5-month-old Quarter Horse filly

Imaging findings

There are bilateral, symmetrical, focal, well circumscribed lesions, measuring approximately 8 mm in diameter at the ventral aspect of the thalamus, in the area of the substantia nigra. Both lesions are hypointense on T1-weighted images with a faint surrounding hyperintense rim. A hyperintense T1 rim is also present in the center of the right lesion, but is not present on the left side. Both lesions are hyperintense on PD, FLAIR and T2-weighted images. The right lesion appears heterogeneous on most sequences. A T2* sequence did not demonstrate any susceptibility artifact in any of the lesions. The remainder of the brain appears within normal limits.

Summary

The symmetric and focal appearance of the lesions is consistent with a toxic, nutritional or metabolic etiology. The specific location of the lesion (substantia nigra) associated with the reported clinical signs is characteristic for nigropallidal encephalomalacia due to ingestion of yellow star thistle (Centaurea solstitialis).

On post mortem examination, the brain had no gross lesions externally. On cut section, a 0.5 cm diameter, soft, friable, well circumscribed, tan foci was present in the area of the right substantia nigra. A similar, more subtle, less demarcated lesion was present on the corresponding left side. Histopathology revealed malacia confined to the substantia nigra.

Nigropallidal encephalomalacia is an equine progressive neurodegenerative disease due to chronic ingestion of yellow star thistle (Centaurea solstitialis) or Russian Knapweed (Centaurea repens). The disease is present in the Northwestern United States and British Columbia, where yellow star thistle grows as a drought-tolerant rangeland weed (Cordy 1954; Fowler 1965).

Clinical signs appear acutely after chronic consumption, for at least several weeks, of large amounts of the plant. Typically paralysis of the lips and tongue is the first clinical sign to occur, leading to dropping food and difficulty eating. Further clinical signs that might be present include facial tremors, severe depression, low head carriage, weakness and emaciation (Cordy 1954; Fowler 1965).

Historically, no specific ante-mortem diagnostic test was available. Presumptive diagnosis was made based on exposure to the plant and ruling out trauma and other neurological diseases such as equine protozoal myelitis, equine herpes virus, botulism, rabies and otitis media (Sanders, Tucker et al. 2001).

Equine nigropallidal encephalomalacia is characterized by a highly selective pattern of lesions localized to brain nuclei rich in dopamine, such as the substantia nigra and globus pallidus. The pathophysiology of the disease appears to be similar to environmentally acquired toxic parkinsonism disease (Chang, Rumbeiha et al.). On post mortem examination, focal areas of malacia are observed in the involved nuclei.

Two reports demonstrated that MRI was a useful diagnostic tool for identification of this disease (Sanders, Tucker et al. 2001; Ferrel, Gavin et al. 2002). Sanders et al. reported a case with T2 and PD hyperintense lesions in the substantia nigra and globus pallidus with the presence of a hyperintense rim on T1 weighted images. These findings are similar to what we observed in the substantia nigra in our case; however, no lesion was observed in the globus pallidus in our patient. In an addendum to their case report, Sanders et al. describe a second case without lesion in the globus pallidus, similar to what we observed. The authors hypothesized that the lack of lesion in the globus pallidus might represent an earlier stage of the disease. The presence of a hyperintense T1 rim in or surrounding the lesions is an interesting finding. No evidence of hemorrhage was present in our case and in the reported cases, ruling out the presence of products of degradation of hemoglobin. Explanation for this finding might be the presence of a high concentration of iron in these nuclei or the production of neuromelanin from the metabolism of dopamine.

References

1. Chang, H. T., W. K. Rumbeiha, et al. (2011) “Toxic Equine Parkinsonism: An Immunohistochemical Study of 10 Horses With Nigropallidal Encephalomalacia.” Vet Pathol. e-publication
2. Cordy, D. R. (1954). “Nigropallidal encephalomalacia in horses associated with ingestion of yellow star thistle.” J Neuropathol Exp Neurol 13(2): 330-42.
3. Ferrel, E. A., P. R. Gavin, et al. (2002). “Magnetic resonance for evaluation of neurologic disease in 12 horses.” Vet Radiol Ultrasound 43(6): 510-16.
4. Fowler, M. E. (1965). “Nigropallidal encephalomalacia in the horse.” J Am Vet Med Assoc 147(6): 607-16.
5. Sanders, S. G., R. L. Tucker, et al. (2001). “Magnetic resonance imaging features of equine nigropallidal encephalomalacia.” Vet Radiol Ultrasound 42(4): 291-6.