Clinical Ultrasound || The infant spine

  • Published on
    23-Dec-2016

  • View
    215

  • Download
    2

Embed Size (px)

Transcript

  • 1315

    67CHAPTER

    The infant spine

    INTRODUCTION 1315

    TECHNIQUE 1315

    INDICATIONSFORSPINALULTRASOUND 1316

    CONTRAINDICATIONSFORSPINALULTRASOUND 1316

    ULTRASOUNDVERSUSMRI 1316

    NORMALANATOMY 1316Extraspinalandspinalanatomy 1316

    Extraspinal 1316Spinal 1316

    Intraspinalanatomy 1318Sacrumandcoccygealregion 1318Lumbarregion 1318Thoracicregionandconus 1319Cervicalregion 1320

    Vascularstructures 1320

    EMBRYOLOGY 1322

    CLINICALAPPLICATIONS 1322Spinaldysraphism 1322

    Openspinaldysraphism(OSD) 1322Closedspinaldysraphism(CSD) 1322

    Trauma 1330Vascularanomalies 1332Tumours 1332

    Extraduraltumours 1333Intraduraltumours 1333Sacrococcygealtumours 1333Currarinostriad 1334

    INTRODUCTION

    Development of real-time ultrasound and high-frequency linear array technology has taken imaging of the spine in early life well beyond early reports of using articulated arm static scanners. Clearer and more detailed information about spinal anatomy and pathology has made the modality increasingly useful.14 Its use in the older child and adult is severely limited by the presence of the surrounding posterior bony vertebral arches, whereas in the neonate these structures are incompletely ossified. At birth the pos-terior portions of the neural arches and spinous processes are still cartilaginous, providing an acoustic window into the spinal canal, which can be imaged in the sagittal (longitudinal) and axial (trans-verse) plane.By the end of the first year of life, fusion of the lamina into a

    bony arch is complete in the lumbar region; ossification progresses cranially and is complete in the cervical region by 2 years of age. Visualisation of the spinal canal in the older child is only possible when a congenital or surgical bony defect is present, but ultrasound scanning rarely provides useful or diagnostic information in such

    Tim Jaspan and Rob Dineen

    cases and there should be no hesitation in proceeding to magnetic resonance imaging (MRI).Spinal ultrasound is most useful in the first few weeks of life,

    when the posterior elements are unossified. The quality of informa-tion can be unrivalled if performed with care and attention to detail.5

    Although MRI is now the modality of choice for spinal imaging, the neonatal spine presents a difficult challenge for MRI because of the small size of the baby, lack of suitable dedicated neonatal spinal surface coils and the high water content and immature myelination of the spinal cord, leading to relatively poor tissue contrast and signal-to-noise ratio compared to older infants and children. Pulsa-tile hyperdynamic cerebrospinal fluid (CSF) flow in the spine is particularly pronounced in infancy and early childhood, presenting additional interpretive problems, generating complex CSF flow pul-sation artefacts on spinal MRI.6 Conversely, ultrasound can reveal exquisite anatomical detail, guiding subsequent imaging triage.

    TECHNIQUE

    The infants back should be inspected for cutaneous stigmata of occult spinal dysraphism. Although such findings may be the trigger for the study, clinicians sometimes miss subtle physical features (such as skin pits). Care should be made to look for a tiny ostium that may mark a dermal sinus track, which can be difficult to identify by ultrasound scanning.A high-quality real-time ultrasound machine is needed. A curved

    linear or sector transducer is preferable for scanning the craniocer-vical junction. Linear array transducers are best for imaging the rest of the spine. In the neonate frequencies of 7.512 MHz are routinely used.The infant should be examined in the prone position, the spine

    slightly flexed to flatten out the natural lower spinal lordotic cur-vature, lying prone over a pillow or bolster placed on the lap of the person holding the infant. Flexion maximises the acoustic window in axial scanning.Performing the examination following a feed may ensure the baby

    is settled and more likely to lie still. Slightly warmed ultrasound gel will reduce the disturbance to the baby. Plenty of coupling gel on the skin should be maintained throughout the examination.The entire spine, from craniocervical junction to coccyx, is

    scanned in the sagittal and axial planes, capturing representative images from each zone. A split-screen facility enables contiguous sagittal sections of the spine to be displayed together, providing a composite image of a longer length of spine.3 This is useful for determining anatomical landmarks, counting spinal vertebral levels and delineating the full extent of more complex pathological struc-tures. The extended field-of-view facility on some scanners achieves the same result.The examination starts at the sacrococcygeal level; identifying the

    five sacral vertebral bodies provides a landmark for anatomical localisation. The presence and direction of spinal cord and nerve root movement should be looked for. A clip store helps in the

  • CHAPTER 67 The infant spine

    1316

    demonstrated a spinal abnormality, ultrasound identified the index lesion. MRI confirmed normal appearances in all spinal ultrasound scans reported as normal, suggesting excellent sensitivity for ultrasound.However, caution must be exercised in evaluating studies that

    include older children and where older generation MRI systems were used. Current state-of-the-art MRI, with superior surface coil technology, gradient systems, CSF and cardiac gating techniques, has improved the quality of MRI in the first few months of life.

