Intradural spinal mass lesions are relatively uncommon, compared to intracranial or extradural masses, and can be challenging to diagnose. Additionally the need for a pre-operative/non-operative diagnosis is in many ways greater as biopsy of lesions within the cord has the potential of devastating neurological impairment. As such a systematic approach to these lesions is required.
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MRI is the modality of choice for the assessment of lesions within the spinal canal as it has exquisite anatomical contrast and structural resolution, able to image all compartments and able to assess for presence of enhancement, cystic change and blood product.
Myelography historically was of prime importance but is now only really used in patients who cannot have an MRI (e.g. pacemaker), who have too much artefact from spinal instrumentation, or occasionally as a problem-solving technique (e.g. spinal arachnoid cyst vs ventral cord herniation). This is usually combined with CT (i.e. CT myelography).
CT remains the best modality to assess the osseous structures and is especially important in planning instrumentation, although this is usually not required for intradural tumours as the vertebral bodies are in almost all cases essentially unaffected.
Angiography is useful in a select group of patients who have vascular lesions (both vascular malformations and vascular tumours) and in some instances endovascular treatment may be useful.
Ultrasound, except in the infant, does not have a role, as it is unable to image the intradural compartment due to the overlying posterior spinal elements.
As such, the rest of this article focuses on MRI.
A standard approach to imaging an intradural mass consists of:
- T1 sagittal and axial
- T2 sagittal and axial
- T1 with contrast sagittal and axial, with one or both planes fat saturated (especially when there is foraminal extension or when there is concern of an extradural process)
Additional sequences may also be useful including:
- CSF flow studies
- gradient echo sequences (for blood products/calcification)
- high resolution
- diffusion weighted imaging
Normal appearance on MRI
The spinal cord begins as a continuation of the medulla oblongata and extends to the conus medullaris. In adults, the tip of the conus typically terminates at L1-L2, however it may terminate as high as mid-T12 or as low as L2-L3. The cord has two enlargements: the cervical enlargement, which extends from C4 to T1 and the lumbosacral enlargement, which extends from T11 to L1. These enlargements correspond to the locations in the cord that supply the spinal nerves for the upper and lower extremities.
The spinal nerve roots arising from the lumbosacral enlargement and conus medullaris comprise the cauda equina, which runs through the lumbar cistern. The filum terminale, a continuation of pia mater, descends from the tip of the conus within the lumbar cistern among the nerve roots of the cauda equina. After it leaves the inferior end of the dural sac, it passes through the sacral hiatus and inserts on the dorsal aspect of the coccyx.
The spinal dura mater is the outermost covering membrane of the spinal cord. Superiorly, it adheres to the margin of the foramen magnum where it is continuous with the cranial dura mater. Its caudal end most commonly occurs at the level of S2. The dura extends for a distance into the neural foramen along each nerve root. The dura is separated from the vertebrae by the epidural space, which contains fat and the epidural venous plexus.
The subdural space, between the dura and arachnoid layers, is a potential space with the dura and arachnoid normally apposed to one another. The subarachnoid space, between the arachnoid mater and pia mater, is continuous with the intracranial subarachnoid space and contains cerebrospinal fluid. It extends a short distance along the nerve roots into the neural exit foramina. The pia mater is applied to the surface of the spinal cord and the roots of the spinal nerves.
Axial MRI scans of the cord demonstrate the anterior median fissure, a prominent cleft in the midline of the ventral surface of the cord. The posterior median sulcus is a shallow indentation on the dorsal aspect of the cord. The ventral nerve roots exit the spinal cord at the anterolateral sulcus and the dorsal nerve roots enter the cord at the posterolateral sulcus. The dorsal intermediate sulcus separates the dorsal funiculus into the gracile and cuneate tracts.
Features of the internal structure of the spinal cord may be visualised on MRI. The central grey matter has a characteristic H-shape. The anterior grey columns are larger than the posterior grey columns. The anterior grey columns are separated from the ventral surface of the cord by the white matter of the anterior funiculus. The dorsal grey matter extends nearly to the surface of the cord. Smaller lateral horns are present from T1 to L2 and from S2 to S4. The central canal of the cord is located within the grey commissure. Although the normal central contains CSF and is lined by ependyma, it is often discontinuous in adults.
Systematic approach to interpretation
As with all studies, having a systematic approach to intradural lesions is essential if subtle lesions are to be detected and the differential adequately narrowed. There is no single correct way to do this, and what is presented is merely a personal approach.
The relationship of the mass to the cord is of prime importance and typically two compartments are considered: intramedullary (ie within the cord), intradural extramedullary (ie. within the theca but outside of the cord). Additionally the cauda equina region is often considered separately as a number of lesions are particular to it. The vast majority of cases will then fall into a relatively small number of more common entities for each region (for a more complete list please refer to neoplasms of the spinal canal):
- Intramedullary (see also intramedullary spinal tumours)
- Intradural extramedullary
- cauda equina and filum terminale
Although in most cases which compartment a lesion is in is obvious, sometimes larger lesions can be challenging, and typically the questions which arise are:
- is this a large extramedullary mass compressing the cord or an exophytic intramedullary mass?
- is this a conus lesion or a lesion just below the conus?
In most cases a careful assessment not so much of the mass but of the cord immediately above or below the mass is helpful. An extramedullary mass will push the cord away from it whereas an intramedullary lesion will expand and thin the cord around it (this is known elsewhere as the claw sign).
