Pictorial review: imaging of the spinal manifestations of achondroplasia

Abstract

Achondroplasia is the commonest hereditary skeletal dysplasia exhibiting dwarfism with characteristic rhizomelic (proximal) shortening of the limbs. It is predominantly linked with an autosomal dominant inheritance, but sporadic mutations can occur which are associated with advanced maternal age. Approximately 1 in every 25 000–30 000 live births are affected, and the overall life expectancy is marginally reduced by ~10 years. Mutations in the fibroblast growth factor receptor causes a decrease in endochondral ossification, which results in stunted growth of cartilaginous bones. A resultant narrowed foramen magnum and a short clivus are seen which predisposes to craniocervical spinal canal stenosis. Apnoeic events arising from the compression of the vertebral arteries at the level of the craniocervical junction lead to fatality in the young, with a death rate as high as 7.5%. Decrease in the caudal inter-pedicular distance is characteristic and a contributory factor for cervical, thoracic and lumbar spinal canal stenosis, most pronounced in the lumbar spine with patients often requiring surgical intervention to ease symptoms. Thoracolumbar kyphoscoliosis and sacral manifestations such as small sacro-sciatic notches and a horizontal pelvis are seen. The aim of this pictorial review is to demonstrate the imaging findings of the spinal and pelvic manifestations of achondroplasia.

Introduction

Achondroplasia is the commonest hereditary skeletal dysplasia manifesting as marked dwarfism, being transmitted in either an autosomal dominant mode or as the result of a sporadic mutation. The prevalence is estimated at 1 in every 25 000–30 000 live births.¹ The homozygous form is lethal resulting in stillbirth, while advanced maternal age has been related to sporadic cases.²

The condition is due to a decrease in endochondral ossification caused by activating mutations in the fibroblast growth factor receptor 3 (FGFR3), with 98% of patients having a G380R substitution in the transmembrane domain of the FGFR3 while other mutations are rare. The typical achondroplastic phenotype is characterised by rhizomelic (proximal) shortening of the limbs, lumbar lordosis, protuberant abdomen, craniofacial deformities, macrocephaly, frontal bossing, short cranial base, and severe midface hypoplasia.³ Bones formed from membranous ossification (skull vault, maxilla, mandible, clavicle, and pelvis) are not affected in achondroplasia.² Narrowing of the foramen magnum (FM) and a short clivus are also characteristic and predispose to craniocervical spinal canal stenosis, which in turn risks cervico-medullary compression. The spine demonstrates posterior vertebral scalloping, reduced pedicle length and a reduced inter-pedicular distance which cause spinal stenosis.⁴

It is estimated that 35–47% of young achondroplastics have neurological problems, including mental and motor developmental delay, hypotonia, feedingand sleeping disorders as well as compressive spinal syndromes. Conversely, adults usually display signs of spinal stenosis.⁵ The average life expectancy is ~10 years less than the general population.¹

Clinical relevance of spinal involvement

FM hypoplasia, which arises from diminished growth of the cartilaginous bone, an abnormal synchondrosis and an unusual “key-hole”-shaped appearance contribute to craniocervical junction (CCJ) abnormalities. Although direct spinal cord compression can occur, it is more conceivable that apnoeic events arising from compression of the vertebral arteries at the level of the CCJ lead to fatality in the young, with the risk of death being as high as 7.5%.¹ A narrowed CCJ, megalencephaly, and elevation of the brainstem can all result in cervico-medullary compression.

Spinal canal stenosis, thoracolumbar kyphosis, and an increased lumbosacral angle are all observed and contribute to cord or nerve root compression at various levels which may manifest as myelopathy or neurogenic claudication.

Imaging findings

Foramen magnum and cervical spine stenosis

Significant shortening of the clivus, narrowing of the FM and reduced sagittal diameter of the CCJ at the level of C2 are seen in achondroplastics (Figure 1). Although the diameter of the FM and size of the clivus increase with age, normality is never achieved.⁶ The discordant development of the basi-cranium results in a large skull and comparatively small base with stenosis of the FM and jugular foramina (JF). Neurological and respiratory complications ensue secondary to compression of the medulla oblongata and upper cervical spinal cord.³ Patients at risk of a narrowed FM or JF may benefit from decompressive surgery.⁶ Often, a more horizontal occipital bone is seen. If the opisthion is directed upward in relation to the occipital plane, then a “cup-shaped” planum occipitale is present, which contributes to further stenosis (Figure 2). Cervical spinal stenosis manifests as a reduction in the anterioposterior canal dimension (Figure 3), which in combination with degenerative disc bulging can result in cervical cord compression (Figure 4), while foraminal stenosis may cause cervical nerve root compression (Figure 5).

Figure 1.

