For any doctor looking after children with epilepsy, an essential skill is interpretation of the MRI. But, even in centers of imaging excellence, accurate clinical and EEG data are required to enable interpretation of an MRI abnormality or, alternatively, to allow a focused search for the common subtle abnormalities that may underlie symptomatic epilepsy. Brain imaging has become an essential part of the evaluation of most children with epilepsy. Yet, MRI has not taken over from the clinical basics. The pillars of a thorough evaluation of a child with seizures still remain; a good clinical assessment (detailed history and physical examination) supplemented by an interictal EEG.
As a consultant dealing with epilepsy, it is clearly a mistake to rely solely on imaging in the absence of good electroclinical data, ideally forming a diagnosis of the child’s epilepsy syndrome. If the initial diagnostic question “Does this child have epilepsy?” is answered in the affirmative this should be followed by “what is the epilepsy syndrome?” The answer to the second question is the rational basis to determine which patients require brain imaging (Fig. 5–1).
This chapter will focus initially on the rationale, indications, and published clinical guidelines for imaging children with epilepsy. A brief discussion on the broad principles of optimal imaging methods for the epilepsy population will be followed by the application of imaging to more common specific clinical situations.
The rationale for neuroimaging in epilepsy is summarized in a position statement by the Commission on Neuroimaging of the International League Against Epilepsy (ILAE) (1997).1,2 They provided two indications:
“To identify underlying abnormalities such as vascular lesions, acute trauma and tumours that require specific treatment.”
“To aid the formulation of syndromic and etiological diagnoses and to give patients, their relatives and physicians an accurate prognosis.”
Other reasons for neuroimaging include:
Metabolic diseases diagnosable on MRI can present with focal seizures such as adrenoleukodystrophy or mitochondrial encephalopathy. Other acute, nonepilepsy disorders such as hydrocephalus, trauma, infection, stroke, and acute-disseminated encephalomyelitis can clearly lead to change in management.
Apart from the above hereditable metabolic and neurodegenerative conditions that have obvious genetic implications, an important group are the malformations of cortical development. An increasing number of the more common abnormalities such as polymicrogyria now have an underlying genetic diagnosis. Well-characterized recessive and X-linked disorders explain the majority of patients with lissencephaly.
One of the most common reasons driving families and clinicians to perform neuroimaging in childhood epilepsy is fear: “could there be a brain tumor?” If one looks at published imaging series low-grade or “benign” brain tumors are not an uncommon discovery. In an otherwise asymptomatic child presenting with epilepsy, the incidence of malignant brain tumors is very low. Looking from another angle, a description of the presenting symptoms of 200 consecutive children diagnosed with brain tumor was collected at one institution.3 Seizures were the presenting symptom in 9% and were often the only symptom with no focal signs. It was not stated whether these children had low-or high-grade tumors.
Ideally, it is recommended that one should obtain structural neuroimaging with MRI in all patients with epilepsy, except in patients with a definite electroclinical diagnosis of idiopathic generalized or partial epilepsy. Those children with idiopathic epilepsy syndromes usually have a characteristic EEG and have a very low yield from imaging. Idiopathic partial epilepsies such as benign rolandic or idiopathic generalized, and childhood absence epilepsy are good examples where imaging is not indicated.
The authors of the ILAE statement1,2 go on to say that MRI is particularly indicated in patients with one or more of the following:
Onset of seizures at any age with evidence of a partial onset on history, examination, or EEG unless clear evidence of a typical benign epilepsy.
Onset of unclassified or apparently generalized seizures in children under two years of age or in adulthood.
Difficulty in obtaining control of seizures with anticonvulsants worsening seizures, changes is seizure manifestations, or developmental regression also merit neuroimaging if not previously performed.
Loss of control of seizures with anticonvulsants or a change in the seizure pattern that may imply a progressive underlying lesion.
Children with characteristics of a symptomatic generalized epilepsy syndrome, including infantile spasms or early Lennox–Gastaut syndrome (e.g., tonic, atonic, mixed seizures), as focal MRI findings may be found in a substantial proportion of these children.
