The Substantia Nigra is also Involved in Japanese Encephalitis

We read with interest the article by Cerna et al published in the AJNR (1), in which they reported that isolated lesions of the substantia nigra suggest the possibility of St Louis encephalitis (SLE). It was also noted that there had been no previous reports of focal signal changes in the substantia nigra for any infectious, metabolic, or degenerative diseases of the brain.

We would like to bring to your attention that the substantia nigra is also involved in Japanese encephalitis (JE). In our study on MR changes in JE, the midbrain was reported to be involved in three of seven patients, and Figure 3 in that report showed involvement of the substantia nigra (2). In a more recent report on 42 patients with JE, the midbrain was involved in 18 of 31 patients in whom MR imaging was performed (3). It is interesting to note that two of these patients had isolated midbrain involvement (Fig 1). It should be realized that many published reports may include substantia nigra involvement under the heading of midbrain or brain stem lesion. The substantia nigra was reported to be involved in JE as early as 1989 (4). It should be understood, however, that JE is also commonly associated with thalamic, basal ganglia, cerebral cortex, and cerebellar involvement.

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fig. Cranial, axial, T₂-weighted MR image shows hyperintense signal changes in the substantial nigra in a patient with JE

Cerna et al (1) referred to several autopsy studies of patients with SLE that suggested “selectively severe involvement of the substantia nigra”. In another autopsy study employing quantitative histologic and immunoflourescent techniques on patients who died in the 1975 SLE epidemic in Illinois, it was found that not only was there involvement of the substantia nigra and spinal gray matter, but other areas, including the thalamus, inferior olivary nucleus, brain stem, and to a lesser extent the cerebellar cortex and pons, were also affected (5). A number of authors have reported widespread involvement of the nervous system in SLE, with involvement of both the thalamus and substantia nigra to a similar degree (6). The thalamus, basal ganglia, substantia nigra, red nucleus, pons, cerebral gray matter, and cerebellum have also been reported to be involved in JE (7). The involvement of these areas has been observed in various MR imaging studies as well (2–4).

The patients reported by Cerna et al had a mild clinical picture, and it is not surprising that MR changes manifested in only two of seven patients. The diagnosis of SLE in cases with isolated involvement of the substantia nigra in a severely ill, tremulus patient seems rather premature, because the clinical and pathologic findings of SLE are indistinguishable from other arbovirus infections of the CNS (8). Further studies are needed on larger and more severely affected patients with SLE before drawing definite conclusions regarding the diagnostic specificity of substantia nigra involvement. The radiologic findings should be interpreted in light of the endemic nature of the disease, clinical context, and virologic studies.