A 26-year-old male with achondroplasia. (a) Sagittal T1W and (b) T2W FSE MR images of the upper cervical spine show foramen magnum stenosis (arrows) with reduction of CFS around the upper cervical cord. Note the short clivus.

Figure 2.

A 28-year-old female with achondroplasia. Sagittal T2W FSE MR image of the cranio-cervical junction shows a “cup-shaped” planum occipitale (arrow).

Figure 3.

A 45-year-old female with achondroplasia. Lateral radiograph of the cervical spine shows marked reduction of the AP canal dimension (arrows). Segmentation anomaly is also present at C5–C6.

Figure 4.

A 57-year-old male with achondroplasia. Sagittal T2W FSE MR image of the cervical spine shows a stenotic canal with degenerative disc bulges at C3–C4 and C4–C5 causing mild cervical cord compression (arrows).

Figure 5.

A 40-year-old female with achondroplasia. Axial T2W FSE MR image of the cervical spine shows severe bilateral foraminal stenosis (arrow).

CHII lesion

Cervical high intensity intramedullary (CHII) lesions are located in the grey matter just below the CCJ, with no associated cord compression (Figure 6). Occasionally, they are associated with focal cord atrophy (Figure 7). Brouwer et al⁴ hypothesized that flexion/extension of the cranium could further narrow a pre-existing stenotic spinal canal causing transient but repetitive compression of the spinal cord. In their study, 39% of volunteers had a CHII lesion unaccompanied by clinical symptoms. A correlation between CHII lesions and compression of the cervical cord could not be established in the neutral, flexed, or extended positions or with clinical symptoms.⁴

Figure 6.

A 38-year-old female with achondroplasia. (a) Sagittal and (b) axial T2W FSE MR images of the upper cervical spine shows a CHII lesion (arrows) at the C2 level with no associated cord compression.

Figure 7.

A 45-year-old female with achondroplasia. Sagittal T2W FSE MR image of the upper cervical spine shows a CHII lesion (arrow) at the C2 level with no associated cord compression, but marked cord atrophy.

Thoracic spine stenosis

Congenital narrowing of the spinal canal is seen throughout the length of the spine, although most prominent in the lumbar region where it can cause neurogenic claudication (Figure 8). An interlaminar medial facetectomy is regularly performed to relieve symptoms. However, there are a group of achondroplastics who complain of a decline in their walking pattern as well as features suggestive of lower spinal cord compression. This is thought to be due to degenerative narrowing of the spinal canal caused by a combination of facet joint hypertrophy, ligament flavum hypertrophy and or disc degeneration as opposed to the underlying condition only. Patients with thoracic spinal cord symptoms are infrequently described in the literature. Indications for surgical decompression include gait disturbance, pain, cramps, spasms, or incontinence. Conversely, it is crucial to realize that the benefits achieved from decompressive surgery are limited, as there is only some improvement in clinical outcome as opposed to a cessation of clinical symptoms, which is the conventionally accepted outcome in other decompressive surgical interventions.⁷

Figure 8.

A 67-year-old male with achondroplasia. Sagittal T2W FSE MR image of the cervical and thoracic spine shows a stenotic thoracic spinal canal (arrows) with mild multilevel spinal cord compression.

Thoracolumbar kyphosis and lumbar hyperlordosis

Transient kyphosis is a recognised entity amongst achondroplastics, with nearly every infant under the age of 1 year having the deformity. This non-congenital deformity is not associated with an underlying structural defect.¹ Khan et al⁸ retrospectively reviewed posteroanterior and lateral radiographs of 459 achondroplastics over a period of 14 years, finding a 60% prevalence of scoliosis (Figure 9) and 79% prevalence of thoracolumbar kyphosis (Figure 10). Thoracolumbar kyphoscoliosis can readily be seen in the sitting position with an exaggerated lumbar lordosis in the standing position, which is usually asymptomatic.⁹ Although there is spontaneous resolution when a child becomes ambulant, 10–15% of adults develop a fixed angular kyphosis with marked deformity of one or more vertebrae (Figure 11). Consequential wedging poses the individual at an increased risk of neurological symptoms such as weakness, paralysis, and bladder or bowel incontinence. As a result of draping and tethering of the spinal cord at the kyphotic apex (Figure 11c), the normal “ascent” of the spinal cord is hindered causing stretching and cord damage. Anterior wedging of vertebrae at the curve apex is suggestive of the development of a fixed deformity (Figure 10). Progression of deformity arises from the unfavourable effects of gravity. Appropriate counselling and early intervention (supported sitting) can halt the progression of a fixed deformity, with very few children requiring corrective surgery.¹

Figure 9.

A 24-year-old female with achondroplasia. AP radiograph of the lumbar spine shows a mild left mid-lumbar scoliosis (arrows).

Figure 10.

A 7-month-old male with achondroplasia. Lateral radiograph of the thoracolumbar spine shows a focal kyphosis with “bullet-shaped” upper lumbar vertebrae (arrow).

