Limbic Involvement May Occur in Cluster Headache

To the Editor.  —Herpes simplex is known to exist in a latent form in a high proportion of trigeminal ganglia obtained at routine autopsy.¹ Therefore, the hypothesis that herpes simplex in the hippocampus may be derived from the trigeminal ganglion is interesting.² In support of this concept, Dr Ball quotes an article in which Dr Rose and myself described the close temporal association of herpes labialis involving the mandibular division of the fifth nerve and ipsilateral cluster headache, a syndrome consistent with involvement of the ophthalmic division of the same nerve.³ In that article, we suggested that herpesvirus in the trigeminal ganglion may activate local axonal reflexes and trigger release from mast cells, known to exist in greater numbers on the side of the headache,⁴ thereby causing ipsilateral extracerebral vasodilatation, which is a feature of cluster headache. Initially it appeared to us that the pathology in