ESPE Abstracts

Background: Foramen magnum stenosis (FMS) is a common, serious complication of achondroplasia in infancy. The Achondroplasia Foramen Magnum Score (AFMS; 1-4) was developed to facilitate early detection of stenosis and inform appropriate neurosurgical management. All infants have a baseline screening MRI, of which approximately half of cases demonstrate AFMS3 (craniovertebral junction (CVJ) narrowing with flattening of the cervical cord) or AFMS4 (CVJ narrowing and cervical cord signal change). Children with AFMS4 are referred for neurosurgery; however, the natural history of children with AFMS3 is currently unclear so repeat MRI is performed after 12 months.

Aim: The aim of this study was to review follow-up clinical and MRI assessments of infants initially classified with AFMS3 to define more clearly the natural history of this degree of stenosis. This knowledge would aid informing parents and refine our screening approach.

Study Design: This single institution retrospective cohort study identified all infants with a confirmed diagnosis of achondroplasia and AFMS3 on initial MRI who had subsequently undergone repeat MRI between February 2016 and March 2021. All MRI images were evaluated by a paediatric neuroradiologist and a specialist achondroplasia paediatrician to identify and grade CVJ changes using the AFMS as an objective assessment of FMS. Neurological examination was assessed at time of initial MRI and at follow-up.

Results: 12 cases of AFMS3 satisfied the inclusion criteria. Median age at first scan was 6.5 months (IQR=4.5-10.3) and at second scan was 18.5 months (IQR=14.8-23.5). Follow-up MRI showed improvement in four cases to AFMS1 (CVJ narrowing with preservation of the cerebrospinal fluid (CSF) space around the cord, n = 3) or AFMS2 (CVJ narrowing with loss of the CSF space, n = 1). There was no change in four cases (AFMS3) and progression in four cases to AFMS4. Neurological examination of these children was normal at baseline and follow-up.

Conclusion: These results support the argument for MRI screening of infants with achondroplasia. Furthermore, the results suggest that all infants with AFMS3 should have a follow-up MRI scan as approximately one third progress to AFMS4 after one year, prompting neurosurgical referral. There is no consensus currently on frequency of screening for FMS; however, we suggest that guidance for follow-up scanning is modified to six months to detect progression earlier in this at-risk cohort.