Radiographic assessment of adult brachial plexus injuries

Summary box

1

Detection of nerve root avulsion injuries is critical for planning surgical treatment of patients with brachial plexus palsies.
2

Traumatic pseudomeningoceles are no longer regarded as the only incontrovertible evidence of nerve root avulsion. Direct visualization of dorsal and ventral nerve rootlets is now possible, and identification of avulsions with or without dural abnormalities is critical.
3

Magnetic resonance imaging (MRI) modalities offer many advantages and should be used for primary evaluation of brachial plexus injuries; Computed tomographic (CT) myelography can be reserved for patients with contraindications to MRI.
4

MRI should not be performed prior to 1 month from injury, because acute hemorrhage can compromise visualization of the thecal sac and pseudomeningoceles take time to develop.
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3D Magnetic Resonance Myelography (MRM) and Magnetic Resonance Neurography (MRN) should be performed on a per-root basis for a complete radiographic evaluation.
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To increase accuracy of diagnosis, reformatted axial sections must be obtained in the direction of nerve roots, which changes from C5 to T1.
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Reformatted sections should be rotated simultaneously in 2 different planes in order to optimally depict their attachment to the spinal cord.
8

Nerve rootlets should be identified on several sections and compared with intact rootlets of the uninjured contralateral side to avoid false positive findings caused by abnormal postures resulting from shoulder trauma.
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Partial avulsions are characterized by absent ventral or dorsal roots with only minimal or no abnormalities of the nerve root sleeves.
10

In stretch injuries, edema and fibrosis of the brachial plexus can manifest as swelling of the injured nerves, which develop a tortuous appearance with diffuse increases in nerve signal intensities over considerable lengths, as demonstrated readily on MRN.

Introduction

Despite the many advances in microsurgical strategies and techniques during the last several decades, treatment of brachial plexus injuries remains challenging. Of paramount importance to surgical planning is a thorough characterization of the injury affecting various elements of the brachial plexus. For instance, accurate detection of nerve root avulsion is crucial. If the nerve is traumatically transected distal to the dorsal root ganglion (postganglionic), nerve repair using sural nerve grafts remains a viable option for restoration of motor function. In contrast, if the lesion is proximal to the dorsal root ganglion (preganglionic), nerve repair is uncommon, and nerve transfer (neurotization) becomes the more useful strategy.

Traumatic lesions can be very complex, and they vary widely in type, location, and severity. Traction (stretch) injuries, which account for the vast majority of brachial plexus palsies, can result in widespread, unpredictable lesions to multiple elements of the brachial plexus. Some types of lesions may even appear normal on intraoperative visual inspection despite being the most severely injured, as evidenced in cases of nerve root avulsions within the spinal canal. Should a nerve repair be undertaken without radiographic knowledge of the intraspinal nerve root avulsion, failure of the surgical intervention is inevitable.

Nerve root avulsions complicate more than 70% of traumatic brachial plexus injuries, so what are the most reliable diagnostic techniques used to define the nature of the lesion(s)? Clinical examination cannot routinely distinguish between pre- and postganglionic injuries, although the presence of Horner’s sign suggests proximal injury to lower elements of the brachial plexus. Electrodiagnostic examinations contribute to accurate diagnosis of brachial plexus injuries, but again the information is inferred, not directly demonstrated. On the other hand, radiographic assessments are beginning to provide methods for visualizing the anatomic defect. In the setting of traumatic injury to the brachial plexus, radiographic studies can be most useful in the detection of preganglionic injuries and will be the focus of this chapter.