    NORMALANATOMY

    Extraspinal and spinal anatomy

    ExtraspinalThe skin surface appears as a narrow echogenic bilaminar stripe in the near field. Below this subcutaneous fat forms an intermediate to low echogenicity plane, immediately deep to which lies the lum-bosacral fascia. This commences in the coccygeal region (Fig. 67.1), forming a strongly echogenic unbroken stripe, which in turn blends with the hypoechoic interspinous ligaments and posterior paraspi-nal musculature in the lumbosacral region (Fig. 67.2). Gas within the rectum appears as an echogenic structure immediately deep to the coccyx (Fig. 67.1).

    SpinalThe spinal column is composed of 33 individual segments, which are unfused in the cervical (7 segments), thoracic (12 segments) and lumbar (5 segments) regions. The 5 sacral and 4 coccygeal segments are partially fused. At term, echogenic ossification centres are present in all the vertebral bodies except the coccyx, and in paired centres that form the posterior neural arches. The latter do not fuse in the midline until the end of the first year of life.On sagittal plane imaging, the vertebral bodies form strongly

    echogenic complexes lying deep to the anechoic CSF-filled thecal sac. The appearance of the posterior neural arches lying superficial to the thecal sac varies with the level of the spine and age of the patient, and they cast variable echogenic shadowing within the

    appreciation of cord and root dynamics. With changes of position and with crying the spinal cord normally shows dorsoventral movement and the echogenic roots of the cauda equina move freely within the transonic CSF in the thecal sac. Pulsations of the anterior spinal artery and dancing motion of individual nerve roots may also be seen.1 The spinal cord does not normally pulsate with the cardiac cycle.2

    In up to 10% of cases, a second (or more) dysraphic abnormality is present elsewhere in the neuraxis. Cord cavitation distant from the site of the lesion should be looked for, emphasising the need to examine the entire length of the cord.

    INDICATIONSFORSPINALULTRASOUND

    Requests for spinal ultrasound often depend upon local experience and availability of appropriate sonographic expertise. Another factor is a lack of awareness amongst many referring clinicians as to the uses and limitations of ultrasound in early life.Ultrasound is recommended as the initial examination in sus-

    pected spinal disease in the neonate. Where ultrasound is inconclu-sive or reveals pathology it should be followed by MRI. Over-reliance on ultrasound carries the risk of diagnostic errors and failure to detect potentially significant pathology. There should be a low threshold for proceeding to MRI.

    CONTRAINDICATIONSFORSPINALULTRASOUND

    The only absolute contraindication is an open neural tube defect because of the risk of contamination and infection. Increasing age is a relative contraindication; visualisation of the spinal canal is limited beyond early infancy. Over-reliance on ultrasound as the primary imaging modality outside of the first few weeks of life should be strongly resisted. Severe spinal deformity and bony ver-tebral anomalies can make ultrasound difficult. Scanning in the axial plane is most useful in these cases.

    ULTRASOUNDVERSUSMRI

    In a study of spinal disorders in infants undertaken in the 1990s, ultrasound was equally sensitive to MRI in the diagnostic yield.7 Thirty children with a mean age of 5.5 months underwent 38 spinal ultrasound scans; in 32 cases ultrasound matched the information seen on MRI. In 5 cases ultrasound showed the main abnormality but MRI gave additional information. In all 24 MRI studies that

    Figure67.1 Sagittalimageofsacrumandcoccyx. The subcutaneous fat is hypoechoic, immediately below which lies the echogenic lumbosacral fascia (LSF). The echogenic sacral vertebral bodies (S) contrast with the hypoechoic coccyx (C). Note echogenic fat (F) in the sacral spinal canal and echogenic bowel gas (BG).

    LSF

    CS5

    S4

    F

    BGS3

    Indications for ultrasound scanning in early infancy

    Highrisk Mediumrisk Lowrisk

    Multiple congenital anomalies

    Complicated sacral dimple (>5 mm diameter, >2.5 cm above anus)

    Uncomplicated sacral pit or dimple (

  • Normal anatomy

    1317

    Figure67.2 Sagittalimageofsacrum. The bilaminar skin surface complex is readily seen (arrowhead), with the underlying hypoechoic subcutaneous fat and echogenic lumbosacral fascia (LSF). The hypoechoic spinous processes (short arrows) are bridged by more echogenic interspinous ligaments. The sacral vertebral bodies (S) lie deep to the epidural fat (F) filling the sacral canal. The sacral cul-de-sac (long arrow) can be difficult to differentiate from the echogenic fat. The hypoechoic L5/S1 disc (D) lies between the bodies of S1 and L5.

