex estimated clinically around the T12-L1 junction. The skin over the apex of the kyphosis was intact without signs of ulceration, erythema, or cutaneous stigmata of occult spinal dysraphism such as dimples, hemangiomas, or hairy patches. There was a compensatory, exaggerated lumbar lordosis distally and a hyperlordotic cervical spine proximally. Standing posture demonstrated a wide-based, waddling gait that had devolved into a pattern characterized by noticeable spasticity and dynamic scissoring of the lower extremities during the swing phase of gait.

Palpation of the spine revealed no tenderness to direct manipulation or percussion, but the thoracolumbar kyphosis felt distinctly stiff and irreducible on attempted passive extension maneuvers (suspension test). Paraspinal musculature was significantly hypertrophied in the lumbar region, representing a biomechanical compensation for the sagittal plane malalignment.

Range of motion assessment of the spine confirmed the rigidity of the thoracolumbar kyphosis. While the cervical spine had full range of motion, and the lumbar spine demonstrated an exaggerated but flexible lordosis, the apex of the kyphosis remained fixed in all planes. Hip and knee joint range of motion was full, with slight hyperlaxity noted in the elbows and knees. Genu varum was noted bilaterally, which is typical for achondroplasia due to fibular overgrowth relative to the tibia.

Neurological examination revealed a distinct upper motor neuron myelopathic pattern:
* Motor: Bilateral lower extremity weakness, predominantly affecting distal muscle groups. Manual muscle testing demonstrated hip flexors 4/5, knee extensors 4/5, ankle dorsiflexors 3+/5, and ankle plantarflexors 4/5. Upper extremity strength was preserved at 5/5 bilaterally, indicating the lesion was distal to the cervical spine.
* Tone: Markedly increased tone in bilateral lower extremities, consistent with spastic diplegia. Upper extremity tone was normal.
* Reflexes: Severe hyperreflexia in patellar and Achilles tendon reflexes bilaterally (Grade 3+), with sustained clonus (greater than 4 beats) at both ankles. Bilateral Babinski signs were positive (upgoing plantar responses). Hoffman's sign was negative bilaterally.
* Sensory: Intact sensation to light touch, pinprick, and proprioception in the upper extremities and upper trunk. There was mildly diminished sensation to pinprick below the L1 dermatome bilaterally, though a distinct sensory level was difficult to map reliably in a 5-year-old. Proprioception in the toes was diminished bilaterally.
* Sphincter Function: Diminished rectal sphincter tone on digital examination. The parents' report of new-onset nocturnal enuresis strongly correlated with neurogenic bladder dysfunction secondary to cord compression.

Vascular assessment revealed palpable and symmetrical dorsalis pedis and posterior tibial pulses in all four extremities. Capillary refill was brisk, and there were no signs of venous insufficiency or arterial compromise.

Imaging and Diagnostics

Radiographic Evaluation

Initial diagnostic imaging consisted of full-length, standing posteroanterior and lateral radiographs of the spine, as well as localized coned-down views of the thoracolumbar junction. The lateral radiograph demonstrated a sharp, angular thoracolumbar kyphosis measuring 65 degrees from T10 to L2 using the Cobb method. The apex of the deformity was centered at T12 and L1.

Morphologic analysis of the vertebral bodies revealed classic achondroplastic changes, including bullet-shaped vertebrae at the apex of the kyphosis (anterior wedging and hypoplasia of the T12 and L1 vertebral bodies) and posterior vertebral body scalloping. The pedicles were notably short and thickened in the sagittal plane. The interpedicular distances in the lumbar spine progressively narrowed from L1 to L5, an inverse of the normal anatomic widening seen in unaffected individuals, confirming the presence of congenital spinal stenosis.

Dynamic flexion and extension lateral radiographs over a bolster demonstrated less than 10 degrees of correction in the kyphotic angle, confirming the rigid, structural nature of the deformity. Bilateral lower extremity radiographs confirmed the expected rhizomelic shortening and genu varum without evidence of acute fracture or destructive osseous lesions.

Magnetic Resonance Imaging

To fully evaluate the neural elements and the extent of myelopathy, a total spine MRI without contrast was obtained. T1-weighted and T2-weighted sagittal and axial sequences were reviewed.

The MRI revealed severe central canal stenosis maximal at the T11-L2 levels. The stenosis was multifactorial, resulting from a combination of congenitally short pedicles, thickened ligamentum flavum, bulging of the intervertebral discs at the apex of the kyphosis, and the angular deformity itself draping the spinal cord over the posterior aspect of the T12 and L1 vertebral bodies.

