PathologyDemyelinating diseases
Demyelination is produced by many different agents from infection to radiation therapy, toxic effect of drugs, autoimmune processes, ischaemia, etc. The term demyelination is then used to indicate a condition in which normally formed myelin is later destroyed: dysmyelination is used to indicate a condition in which, usually due to genetic causes, myelin never forms properly.
The clinical presentation of demyelinating and dysmyelinating diseases is very different: dysmyelination usually manifests earlier, with progressive signs.
Demyelination is either clinically related to the underlying disease (vasculitis, viral infections) or, as in the case of the most common demyelinating condition, multiple sclerosis, the course may be as a series of relapses and remissions of neurological signs or it may be progressive.
The role of neuroradiological diagnosis in multiple sclerosis has significantly changed with the advent of CT in 1972, and subsequently with MRI in the early eighties.
Before CT the neuraradiologist was involved only in the differential diagnostic process, to rule out by means of angiography or pneumoencephalography tumours, vasular ischaemic lesions or any other disease affecting the brain and the spinal cord.
With CT the brain parenchyma is directly imaged: white matter is slightly hypodense with respect to the slightly hyperdense gray matter. Foci of demyelination may be recognized as hypodense areas against the already hypodense background of the white matter.
MRI has proven to be far superior to CT in the diagnosis of white matter lesions, due to its sensitivity to white matter changes that appear as hyperintense areas against the relatively isointense background of the white matter on PD and T2-weighted images.
Gadolinium injection shows the disruption of the blood-brain barrier when present and has proven to be of great value in better diagnosing MS and understanding its clinical evolution.
Multiple sclerosis
CT is usually positive in about 50% of MS patients non-selected for type or phase of the disease. Plaques are visible as foci of hypodensity (Fig. 38), mainly in the supratentorial periventricular white matter. Small plaques, particularly in the corpus callosum, in the subcortical areas, in the brain stem and posterior fossa are in general not easily recognized.
There is no way to distinguish recent from old plaques when they co-exist in the same patient.
MRI is definitely superior to CT: much smaller plaques can be detected and critical areas such as the corpus callosum and the brain stem are easily investigated employing sagittal or coronal sections.
The positivity of MRI is around 90% in patients non-selected for type or phase of the disease; the positivity rises up to 100% in cases of definite MS. MRI is more sensitive than laboratory tests like evoked potentials or CSF oligoclonal bands in patients with definite MS.
Plaques tend to be periventricular, frequently have an oval shape with a radial distribution along medullary veins, and the major axis oriented perpendicular to the lateral wall of the ventricle.
The most common locations are in the deep white matter of the cerebral hemispheres (Fig. 39); the cerebral or cerebellar peduncles, brain stem (Fig. 40, 41), medial longitudinal fasciculus and corpus callosum are also frequently involved. As with CT, it is not possible with routine SE sequences to differentiate new from old plaques.
Gadolinium injection provides the only way to differentiate recent from old plaques; recent plaques show a focal hyperintensity on T1. Sometimes Gd allows identification of hyperacute plaques that have not yet produced a signal change in T1 or T2. Gd is in fact able to demonstrate the disruption of the blood brain barrier before the formation of the inflammatory infiltrate and subsequent formation of oedema.
Gd enhancement may have different appearances; ring enhancement probably reflects reactivation of an old plaque while a nodular homogeneous enhancement indicates a recent plaque.
The demonstration in the same patient at the first clinical presentation of acute enhancing plaques together with chronic non-enhancing plaques confirms that MS is not a monophasic disease and provides useful criteria for differential diagnosis with other diseases affecting the white matter such as Acute Disseminated Encephalomyelitis (ADEM).
Some authors have performed Gd enhanced scans every two weeks and have demonstrated a high subclinical activity in the formation of plaques. The knowledge of this phenomenon has provided a useful indication for clinical models in therapeutic trials.
Optic neuritis
MRI is particularly useful in patients that present with a first episode of optic neuritis; not infrequently asymptomatic plaques may be seen in the brain parenchyma. They may either be contemporary to the episode of optic neuritis or indicate previous episodes.
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Figure 39.
Definite MS
a) MRI, axial T2-weighted image. Plaques are mainly periventricular, oval shaped with a major transverse axis, hyperintense with respect to normal parenchyma. There is no way in this patient with a relapse to tell which is the active plaque.
b) Following Gd injection a ring-like enhancement around a plaque in the left centrum semiovale is seen.
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Figure 40.
Plaque in the pons, along the left fifth nerve, in a patient with trigeminal neuralgia.
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Figure 42.
Spinal cord plaque. T2-weighted sagittal image. Large focus of increased signal at C2-C3, with mild cord widening.
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The same is true for other clinical presentations; many more plaques, frequently inactive, are found in patients at the first clinically recognized episode of MS.
Spinal cord MS (Myelitis)
Plaques in the spinal cord are less easily seen than in the brain since MRI is less sensitive in demonstrating small lesions in the cord than in the brain.
Cervical plaques, particularly when they are sufficiently large and produce some swelling of the cord, may be detected (Fig. 42).
The positive yield is much lower for the thoracic cord. A commonly used protocol in a patient suspected of having cord localization of MS, is to scan the brain in search for asymptomatic plaques. If this test is positive, the diagnosis is easily made; a negative brain MRI, however, does not rule out a diagnosis of MS.
The higher frequency of detection of spinal cord MS plaques in the cervical region probably reflects the better resolution of MRI for the topographical region rather than a significantly higher frequency of occurrence of plaques in the cervical cord.
Differential diagnosis
Diagnosis of MS is a clinical diagnosis that must be supported by clinical history, age of the patient, other laboratory data such as evoked potentials and CSF oligoclonal bands.
The MR appearance of a plaque is per se non specific. In the presence of multiple, diffuse white matter focal lesions, other disease entities such as vasculitis, radiation damage and subcortical atherosclerotic encephalopathy (Binswanger's disease ) must be taken into consideration.
Acute disseminated encephalomyelitis (ADEM) has a very similar appearance but knowledge of a previous viral infectious episode or vaccination and the monophasic aspect of the demyelinating foci will lead to the diagnosis.
Pseudotumoural plaques
In some rare cases, plaques may be very large and simulate the diagnosis of a tumor both clinically and morphologically (Fig. 43). Short term follow-up, close clinical observation, MR spectroscopy will lead to the correct diagnosis.
Kjell Bergström and Giuseppe Scotti