Figure 11.

A 47-year-old male with achondroplasia. (a) Lateral radiograph, (b) sagittal CT MPR and (c) sagittal T2W FSE MR images showing a severe thoracolumbar kyphosis due to anterior wedging of the L1 and L2 vertebrae (arrows-a,b) with kinking of the cord at the cord at the apex (arrow-c). Note also lumbar hyperlordosis.

Although an exaggerated lumbar lordosis is usually asymptomatic and requires no treatment, when marked there may be increased pain at the curve apex. In severe cases, there may be intermittent spinal claudication or symptomatic spinal canal stenosis.

Lumbar spine stenosis

The inter-pedicular distance normally increases from L1 to L5. However, a progressive decrease is seen in achondroplastics, which is due to premature fusion of the pedicles to the vertebral bodies at the neurocentral synchondroses resulting in shortened pedicles and a decreased inter-pedicular distance (Figure 12a). This in turn causes a narrowed spinal canal cross-sectional area leaving less space available for the neural elements (Figure 13).⁹ Posterior vertebral scalloping is common (Figure 12b), with a rounded appearance anteriorly giving rise to a “bullet-shaped” configuration (Figure 10).⁹ Such changes are usually seen in late adolescence and adulthood, with an average onset in the fourth decade of life.¹

Figure 12.

A 21-year-old male with achondroplasia. (a) AP and (b) lateral radiographs of the lumbar spine show reduction of the inter-pedicular distance from L3 to L4 (arrows-a). Posterior vertebral scalloping is also present (arrows-b). Short pedicles are contributing to severe lumbar spinal stenosis.

Figure 13.

A 39-year-old female with achondroplasia. (a) Sagittal and (b) axial CT showing short broad pedicles (arrows-a,b) resulting in spinal stenosis. (c) Axial T2W FSE MR image shows loss of CSF from around the cauda equina (arrow).

Jeong et al⁵ studied 15 achondroplastic patients for signs of lumbar canal stenosis on MR, eight of whom were symptomatic and seven asymptomatic. The mean sagittal diameter of the spinal canal, thecal sac, and the inter-pedicular distance at the mid-body and mid-disc levels were not significantly different in the two groups, except at the L1 mid-body level. The cross-sectional area was compromised at the level of the intervertebral disc in the symptomatic group with a combination of spinal canal and lateral recess stenosis, most commonly at L1/L2 and L3/L4. There was insignificant dural compression at the level of the vertebral body. They found a significant difference in the cross-sectional area of the spinal canal between the symptomatic and asymptomatic groups at levels L1 and L3 mid-disc and L2 mid-body. Both groups demonstrated a progressive decrease in cross-sectional area of the thecal sac at the level of the mid-body from L1 to L5, while progressive decrease at the level of the disc was seen from L1 to L3 which then increased at L5 in both groups. They concluded that a developmentally narrow spinal canal and early degenerative disc changes appear to be the principal factors for the development of symptomatic stenosis in the lumbar spine (Figure 14).

Figure 14.

A 53-year-old female with achondroplasia. (a) Sagittal and (b) axial T2W FSE MR images show degenerative change with a central disc prolapse at L2-L3 (arrows) resulting in severe thecal sac and cauda equina compression. Note relatively horizontal orientation of the sacrum (arrowheads-a).

Pelvic and sacral manifestations

The pelvic cavity is short and broad, also described as champagne-glass in appearance. There is squaring of the iliac wings and some rounding of the corners which is referred to as “elephant ear” shaped iliac wings. The inferior margins of the iliac wings and the acetabular roofs are flat and horizontal (Figure 15a).⁹ The sacro-sciatic notches are small with an exaggerated sacral tilt and a large anteriorly protruding sacral promontory (Figure 15b).⁹ As a result of the horizontal sacrum, children may occasionally develop pressure-induced chronic coccydynia, which may be managed conservatively with padding of the underwear.¹

Figure 15.

A 32-year-old female with achondroplasia. (a) AP radiograph shows broadening of the pelvis with flat horizontal acetabular roofs (arrows). (b) Sagittal T1W FSE MR image shows a lumbosacral hyperlordosis (arrow) and horizontally angulated sacrum (arrowhead).

Conclusions

Achondroplasia may present with a wide variety of spinal and pelvic manifestations, most marked at the level of the FM, CCJ and lumbar spine where stenosis is evident and often symptomatic. The young usually succumb to apnoeic events caused by compression of the vertebral arteries and/or medulla oblongata. Early counseling and supported sitting are advised to prevent a fixed thoracolumbar kyphoscoliosis and consequential stretching and tethering of the spinal cord. CHII lesion is present; however, they are not clinically relevant and non-progressive. Surgical intervention can provide some symptomatic relief, but cessation of the symptoms is not achieved.