At the vertebral foramen, the anterior and posterior rootlets (originating from the spinal cord) merge within a cone-shaped dural envelope that continues distally as the epineurium of the spinal nerve. When excessive traction is applied to a spinal nerve, the rootlets can be torn away from the spinal cord. The ventral rootlets are shorter than the dorsal ones and are therefore more prone to rupture. The C8 and T1 roots are more prone to avulsion because the C5, C6, and C7 roots are fixed to the vertebral column by small, fibrous epineurial attachments to the well-formed transverse processes, which are absent at the levels of the lower nerve roots. The traction force can also be transmitted to the dural sleeve to produce a partial or full-thickness tear in the dura through which the arachnoid membrane bulges, thereby resulting in the formation of a sac or diverticulum, commonly referred to as a traumatic pseudomeningocele.

The advent of magnetic resonance imaging (MRI) greatly augmented the diagnostic workup of traumatic brachial plexus injuries. For many decades, identification of avulsed nerve roots relied solely on invasive examinations such as myelography and computed tomographic (CT) myelography (CTM). The increasing sophistication in MRI techniques now offers additional diagnostic modalities such as MR myelography (MRM), which has completely replaced invasive myelographic examinations at some institutions. Another alternative method for the radiographic assessment of brachial plexus injuries employs the technique of MR neurography (MRN). However, in our opinion, CTM and, more recently, MRM, remain the most reliable tools for radiographic assessment and should be performed as part of the routine diagnostic/preoperative evaluation of patients with brachial plexus injuries.

Myelography

Radiographic detection of nerve root avulsion in brachial plexus injury was essentially a chance discovery. In 1947, Murphey et al. performed myelography on a patient with severe pain and a brachial plexus injury in order to exclude the presence of a concomitant herniated cervical disk. The resultant myelogram demonstrated mushroom-shaped leakages of iodinated contrast material bulging from the dural sac extending beyond the vertebral foramina ( Box 18.1 ). In 1949, the same author described similar myelographic examinations on 7 patients, 3 of which submitted to a surgical exploration that revealed undeniable nerve root avulsions. Subsequently, myelography became routinely used for the diagnosis of nerve root avulsion in brachial plexus palsy.

Box 18.1

Advantages

1

High spatial resolution
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Readily demonstrates complete nerve root avulsion
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No artifacts due to cerebrospinal fluid (CSF) flow
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Excellent visualization of bone structures
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More useful than CTM for assessing C8–T1 nerve roots

Disadvantages

1

Almost completely replaced by CTM and MRM
2

Invasive diagnostic procedure
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Possible reaction to contrast medium
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Dilution of contrast medium
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Poor filling of meningoceles in the presence of intradural scar
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Intraprocedural complications (epidural/subdural hematomas)
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Postprocedural complications (headache, nausea, vomiting, epileptic seizures)
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Poor evaluation of soft tissue elements

Traditional myelography is an invasive examination during which water-soluble iodinated contrast medium (10–15 mL, 22-gauge needle) is injected intradurally through cervical (C1–C2) or lumbar puncture. The contrast medium must migrate within the spinal fluid from the site of injection to the cervical subarachnoid spaces while avoiding intracranial migration, to minimize adverse effects such as aseptic meningitis and seizures. Conventional X-ray myelography is well-established in its ability to demonstrate the integrity of nerve roots with high diagnostic accuracy, especially when performed in combination with a post-myelogram CT scan.

Several myelographic patterns have been described to accurately reflect the corresponding anatomic lesions. The most common is the traumatic pseudomeningocele, which appears as mushroom-shaped leakage of iodinated contrast medium bulging from the dural sac, which may extend beyond the vertebral foramen ( Figure 18.1 ). The vast majority of nerve root avulsions are associated with traumatic pseudomeningoceles, yet rootlet avulsions in the absence of dural abnormalities and pseudomeningoceles have been well-documented ( Figure 18.2 A–D ). Therefore, the presence of a pseudomeningocele strongly suggests an avulsion injury, but is no longer regarded as incontrovertible evidence of nerve root avulsion.