Finally, new-onset seizures/epilepsy presenting with evidence for a medical emergency such as increased intracranial pressure or status epilepticus always merit emergency imaging.
The latter recommendation highlights the need for reimaging in those patients who follow a negative course to exclude diseases such as Rasmussen encephalitis or inherited neurodegenerative condition.
The American Academy of Neurology (AAN) published a practice parameter in 2000 for evaluating a first nonfebrile seizure in children.4 Class 1 and Class 2 evidence from studies of CT or MRI reports (all from the 1990s) in over 1290 children identified a low rate (1.9%) of relevant abnormalities that led to a change in clinical intervention. Consistently MRI had a far greater yield than CT on direct comparison. A well-known example of one of these studies is a prospective cohort5 of 613 children with newly diagnosed epilepsy. Eighty-six percent had neuroimaging (predominantly MRI); 16% were abnormal but did not influence immediate management decisions.
The conclusions of the AAN practice parameter recommendations for imaging the first nonfebrile seizure were:
If a neuroimaging study is indicated, MRI is the preferred modality.
Urgent neuroimaging should be performed in a child of any age who exhibits a postictal focal deficit not quickly resolving, or who has not returned to baseline within several hours after the seizure.
Nonurgent imaging with MRI should be seriously considered in any child with a significant cognitive or motor impairment of unknown etiology, unexplained abnormalities on neurologic examination, a seizure of focal onset, an EEG that does not represent an idiopathic partial or generalized epilepsy, or in children under 1 year of age.
The National Institute for Clinical Excellence (NICE) of Great Britain guidelines for epilepsy management6 reaffirmed the AAN recommendations. In addition, they recommend that neuroimaging when indicated should be done within 4 weeks of being requested.
Given this rationale why not perform imaging on all children with epilepsy? There are negative aspects to neuroimaging including the risk of sedation or general anesthetic that is required for young or uncooperative children. Another common problem is the enormous anxiety that the family must go through waiting (sometimes) months for the exclusion of a potential life-threatening condition. Cost and access is the most important issue for most of the world’s population.
Apart from these practical concerns, the identification of incidental findings on MRI is of limited or no relevance to the diagnosis of epilepsy and can have considerable negative consequences worsening seizures, changes is seizure manifestations, or developmental regression also merit neuroimaging if not previously performed.
The rates of MRI abnormality in normal children7 and adults8 are now recognized. A prospective study8 of 2536 healthy young male military applicants, with a mean age of 20 years revealed potentially, clinically relevant abnormalities in 6.5% on limited sequences using 1.0T MRI. The most common findings were arachnoid cysts in 1.7%, Chiari malformations in 1.7%, vascular abnormalities in 0.5%, and tumors in 0.2%. Overall, 0.5% required “urgent” referral. In a pediatric study,7 a 6% rate of brain abnormalities was found in 225 healthy children. These included posterior fossa abnormalities, focal white matter lesions of uncertain significance, and arachnoid cysts. Not reported in the pediatric series, benign pineal cysts are also very common and can raise unnecessary concern in families. At the minimum these findings often lead to a repeat or serial MRI’s over months to years to ensure the benign nature of the incidental “abnormality” with clear potential for harmful psychological effects on families. Thus clinicians, when considering ordering MRI, as with other investigations, should anticipate and discuss the potential impact of the incidental finding as these are not rare.
The potential negative aspects of CT irradiation are beyond the scope of this chapter, but there have been several excellent reviews.9 It is estimated that use of CT may be associated with 1.5% to 2% of all cancers in the United States and many authors are advocating that informed consent be obtained before imaging with CT.
Many studies have demonstrated the superior value of MRI over computed tomography. There are some exceptions to this, as in acute trauma and where calcification is prominent. Many studies10 and reviews have suggested the optimum MRI acquisition sequences to try and increase yield. Collection of MRI sequences along or perpendicular to the longitudinal axis of the hippocampus enhances coronal and axial views of mesial temporal pathology. Sequences acquired perpendicular to the anterior–posterior commissural line are often used for extratemporal pathology. Emphasis is also placed on fine slice, contiguous MRI to include the whole brain utilizing both T1 and T2 weighted scans.
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