Herpes zoster most commonly involves:

Ref: Harrison's l8th editionExplanation:The clinical syndrome caused by the reactivation of VZV from sensory ganglia is herpes zoster.The commonly involved dermatomes are:The thoracic dermatomes, especially T4 to T12. are involved in about 50% of casesThe lumbosacral dermatomes in about 16%The cranial nerves in 14 to 20% ot patientso Opthalmic division of trigeminal in 10 to 15 percent ando Other cranial nerves causing 4 to 5 percentThere are two rather characteristic cranial herpetic syndromes are ophthalmic herpes and geniculate herpes.In ophthalmic herpes, which accounts for 10 to 15 percent of all cases of zoster, the pain and rash are in the distribution of the first division of the trigeminal nerve, and the pathologic changes are centered in the gasserian ganglion. The main hazard in this form of the disease is herpetic involvement of the cornea and conjunctiva, resulting in corneal anesthesia and residual scarring. Palsies of extraocular muscles, ptosis, and mydriasis are frequently associated, indicating that the third, fourth, and sixth crania] nerves are affected in addition to the gasserian ganglion.The less common but also characteristic cranial nerve syndrome called Ramsay Hunt syndrome - consists of a facial palsy in combination with a herpetic eruption of the external auditory meatus, sometimes with tinnitus, vertigo, and deafness. This is due to the reactivation of herpes from the geniculate ganglion.