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PubMed This is a summary of 12 peer-reviewed journal articles Updated
Pediatrics

Diagnostic Imaging, Tests & Understanding Your Reports

At a Glance

For infants with achondroplasia, an MRI and sleep study are critical tests. The MRI checks for structural narrowing at the base of the skull, known as foramen magnum stenosis. The sleep study monitors for breathing issues like central apnea to ensure your child's brainstem and airway are safe.

Navigating the medical tests required after a diagnosis of achondroplasia can feel like learning a new language. The two most critical diagnostic tools for an infant or young child are Magnetic Resonance Imaging (MRI) and Polysomnography (PSG) [1][2]. These tests provide the “map” and the “vital signs” your medical team needs to ensure your child’s nervous system and airway are protected.

The Baseline MRI: Mapping the Base of the Brain

The American Academy of Pediatrics (AAP) and international guidelines recommend a baseline MRI of the craniocervical junction—the specific area where the skull meets the spine—usually within the first 6 months of life [3][1].

The goal of this scan is to look for foramen magnum stenosis, which is a narrowing of the opening at the base of the skull [4]. Because the bone grows differently in achondroplasia, this opening can sometimes be too small, leading to cervicomedullary junction compression (pressure on the area where the brainstem becomes the spinal cord) [5].

Doctors look for specific markers on these reports:

  • Space Available for the Cord (SAC): A measurement of how much room the spinal cord has to “breathe” within the bony opening [6].
  • T2 Signal Change: If the report mentions “hyperintensity” or “increased signal,” it may indicate that the pressure has caused some swelling or stress within the cord itself [7].
  • AFMS Grade: Some specialists use the Achondroplasia Foramen Magnum Score, a grading system from 1 (mild) to 4 (most severe), to track changes over time [7].

Polysomnography: The “Sleep Study”

While an MRI shows the structure of the body, a polysomnography (PSG) shows how that structure is functioning during sleep [8]. Children with achondroplasia have a high risk of sleep-disordered breathing, which can be either “obstructive” (a physical blockage) or “central” (a signaling issue from the brain) [9].

When reading a sleep study report, look for the Apnea-Hypopnea Index (AHI). This number represents the average number of times per hour your child’s breathing stops or slows down [10].

  • Central Apnea Index (CAI): In achondroplasia, a high central apnea index can sometimes be a “red flag” that the brainstem is being compressed, even if the child seems fine during the day [8][11].
  • Oxygen Desaturation: This tracks how low the oxygen levels in the blood drop during these pauses [NBK1152].

Completeness Checklist for Parents

When you receive a copy of your child’s medical reports, use this checklist to ensure you have the full picture to discuss with your specialists:

Test What to Look For in the Report
MRI [ ] Does it specifically mention the foramen magnum?
[ ] Is there any mention of cord compression or indentation?
[ ] Does it state if there is normal signal intensity in the spinal cord?
[ ] Is there a measurement of the anteroposterior diameter (the width of the opening)?
Sleep Study [ ] What is the Total AHI?
[ ] Is the Central Apnea Index (CAI) listed separately from the obstructive index?
[ ] What was the nadir (the lowest point) of the oxygen saturation?
[ ] Does the report specify the percentage of time spent below 90% oxygen?

It is common for there to be a “mismatch” between these tests—for example, a child may have a narrow opening on an MRI but a normal sleep study, or vice versa [11][12]. This is why having both tests is essential for your doctor to make the best decision for your child’s care.

Common questions in this guide

Why does my baby with achondroplasia need an MRI?
Infants with achondroplasia need a baseline MRI of the craniocervical junction, typically within the first 6 months of life. This scan checks for foramen magnum stenosis, a dangerous narrowing at the base of the skull that can compress the spinal cord.
What is foramen magnum stenosis in achondroplasia?
Foramen magnum stenosis is a narrowing of the bony opening where the skull meets the spine. Because bone grows differently in achondroplasia, this opening can become too small and put physical pressure on the brainstem and spinal cord.
What does T2 signal change mean on my child's MRI report?
A T2 signal change or hyperintensity on an MRI report indicates an increased signal in the spinal cord. This typically means that pressure from a narrowed skull base has caused swelling, stress, or damage within the cord itself.
Why is a sleep study important for a child with achondroplasia?
A sleep study, or polysomnography, evaluates how well your child's airway and nervous system function during sleep. Children with achondroplasia have a high risk of sleep-disordered breathing, including central sleep apnea, which can be a key warning sign of brainstem compression.
What does the Central Apnea Index (CAI) mean on a sleep study?
The Central Apnea Index measures how often your child's breathing pauses due to brain signaling issues, rather than a physical airway blockage. In achondroplasia, an elevated CAI can be a red flag that the brainstem is experiencing compression.
What if my child's MRI shows narrowing but their sleep study is normal?
It is very common to have a mismatch between structural MRI findings and functional sleep study results. Both tests are required because they provide different pieces of information that help your medical team decide the safest monitoring and care plan.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does this MRI report use the Achondroplasia Foramen Magnum Score (AFMS) to grade the narrowing?
  2. 2.Is there any evidence of T2-weighted signal change or hyperintensity within the spinal cord?
  3. 3.How does the measurement of the 'space available for the cord' (SAC) on this report compare to achondroplasia-specific standards?
  4. 4.Given the central apnea index on the sleep study, do you recommend a follow-up MRI to check for changes at the base of the brain?
  5. 5.Was the sleep study interpreted by someone with experience in pediatric achondroplasia, accounting for age-specific breathing patterns?