There was complete effacement of the ventral and dorsal cerebrospinal fluid spaces at the apex of the deformity. Crucially, T2-weighted sagittal sequences demonstrated a hyperintense intramedullary signal within the conus medullaris and distal spinal cord at the T12-L1 level, consistent with myelomalacia, edema, and chronic compressive myelopathy. The conus medullaris terminated at the L2-L3 level, which is lower than typical but frequently observed in achondroplasia. The previously decompressed craniocervical junction demonstrated adequate CSF flow without recurrent stenosis or syrinx formation.

Computed Tomography and Templating

A fine-cut (1.0 mm slice thickness) Computed Tomography scan of the thoracic and lumbar spine with multiplanar and 3D reconstructions was obtained to delineate the complex osseous anatomy and facilitate precise surgical templating.

The CT scan confirmed the severe narrowing of the spinal canal and the anterior wedging of the apical vertebrae. Axial cuts were utilized to measure pedicle width, length, and trajectory. As anticipated in achondroplastic dysplasia, the pedicles were profoundly narrow, particularly in the lower thoracic and upper lumbar spine. Pedicle widths at T11, T12, and L1 measured between 2.5 mm and 3.2 mm, indicating that standard pediatric pedicle screws (typically 4.0 mm or 4.5 mm) would likely result in cortical breaches if placed using traditional freehand techniques. The CT data was subsequently loaded into a surgical navigation platform to plan for customized, trajectory-specific instrumentation and to evaluate the feasibility of pedicle subtraction osteotomy versus multiple posterior column osteotomies.

Differential Diagnosis

While the underlying genetic diagnosis of achondroplasia was established, the specific etiology of the progressive myelopathy and thoracolumbar kyphosis in a pediatric patient requires differentiation from other skeletal dysplasias and syndromic spinal deformities that present with similar neurological and biomechanical collapse.

Surgical Decision Making and Classification

Indications for Operative Intervention

The decision to proceed with operative intervention in this 5-year-old patient was unequivocal, driven by the presence of progressive neurological deficits, upper motor neuron signs, and MRI evidence of severe cord compression with myelomalacia.

In infants with achondroplasia, a flexible thoracolumbar kyphosis is present in up to 90% of cases, primarily due to generalized hypotonia, a large head mass, and delayed independent sitting. The standard of care for infantile achondroplastic kyphosis is non-operative, involving avoidance of unsupported sitting and the use of a custom thoracolumbosacral orthosis. In the vast majority of patients, this flexible deformity resolves once the child begins to walk and develops compensatory lumbar lordosis and paraspinal muscle strength.

However, in approximately 10% to 15% of patients, the kyphosis persists, becomes structural, and progresses. Surgical intervention is absolutely indicated when the kyphosis becomes rigid and exceeds 50 degrees, when there is anterior wedging of the apical vertebrae (bullet vertebrae) that fails to remodel, or, most critically, when there is evidence of neurological compromise. In this patient, the combination of a rigid 65-degree deformity and progressive spastic paraparesis with bowel/bladder involvement represented an impending neurological catastrophe, necessitating urgent decompression and stabilization.

Anatomic Considerations in Achondroplasia

Surgical planning in achondroplasia requires a profound understanding of the altered osseous anatomy resulting from the FGFR3 mutation. The primary defect lies in endochondral ossification, which severely affects the neurocentral synchondroses. Premature fusion of these synchondroses halts the posterior growth of the vertebral body and the pedicles, resulting in a spinal canal that is congenitally stenotic in both the sagittal and coronal planes.

Because the pedicles are abnormally short and narrow, the neural elements are highly vulnerable during decompression. The ligamentum flavum is often hypertrophied and buckled into the canal due to the loss of disc height and kyphotic angulation. Furthermore, the dura in achondroplastic patients is frequently adherent to the surrounding bony structures and ligamentum flavum, significantly increasing the risk of incidental durotomy during laminectomy. The epidural venous plexus is also typically engorged due to chronic compression, leading to increased intraoperative hemorrhage.

Given the rigidity of the deformity, a simple laminectomy would be insufficient and biomechanically detrimental. Laminectomy alone in the setting of kyphosis removes the posterior tension band, inevitably leading to rapid progression of the deformity, further anterior cord draping, and worsening neurological status. Therefore, the surgical strategy must include a wide posterior decompression combined with a deformity correction via osteotomies and rigid posterior segmental instrumentation and fusion.

