The encephalitic form is expressed more fully in children than in adults. As an acute infectious illness is resolving or after a latency of several days or longer, there is the abrupt onset, over hours or a day or two, of confusion, somnolence, and sometimes convulsions with headache, fever, and varying degrees of neck stiffness. Ataxia is common, but myoclonic movements and choreoathetosis are observed less frequently. In severe cases, stupor, coma, and at times decerebrate rigidity may occur in rapid succession. In many cases, the disease is less severe and the patient suffers a transient encephalitic illness with headaches, confusion, and slight signs of meningeal irritation.
The latency between infection and the first neurologic symptoms that it is acceptable for this diagnosis is a matter of debate, but there are convincing cases (postexanthematous) in which the 2 phases of illness are separated by 3 or 4 weeks; several days is more typical, as noted below. Curiously, in the encephalitic form, new signs may continue to appear for up to 2 or 3 weeks from the onset. This is emphasized in the series of affected children collected by Hynson and colleagues. The imaging changes may also display delayed or continued evolution. These authors note that ataxia was the most common initial feature in their cases, which is not entirely in accordance with our experience.
In the myelitic form (postinfectious myelitis, acute transverse myelitis), there is partial or complete paraplegia or quadriplegia, diminution or loss of tendon reflexes, sensory impairment, and varying degrees of paralysis of bladder and bowel. A syndrome that simulates anterior spinal artery occlusion (spastic paraplegia and loss of pain sensation below a level on the trunk but tending to spare large-fiber sensibility) is not uncommon in our experience. Also, we have cared for a few patients with a limited sacral form of postinfectious myelitis. Midline back pain may be a prominent symptom at the onset.
In both the encephalitic and myelitic types, there may be slight fever, particularly in the more aggressive cases and in younger individuals, where we have seen temperatures reaching 39.4°C (103°F), but the peripheral white blood cell count is normal if the initiating infection has resolved. A few of our patients have had elevated sedimentation rates, but it is not possible to know whether this reflects the precipitating infection. Nonetheless, separating these 2 entities may be difficult, especially in children who have a greater tendency to develop fever and convulsions with ADEM. Either process may be associated with aseptic meningitis.
The CSF shows a slight increase in lymphocytes and protein content, but these are variable, with a few of our patients having only an increase in protein and no cells and others having up to several hundred cells. The MRI shows several bilateral confluent white matter lesions in both cerebral hemispheres early in the course of ADEM (Fig. 35-4); when these are large and numerous, the diagnosis is more certain. The lesions all appear to be of similar age, but we cannot account for several cases we have seen in which serial MRIs show new lesions accumulating over 2 or 3 weeks, as already noted. Moreover, as pointed out by Honkaniemi and colleagues, there may be a delay of several days between the clinical manifestations and the first appearance of changes in the MRI, a situation to which we can attest. Whether a single lesion on MRI can be considered compatible with ADEM is unclear.