Questions For You

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References

References (12)
  1. 1

    Recommendations for neuroradiological examinations in children living with achondroplasia: a European Society of Pediatric Radiology and European Society of Neuroradiology opinion paper.

    Wright J, Cheung M, Siddiqui A, et al.

    Pediatric radiology 2023; (53(12)):2323-2344 doi:10.1007/s00247-023-05728-0.

    PMID: 37674051
  2. 2

    Sleep disordered breathing in a cohort of children with achondroplasia: correlation between clinical and instrumental findings.

    Zaffanello M, Lo Tartaro P, Piacentini G, et al.

    Minerva pediatrica 2017; (69(6)):481-488 doi:10.23736/S0026-4946.16.04282-1.

    PMID: 26041006
  3. 3

    Editorial: Neonatal management of achondroplasia: one hospital's geosocial approach to improve patient outcomes.

    Gooch C, Robin NH, Hurst ACE

    Current opinion in pediatrics 2019; (31(6)):691-693 doi:10.1097/MOP.0000000000000814.

    PMID: 31693574
  4. 4

    Alteration of major venous drainage routes in a patient with achondroplasia after ventriculoperitoneal shunt placement.

    Rai Y, Ogiwara H

    Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 2024; (40(3)):913-917 doi:10.1007/s00381-023-06209-3.

    PMID: 38091071
  5. 5

    Centrally mediated obstructive apnoea and restenosis of the foramen magnum in an infant with achondroplasia.

    Cocca A, Thompson D, Rahim Z, et al.

    British journal of neurosurgery 2023; (37(3)):409-412 doi:10.1080/02688697.2020.1817315.

    PMID: 32924616
  6. 6

    Dynamic MRI in the Evaluation of the Cervical Spine in Pediatric Patients With Achondroplasia.

    Masarwy A, Tuchman A, Huang J, et al.

    American journal of medical genetics. Part A 2025; (197(10)):e64143 doi:10.1002/ajmg.a.64143.

    PMID: 40464123
  7. 7

    The (extended) achondroplasia foramen magnum score has good observer reliability.

    Jenko N, Connolly DJA, Raghavan A, et al.

    Pediatric radiology 2022; (52(8)):1512-1520 doi:10.1007/s00247-022-05348-0.

    PMID: 35396670
  8. 8

    Polysomnography as an indicator for cervicomedullary decompression to treat foramen magnum stenosis in achondroplasia.

    Sano M, Takahashi N, Nagasaki K, et al.

    Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 2018; (34(11)):2275-2281 doi:10.1007/s00381-018-3880-0.

    PMID: 29959505
  9. 9

    Evaluation of polysomnography findings in children with genetic skeletal disorders.

    Nayır Büyükşahin H, Emiralioglu N, Simşek Kiper PÖ, et al.

    Journal of sleep research 2023; (32(5)):e13914 doi:10.1111/jsr.13914.

    PMID: 37128177
  10. 10

    Ensemble-learning regression to estimate sleep apnea severity using at-home oximetry in adults.

    Gutiérrez-Tobal GC, Álvarez D, Vaquerizo-Villar F, et al.

    Applied soft computing 2021; (111()) doi:10.1016/j.asoc.2021.107827.

    PMID: 39544517
  11. 11

    Is there a correlation between sleep disordered breathing and foramen magnum stenosis in children with achondroplasia?

    White KK, Parnell SE, Kifle Y, et al.

    American journal of medical genetics. Part A 2016; (170A(1)):32-41 doi:10.1002/ajmg.a.37385.

    PMID: 26394798
  12. 12

    Cervical spinal cord compression in infants with achondroplasia: should neuroimaging be routine?

    Sanders VR, Sheldon SH, Charrow J

    Genetics in medicine : official journal of the American College of Medical Genetics 2019; (21(2)):459-463 doi:10.1038/s41436-018-0070-0.

    PMID: 29872110

This page explains achondroplasia imaging and sleep study terminology for educational purposes. Your child's pediatric specialists and multidisciplinary medical team are the best sources for interpreting their specific test results.

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