A more reliable indicator of nerve root avulsion is the aspect of nerve root shadows within the dural sleeve. Nagano et al. studied 90 adult patients with traumatic brachial plexus injuries and correlated their radiographic appearance with intraoperative findings. They reported a wide spectrum of myelographic abnormalities, ranging from slightly abnormal root sleeve shadows (indicating a reduction in number or size of nerve rootlets or partial avulsion) to complete obliteration of the nerve root sleeve tip with complete absence of the filling defect of the nerve root (complete avulsion). The authors concluded that the complete absence of filling defects of the nerve roots within the root sleeve is associated with a preganglionic injury in 96.5% of cases. Additionally, evaluation of nerve rootlets can be assisted by identification of deformities of the subarachnoid spaces, which are often enlarged or irregular when partial or complete avulsions occur.

The importance of a meticulous myelographic evaluation of the individual nerve rootlets has been recently emphasized in a report documenting the usefulness of X-ray myelography in traumatic injuries of the brachial plexus. Complete or partial nerve root avulsions can occur in the absence of pseudomeningoceles in approximately 20% of cases visualized by X-ray myelography; however, when this technique is combined with CTM, the overall accuracy of detection increases to 92%. Note that for evaluation of intradural nerve roots, X-ray myelography can be more sensitive in detecting root avulsions at C8 and T1 levels than CTM; however, it has limited value in the identification of partial avulsions, because ventral and dorsal nerve roots cannot be evaluated separately.

X-ray myelograms may be inadequate for evaluation of all of the nerve rootlets comprising the brachial plexus because of poor contrast opacification of the cervical subarachnoid space and consequent inability to visualize the rootlets. In the case of dural scarring, irregular filling of the subarachnoid spaces can apparently occur at several nerve root levels, resulting in poor myelographic definition of the thecal sac and incorrect assessment of nerve root integrity. Furthermore, significant image degradation can also result from inadvertent subdural injection of contrast medium.

CT myelography

With the advent of CT, CTM has rapidly become the standard examination for traumatic injuries of the brachial plexus ( Box 18.2 ). Compared to standard myelography, CTM requires less intrathecal volume of the water-soluble iodinated contrast medium, which decreases incidence of adverse effects while maintaining high contrast resolution. Other advantages include high spatial resolution, reduced movement artefacts, and excellent visualization of bone structures. Since the introduction of the multidetector CT, a large number of sub-millimeter sections can be acquired in a very short time, thereby facilitating immediate postprocessing of the study. Disadvantages of CTM include increased radiation exposure (compared to standard myelography), occasional artifacts due to beam hardening, and poor visualization of soft tissue elements such as the distal brachial plexus.

Box 18.2

Advantages

1

Primary radiographic examination when MRM is not available
2

Can be used for evaluation of claustrophobic patients
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Can reveal partial root avulsions
4

No CSF flow artifacts
5

Excellent visualization of bone structures
6

Multidetector CT allows fast acquisition of large imaging volumes with high resolution multiplanar reconstructions

Contraindications and disadvantages

1

Invasive diagnostic procedure
2

High radiation exposure
3

Risk of adverse reaction to contrast medium
4

Intraprocedural complications (epidural/subdural hematomas)
5

Postprocedural complications (headache, nausea, vomiting, epileptic seizures)
6

Artifacts due to radiation beam hardening
7

Poor evaluation of distal brachial plexus

CTM procedural algorithm

1

Same-day hospital procedure
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Requires Tilt-table and fluoroscopy
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Hydrate patient prior to procedure
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Acquire peripheral intravenous access
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Place patient in the lateral position
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Inject local anesthesia
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Perform lateral cervical (C1–C2) or lumbar puncture with a 22-gauge needle
8

Inject 10–15 cc of low concentration water-soluble iodinated contrast medium (200 mg/mL)
9

Move the patient to allow contrast medium to migrate from the site of injection to the cervical subarachnoid spaces without entry into the intracranial compartment
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Transfer the patient to the CT scanner, and accurately position neck and shoulders
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Acquire the CT volume from C3 to T2
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Use a scanning time of 10s (64 row multidetector CT)
13

Hydrate patient after procedure