    S5

    LSF

    S4

    S3

    F

    F

    FS2

    S1L5 D

    F

    Figure67.3 SagittalimageofthelumbarspinewithcolourDopplerimaging. The vertebral bodies (VB) form strongly echogenic structures immediately deep to the anterior wall of the thecal sac. The intervertebral disc (D) is hypoechoic but contains a central echogenic stripe. CE, cauda equina. The basivertebral veins are readily identified on colour Doppler imaging (arrows).

    CECE

    VBD

    Figure67.4 Craniocervicalregion.A: Sagittal image of upper cervical spine with a sector transducer. Note the lower brainstem (P, pons; M, medulla), craniocervical junction (arrowhead) and cervical cord (long arrow). The spinous processes are more echogenic (black arrows). Short arrow indicates the central echo complex. CL, clivus; C, cervical vertebral body. B: Sagittal T2 MR image (displayed horizontally). The internal architecture of the cord is not as well visualised as on the corresponding ultrasound image. Note good correlation of the anatomy of the brainstem. P, pons; M, medulla; CM, cisterna magna; CL, clivus; craniocervical junction indicated by arrowhead and upper cervical cord by arrow.

    A B

    CM

    M

    C2

    P

    C3C4

    C5

    CL

    CM

    CL

    M

    P

    C2

    P

    canal. With increasing age, they become more echogenic and an increasing barrier to insonation of the underlying structures. In the first few weeks of life, the unossified coccyx appears as a tubular hypoechoic structure extending caudally from the fifth sacral body (Fig. 67.1). Identification of the five echogenic, rectangular ossified sacral vertebral bodies, forming a curved sweep in the sagittal plane, is the basis for localisation of the spinal level (Fig. 67.2).

    The lumbar vertebrae have a cleft in the midbody region, contain-ing the basivertebral veins (Fig. 67.3). Intervening intervertebral discs are hypoechoic, reflecting the gel-like nucleus pulposus. The nucleus pulposus is particularly large in the first year of life, there-after gradually decreasing in size.The vertebral bodies of the upper cervical spine and ossification

    centres of the clivus form constant strongly echogenic landmarks. The intervening anterior arch of C1 is poorly visualised. The spinous processes are cartilaginous in the first few months of life; however, in the cervical spine they ossify earlier than the remainder of the spinal column and may appear echogenic (Fig. 67.4).

  • CHAPTER 67 The infant spine

    1318

    Lumbar regionThe echogenic vertebral bodies form the deep extent of the spinal canal on sagittal scanning, separated from the echogenic anterior thecal sac wall by a thin band of intermediate echogenicity fat in the anterior epidural space. The echogenic posterior thecal sac wall lies immediately deep to the posterior neural arches. The CSF within the thecal sac contains the linear echogenic nerve roots of the cauda equina (Fig. 67.7).In the axial plane the roots of the cauda equina form an amor-

    phous mass of echogenic structures filling much of the thecal sac in the lower lumbar spinal canal (Fig. 67.8). In the upper lumbar spine they become progressively more symmetrical, clustered either side of the midline (Fig. 67.9).The filum terminale, a single strand of neuroglial tissue, may be

    identified on sagittal images as a fine linear midline echogenic structure within or posterior to the cauda equina. The filum extends caudally from the tip of the conus medullaris to pierce the thecal sac and attach to the posterior aspect of the coccyx.8 No nerve roots arise from the filum terminale.

    In the axial plane the posterior neural arches are delta- or gull-shaped echogenic structures casting variable acoustic shadowing (Figs 67.5 and 67.6). In the thoracic region, the paired ribs appear as posteriorly convex echogenic structures sweeping away from the vertebral bodies, overlying the echogenic lungs (Fig. 67.6).

    Intraspinal anatomy

    Sacrum and coccygeal regionEchogenic epidural fat fills the sacral spinal canal, blending with the theca and its contained anechoic cerebrospinal fluid (CSF) at the sacral cul-de-sac (Fig. 67.1).

    Figure67.5 Axialimagethroughmidthoraciccord. The spinal cord lies towards the anterior half of the spinal canal, suspended by the echogenic dentate ligaments (arrows) in the anechoic CSF. T, transverse process; La, lamina; S, spinous process; VB, vertebral body; arrowheads indicate ventral and dorsal nerve rootlets.

    La

    S

    T

    VB

    Figure67.6 Axialimagethroughtheupperthoracicspine. The spinal cord is round and hypoechoic with a clearly defined central echo complex. D, deep fascial layer; La, lamina; T, transverse process; M, paraspinal muscle; R, rib; Lu, lung; VB, vertebral body.

    LaM

    D

    TR

    VBLu

    Figure67.7 Sagittalimageofthelumbarspine. The cauda equina (CE) fills the thecal sac. The thecal sac wall (arrowheads) is separated from the echogenic cauda equina by anechoic CSF. VB, vertebral body; D, intervertebr...