Clinical guidance for MRI referral

MRI of the head

Unexplained seizure(s)

Clinical guidance for MRI referral
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MBS item description

Referral by a medical practitioner (excluding a specialist or consultant physician) for a scan of head for a patient 16 years or older for unexplained seizure(s) (R) (K) (Contrast) (Anaes.)

Key information

  • Although neuroimaging is indicated in most new onset seizures, there is a lack of evidence that imaging improves health outcomes.
  • The type of seizure will influence decisions about neuroimaging.
  • MRI is the imaging investigation of choice in most cases and CT can be considered where MRI is not available.
  • There is potential to reveal incidental findings with brain MRI, which can result in unnecessary anxiety and intervention.

Explanation
Probably effective, ineffective or harmful (or probably useful/ predictive or not useful/predictive) for the given condition in the specified population. (Level B rating requires at least one Class I study or at least two consistent Class II studies.)

Recommendation
Brain imaging with CT or MRI should be considered as part of the routine neurodiagnostic evaluation of adults presenting with an apparent unprovoked first seizure

  1. Krumholz A, Wiebe S, Gronseth G, et al. Practice parameter: Evaluating an apparent unprovoked first seizure in adults (an evidence-based review): report of the quality standards subcommittee of the American Academy of Neurology and the American Epilepsy Society. Neurology 2007;69(21):1996–2007.

Explanation
D – Evidence level 3 or 4 (nonanalytic studies or expert opinion)
or
Extrapolated evidence from studies rated as 2+ (well-conducted case–control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal)

Recommendation
CT has a role in the urgent assessment of seizures, or when MRI is contraindicated or unavailable

  1. Scottish Intercollegiate Guidelines Network (SIGN). Diagnosis and management of epilepsy in adults: a national clinical guideline. Edinburgh: SIGN, 2003.

Grade: None given

Recommendation
MRI is the imaging investigation of choice for most unexplained seizures

  1. National Institute for Health and Clinical Excellence (NICE). The epilepsies: diagnosis and management of the epilepsies in adults in primary and secondary care. London: NICE, 2012.

There are two main seizure types: generalised and focal (or partial).

Table 2.1 Generalised seizures

Table 2.1

Generalised seizures

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Table 2.2 Focal (partial seizures)

Table 2.2

Focal (partial seizures)

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The type of seizure will influence decisions regarding neuroimaging. Determining seizure type is best done by close attention to patient and eyewitness descriptions of the event and the findings on an electroencephalography (EEG) done in close proximity to the seizure.

It is important to make the distinction between idiopathic generalised epilepsies (IGEs) and focal (localisation-related) epilepsies, as this affects further investigations, treatment choices, prognosis and counselling.8 An Australian study at a first seizure clinic found epileptogenic lesions in 17% of patients presenting with focal onset seizures.10

Patient history can often distinguish epileptic seizures from non-epileptic disorders by identifying the events directly preceding the seizure, associated conditions and details of the event, including possible triggers, duration and type of movements.11There can be difficulty in differentiating syncope from seizures based on patient self-reporting.

More than 50% of patients who present with a first seizure never have another. If patients do not have recurrence after 2 years, the risk falls to <10%. Patients with epileptic discharges on EEG or congenital neurological deficits have the highest recurrence rates (up to 90%).12

Additional investigations may be required after new onset seizure. EEG should be considered as part of the routine neurodiagnostic evaluation of adults presenting with an apparent unprovoked first seizure.7 Other testing should be performed based on clinical judgement.

An MRI is not always required for a patient with new onset seizure. Idiopathic generalised epilepsy is not associated with an increased prevalence of brain lesions; therefore, if this is confidently diagnosed, the patient does not require imaging.8However, as neuroimaging is recommended in most situations of new onset seizure,11,13 many patients who are later diagnosed with idiopathic generalised epilepsy will have neuroimaging after their first seizure.

Neuroimaging (preferably MRI) is recommended after a first unprovoked seizure for all adults with risk factors.11,14

Note that seizures are a feature of some brain tumours and may precede tumour diagnosis by years.15

CT

CT has a major advantage because of the speed with which it can be obtained, so its value is mainly in the emergency situation. For emergency patients presenting with seizure, immediate non-contrast CT may be useful to guide appropriate acute management, especially where there is an abnormal neurological examination, predisposing history or focal seizure onset.16

CT of the head has demonstrated structural lesions in about one-third of adults who present to the emergency department with a first seizure.11 Further, identification of lesions has altered the acute medical or surgical management in up to 17% of adults presenting with first seizure.11

MRI

MRI is regarded by expert opinion as having a higher yield and is the preferred procedure in non-emergency or elective situations. It should only be ordered after obtaining a good clinical history, preferably including an eyewitness description, and where possible, after an EEG that does not show generalised epileptic discharges.

MRI is the preferred modality for high-resolution structural imaging in epilepsy17 and is more sensitive than CT for detecting intrinsic brain tumours, stroke, focal cortical dysplasia, mesial temporal sclerosis, vascular malformations and cerebral dysgenesis.11

Although MRI is generally preferred to CT because of its greater sensitivity for intracranial pathology, CT should be performed if intracranial bleeding is suspected because of recent head trauma, coagulopathy or severe headache. This is because of the advantage of speed, as mentioned above.11

MRI is particularly indicated in those who develop epilepsy in adulthood and have any suggestion of a focal onset on history or examination.9

Appropriate imaging and the identification of lesions have been shown to alter the acute medical or surgical management of patients presenting with seizures.17 Whether the alteration of management led to better health outcomes is less clear.

Brain imaging has been shown to detect lesions in 21–37% of patients presenting with epilepsy.8 Note that the prevalence of incidental findings on standard brain MRI is 2.7% (increasing to 4.3% using high-resolution MRI).18

Not all MRI abnormalities are associated with epileptic seizures. Punctate foci of T2 signal change in the white matter, many cystic lesions (arachnoid cysts, choroidal fissure cysts), lacunar strokes, ventricular asymmetry, diffuse atrophy and isolated venous anomalies (i.e. those not associated with arteriovenous malformation or cavernous angioma) are not known to be epileptogenic, and should be considered incidental to a seizure diagnosis.19 The lesions detected in patients presenting with epilepsy only require treatment in a small minority of patients.8

The prevalence of incidental findings increases with age.18

Figure 2.1 Prevalence of incidental findings in various age categories

Figure 2.1

Prevalence of incidental findings in various age categories

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Reproduced with permission from British Medical Journal Publishing Group Ltd from Morris et al. Incidental findings on brain  magnetic resonance imaging: systematic review and meta-analysis. BMJ 2009;339:b3016 doi:10.1136/bmj.b3016.

Potential harm arising from the further treatment and investigation of incidental findings has to be balanced against the likelihood of benefit from detecting a significant abnormality.

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  8. Scottish Intercollegiate Guidelines Network (SIGN). Diagnosis and management of epilepsy in adults: a national clinical guideline. Edinburgh: SIGN, 2003.
  9. National Institute for Health and Clinical Excellence (NICE). The epilepsies: diagnosis and management of the epilepsies in adults in primary and secondary care. London: NICE, 2012.
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