Surgical Technique and Intervention

Anesthesia and Patient Positioning

The patient was brought to the operating room and induced with general endotracheal anesthesia. Intubation in achondroplastic patients requires extreme caution due to the high prevalence of craniocervical stenosis and potential instability. Although this patient had a prior foramen magnum decompression, manual in-line stabilization was maintained during intubation, and a video laryngoscope was utilized to minimize cervical extension.

Baseline somatosensory evoked potentials and motor evoked potentials were obtained prior to positioning. The patient was then carefully transitioned to the prone position onto a specialized pediatric spine frame, ensuring the abdomen was entirely free to prevent elevated intra-abdominal pressure, which would exacerbate epidural venous bleeding. All bony prominences were meticulously padded. Total intravenous anesthesia was maintained throughout the procedure to optimize neuromonitoring signal reliability. Mean arterial pressure was strictly maintained above 75 mmHg to ensure adequate spinal cord perfusion, particularly during the decompression and deformity correction phases.

Surgical Approach and Decompression

A standard posterior midline incision was made from T8 to L4. Subperiosteal dissection of the paraspinal musculature was performed bilaterally out to the tips of the transverse processes. Extreme care was taken during the exposure, as the laminae in achondroplasia can be paper-thin, and the interlaminar spaces are virtually non-existent due to the shingling effect of the kyphosis.

Once the posterior elements were exposed, intraoperative fluoroscopy confirmed the operative levels. The decompression phase commenced with a wide, bilateral laminectomy extending from T11 through L2. A high-speed burr with a diamond matchstick tip was utilized to thin the laminae until only a cortical shell remained, which was then carefully elevated using micro-curettes and Kerrison rongeurs. The use of large Kerrison rongeurs was avoided to prevent plunging into the critically stenotic canal.

As anticipated, the ligamentum flavum was severely hypertrophied and adherent to the underlying dura. Microdissection techniques were employed to separate these layers. The dura itself was noted to be tightly constricted and pulsatile only after the complete removal of the compressive osseous and ligamentous elements. Complete unroofing of the nerve roots at the apical levels was achieved by performing bilateral medial facetectomies and complete foraminotomies.

Osteotomy and Deformity Correction

To mobilize the rigid 65-degree kyphosis, posterior column osteotomies (Ponte-type osteotomies) were performed at the T11-T12, T12-L1, and L1-L2 levels. This involved aggressive resection of the superior and inferior articular processes, the ligamentum flavum, and the interspinous ligaments, effectively destabilizing the posterior and middle columns to allow for sagittal plane correction.

Given the patient's young age and the severity of the apical wedging, consideration was given to a Pedicle Subtraction Osteotomy at T12. However, the multi-level posterior column osteotomies provided sufficient flexibility to correct the deformity without the increased blood loss and neurological risk associated with a three-column osteotomy in a highly stenotic, dysplastic pediatric spine.

Spinal Instrumentation and Fusion

Given the profound narrowing of the pedicles identified on preoperative CT templating, traditional freehand pedicle screw placement carried an unacceptable risk of medial breach and spinal cord injury. Therefore, intraoperative 3D fluoroscopy (O-arm) and stereotactic navigation were utilized.

Reference arrays were clamped to the spinous process of T9, and a spin was performed. Using the navigated probe, pedicle trajectories were mapped. Due to the diminutive size of the pedicles (2.5 mm to 3.2 mm width), specialized 3.5 mm and 4.0 mm pediatric cortical-cancellous screws were utilized. Screws were placed bilaterally at T9, T10, L3, and L4. At the apex of the deformity (T11, T12, L1, L2), pedicle screws were omitted to allow for a massive decompression and to avoid crowding in the highly dysplastic apical pedicles. Instead, the construct spanned the apex.

Pre-contoured 4.5 mm titanium rods were selected. The rods were contoured to restore a physiological sagittal profile, incorporating a gentle kyphosis in the thoracic spine and restoring lumbar lordosis. The rods were seated into the proximal anchors and gradually cantilevered into the distal anchors. Compression was applied across the osteotomy sites to shorten the posterior column, thereby correcting the kyphosis and indirectly decompressing the anterior spinal cord by relaxing the neural elements.

Throughout the correction maneuver, MEPs and SSEPs were monitored continuously; signals remained stable and actually demonstrated an improvement in amplitude following the decompression and correction. Final fluoroscopy demonstrated excellent hardware placement and correction of the kyphosis to approximately 15 degrees.

The posterolateral gutters and the transverse processes were decorticated using a high-speed burr. A copious amount of local autograft, harvested from the laminectomy and osteotomy bone, was mixed with cancellous allograft chips and packed meticulously into the lateral gutters to promote a robust posterolateral fusion. The wound was closed in multiple layers over a subfascial drain to prevent postoperative hematoma formation.

