Craniocervical junction anomalies in achondroplasia

General

The most severe neurological complications occurs at young

Normal

The cranial base and the neural arches grow and enlarge by endochondral ossification.

Normal foramen magnum growth

Usually small at birth, the foramen magnum remains small, particularly in the transverse diameter. Growth of the skull is particularly robust and important in the first 18 months of life with achondroplastic patients growing significantly less rapidly

There is very little difference in the actual size of the foramen between symptomatic and asymptomatic patients; the average adult foramen in patients is the size of the average normal newborn in the transverse diameter and the size of the average 2-year-old in the sagittal diameter

Mechanism


flowchart TD
    A["Defective endochondral<br>ossification"] --> B["Small or deformed foramina"] & D["Relative lack of rigidity of the bones"] & I["Premature fusion +<br>abnormal development of<br>the posterior skull<br>base synchondroses"]
    B --> C["Venous sinus obs(s)"] & n3["Stenosis of foramen magnum/retroflexed dens"]
    D --> E["Occipital condyle may<br>distort over time"] & n4["Base of the skull is<br>constantly under<br>pressure (from the<br>weight of the head and<br>the tension of the cervical<br>muscles)"]
    E --> F["Reducing size<br>of foramen magnum"]
    G["Increase likelihood of<br>medullary and upper<br>spinal cord compression"] --> H["Early death/neuro deficits/<br>Sleep Apnea/Respiratory<br>failure/Myelopathy/<br>Syringobulbia/<br>Syringomyelia"]
    I --> J["Hypertrophied margin<br>of the posterior aspect<br>of the foramen magnum"]
    J --> K["Appears as a bony<br>shelf radiologically<br>and surgically"]
    n4 --> n5["As a result of these forces<br>+ the “softness” of the bone"]
    C --> n6["Hydrocephalus (RARE-Most<br>patient do not have HCP)"]
    n3 --> n7["Blockage of CSF flow"] & G
    n7 --> n6
    K --> n3
    F --> n3
    n5 --> n8["Foramen magnum<br>invaginates upward into the<br>skull"]
    n8 --> n3

    n1@{ shape: rect}
    n3@{ shape: rect}
    n4@{ shape: rect}

Clinical features

Present as

Sudden infant death

4x Inc of infant sudden death below 4 years old

Sleep apnea syndrome
Respiratory failure, myelopathy
Syringobulbia/syringomyelia
Hydrocephalus

Check for

Ophistotonus
Snoring

Use sleep study to look for

Mixed
Airway obstruction which could also be due to enlarged adenoids
Central sleep apnea (60% have it)

PT do not move their abdomen when breathing at sleep

Choking

Imaging

Left is an axial MRI image at the level of the foramen magnum. Note the narrow and distorted shape of the foramen. In b which is a coronal image through the base of the skull showing elevation of the structures adjacent to the foramen due to distortion of the normal position of these structures as a result of pressures on the softened bone over time.

(Left) The T-1 weighted MRI image of an infant with achondroplasia demonstrates a small posterior fossa and small short vertebral bodies. (Right) Lateral skull film of a patient with achondroplasia demonstrating macrocephaly, platybasia, and small short vertebrae.

Treatment

Foramen magnum stenosis

Operative

Surgical decompression of foramen magnum

Indications for surgery

myelopathy with upper motor neuron signs such as clonus and hyperreflexia,
Central apnea as documented on sleep study
Presence of a syrinx, with evidence of a narrow foramen magnum
T2 signal change in the spinal cord on MR imaging.

Technique

Only do bony decompression as opening the Dura and damaging circular sinus can lead to death via exangunsion

If open dural venous circular sinus will bleed profusely due to raised sinus venous pressure secondary to small skull base foramen