Sometimes one must also consider whether a mass is actually extradural, and again examination of the dura above and below the mass is useful. Other than extension into the foramen as is seen in nerve sheath tumours and sometimes meningiomas (so-called dumbbell lesions), it is rare for intradural masses to extend into the extradural compartment, and thus vertebral body or clear epidural involvement usually means that the mass is extradural.
Once a lesion is determined to be within the cord substance, its location within the cord is important. Ependymomas, for example, are usually located centrally whereas astrocytomas are usually eccentric.
Intradural extramedullary lesions
Intradural extramedullary lesions may be related to nerve roots and may extend into the foramen (e.g. schwannomas and neurofibromas) or they may have a broad dural attachement (e.g. meningiomas) or be attached to the cord (leptomeningeal metastases).
Cauda equina/filum terminale lesions
It is sometimes possible, especially with smaller lesions, to identify whether or not it arises from the filum terminale (e.g. myxopapillary ependymoma) or from one of the nerve roots (e.g schwannoma). The filum terminale is usually slightly thicker than nerve roots and may have a small amount of fat. If one carefully traces nerve roots from below, one can trace them rostrally on each side as paired structure, as they ascend through the sub-articular recesses.
Most intradural tumours are isointense or hypointense on T1 weighted images. T1 hypointensity may also reflect haemorrhage, cyst formation, fibrous tissue and calcification. T1 hyperintensity may be present if the lesion is haemorrhagic or if it is a fat-containing mass such as a dermoid, teratoma or lipoma. Melanin also results in T1 shortening, even in the absence of haemorrhage. Myxopapillary ependymomas are usually isointense, however a prominent mucinous component occasionally results in T1 hyperintensity. Ganglioglioma typically demonstrate mixed signal intensity on T1 weighted images. This is due to the dual cellular elements of the tumour (ganglion cells and glial elements) and is a unique finding among spinal cord tumours.
Spinal cord tumours are usually hyperintense on T2 weighted images. An exception is solitary fibrous tumours which are markedly hypointense on T2 weighted studies. The T2 hypointensity of these tumours is thought to be due to the presence of abundant collagen fibres. Tumour haemorrhage leads to a hypointense haemosiderin rim on T2 weighed images (“cap sign”). This sign is suggestive of but not pathognomonic for ependymoma. T2 hypointensity may also reflect tumour calcification.
Peritumoural edema manifests as T2 hyperintensity surrounding the tumour nodule. Although this is a nonspecific finding, extensive oedema which is disproportionate to the size of the mass is typical of a metastasis.
Care should be taken to assess for the presence of prominant vascular flow voids, which may either indicate a vascular tumour (e.g. haemangioblastoma) or a vascular malformation (e.g. spinal dural arteriovenous fistula).
Most spinal neoplasms demonstrate contrast enhancement, although the pattern of enhancement is highly variable. Lack of enhancement, however, certainly does not exclude a neoplasm as minority of tumours do not enhance at all. Conversely, intradural lesions which mimic spinal neoplasms may demonstrate variable contrast enhancement. Such lesions include cavernous malformations, dural arteriovenous fistulas, spinal cord infarction, demyelinating lesions, transverse myelitis and spinal cord abscesses.
Depending on age, blood products have a variable appearance on MRI. Chronic blood appears hypointense on T1 and T2 weighted images and demonstrates blooming on susceptibility weighted images. Haemorrhage within a spinal tumour suggests ependymoma, hemangioblastoma, paraganglioma or a haemorrhagic metastasis (thyroid, melanoma). Cavernomas and cord contusions also demonstrate blood product of varying ages.
Depending on the likely diagnosis, examination of the rest of the neuraxis may be required, and should be recommended especially when appearances suggest leptomeningeal metastases or ependymoma. Similarly the presence of multiple lesions (e.g. spinal nerve sheath tumours) may raise concern of a phakomatosis (e.g. NF1 or NF2).
What clinicians want to know
If an intradural spinal mass is identified, the following features should be described as they may help narrow the differential diagnosis or have clinical implications:
- signal intensity on T1 and T2 weighted images +/- appearance on other sequences
- presence and pattern of contrast enhancement
- intramedullary or intradural extramedullary
- spinal cord segment(s) affected (see below)
- for intramedullary tumours, location within spinal cord (central vs eccentric)
- exophytic component
- single vs multiple lesions
- presence of: haemorrhage, calcification, necrosis
- associated cysts (tumoural, non-tumoural) or syringomyelia
- surrounding abnormalities
- peritumoural oedema
- prominent flow voids
- leptomeningeal enhancement
- bony changes (although these are relatively rare)- widening of the spinal canal, scalloping of the posterior vertebral bodies, neural exit foraminal enlargement, scoliosis
- compression of the spinal cord or displacement of nerve roots
Spinal level nomenclature
Although anatomy textbooks always describe standard 7 cervical, 12 thoracic and 5 lumbar vertebrae, the reality is that in a significant number of patients some degree or variation on this general layout is present. For example it is common for the S1 level to be lumbarised (i.e 6 lumbar type vertebrae are present) or for an additional pair of ribs to be present. Such variation results in potentially discrepant numbering of levels, depending on interpretation and whether the levels are counted from above or below.
It is thus essential that in all cases where anatomy is variant, a pedantic, careful and explicit description of the way the levels have been numbered be included in the report.