Post Operative Protocol and Rehabilitation

Immediate Postoperative Care

The patient was transferred intubated to the Pediatric Intensive Care Unit for close hemodynamic and neurological monitoring. He was successfully extubated on postoperative day one without respiratory compromise. Strict mean arterial pressure parameters (>75 mmHg) were maintained for the first 48 hours to ensure optimal spinal cord perfusion and mitigate the risk of ischemic reperfusion injury following chronic compression.

The subfascial drain was removed on postoperative day three once output decreased to less than 30 cc per 24 hours. Intravenous patient-controlled analgesia was transitioned to oral multimodal pain management, including scheduled acetaminophen, NSAIDs (instituted after 48 hours to balance pain control with fusion biology), and a short course of oral opioids.

Neurological examinations in the immediate postoperative period demonstrated stable motor function with a subjective decrease in lower extremity spasticity. The hyperreflexia remained present but was less pronounced. Bowel and bladder function were monitored closely, and the nocturnal enuresis resolved prior to discharge, indicating successful decompression of the conus medullaris.

Long Term Rehabilitation

Mobilization was initiated on postoperative day two with the assistance of pediatric physical therapy. The patient was fitted with a custom-molded Thoracolumbosacral Orthosis to be worn at all times when out of bed for the first three months postoperatively. This brace served as an external adjunct to protect the internal fixation during the initial phases of osseous fusion, particularly given the altered biomechanics and osteopenia often associated with achondroplasia.

The patient was discharged to an inpatient pediatric rehabilitation facility on postoperative day seven to undergo intensive physical and occupational therapy. Rehabilitation focused on gait retraining, core strengthening, and spasticity management.

At the three-month follow-up clinic visit, standing radiographs demonstrated maintenance of the sagittal correction with no evidence of hardware failure, screw pullout, or junctional kyphosis. Clinically, the patient had regained the ability to ambulate independently without a walker, demonstrating a significant resolution of his preoperative spastic diplegia. By one year postoperatively, CT imaging confirmed a solid, continuous posterolateral fusion mass from T9 to L4, and the patient had returned to all age-appropriate baseline activities.

Clinical Pearls and Pitfalls

• Pitfall - Underestimating the Rigidity of Achondroplastic Kyphosis: A common error is assuming a thoracolumbar kyphosis in an achondroplastic child is purely postural. While true in infancy, persistence past walking age with anterior vertebral wedging signifies a structural, rigid deformity. Dynamic imaging (flexion/extension) is mandatory. Attempting to correct a rigid deformity without adequate osteotomies will result in hardware failure or iatrogenic neurological injury.
• Pearl - The Danger of Laminectomy Alone: In the setting of achondroplastic thoracolumbar kyphosis, performing a laminectomy for decompression without concomitant instrumented fusion is an absolute contraindication. Removal of the posterior tension band will precipitate rapid, catastrophic progression of the kyphosis and subsequent spinal cord transection.
• Pitfall - Dural Adhesions and Durotomy: The dura in achondroplasia is notoriously thin, redundant, and densely adherent to the hypertrophied ligamentum flavum and dysplastic lamina. Surgeons must anticipate a high risk of incidental durotomy. The use of a high-speed burr to thin the bone to a translucent shell before using micro-curettes is vastly superior to using Kerrison rongeurs, which can easily tear the dura or contuse the underlying cord.
• Pearl - Advanced Imaging and Navigation: The pedicular anatomy in achondroplasia is highly abnormal, featuring extremely narrow, short pedicles with altered trajectories. Freehand screw placement relies on anatomic landmarks that are severely distorted in these patients. The use of preoperative CT templating combined with intraoperative 3D navigation (O-arm) is strongly recommended to safely place pedicle screws and avoid medial breaches into the stenotic canal.
• Pitfall - Cervical Spine Vulnerability During Positioning: Even in patients with a history of prior foramen magnum decompression, the entire cervical spine remains vulnerable due to generalized ligamentous laxity and potential residual stenosis. Extreme care must be taken during intubation and prone positioning to maintain a neutral cervical alignment, avoiding hyperextension which could precipitate acute cervical myelopathy.
• Pearl - Hemodynamic Management for the Chronically Compressed Cord: The spinal cord in these patients has been subjected to chronic, insidious compression. Sudden decompression alters the local hemodynamics. Maintaining a high normal mean arterial pressure (MAP > 75-80 mmHg) postoperatively is critical to ensure adequate perfusion and prevent ischemic reperfusion injury (white cord syndrome), which can manifest as delayed postoperative paralysis.