Bone Abstracts

Introduction: Achondroplasia, the commonest skeletal dysplasia, is caused by specific variant(s) in the fibroblast growth factor receptor 3 (FGFR3) gene that cause abnormal spine, skull and limb bone growth. Surgical indications for foramen magnum stenosis in this population vary widely in the literature.

Methods: We performed a retrospective analysis of patients with achondroplasia aged >20 years (n=33) in our regional skeletal dysplasia clinic. Where MRI scans were performed, the first scan was used for analysis of skull base landmarks and angles. Foramen magnum anatomy was categorised according to a recently proposed Achondroplasia Foramen Magnum Severity Score (AFMSS). Indications for MRI, surgical details, and clinical outcome data were collated from patients’ files.

Results: MRI scans were performed in 63.6% (21/33) patients, mean age of first scan 4.8 years (range 3 months – 16 years). Clinical indications for MRI: upper motor neuron signs (24%), respiratory problems, e.g. apnoea (19%), increasing head circumference (19%); and alongside spine MRI for spinal deformities (19%), screening (19%). No patients who did not undergo MRI scanning had, nor developed, abnormal neurology. Scans showed FM narrowing on all and were classified AFMSS grade 1 (craniovertebral junction narrowing alone, n=5), grade 2 (loss of CSF space, n=8), grade 3 (cervical spinal cord flattening, n=4) and grade 4 (with cervical spinal cord signal change, n=4). Foramen magnum decompression (FMD) was undertaken due to clinical signs of myelopathy in 3 patients (9.1% of cohort); these patients’ MRI all showed cervical cord signal change (AFMSS grade 4). Clinical outcomes post FMD neurosurgery: improved motor power (n=1), unchanged neurology (n=2). Patients with sleep apnoea (n=3) had AFMSS Grade 3 and no surgical intervention: one progressively improved and two showed no deterioration.

Conclusion: 90% of our cohort had good outcome with no neurosurgical intervention. All those scanned had abnormal foramen magnum on MRI, all except those with clinically evident myelopathy were treated conservatively, including the 4 with cervical cord flattening without signal change. Intra-spinal cord signal change was the only radiological abnormality directly associated with FMD (P=0.02) which was accompanied by clinically evident myelopathy in all. Clinical abnormalities, rather than radiological scores, give reliable indications for neurosurgical intervention.

Disclosure: The authors declared no competing interests.