A lesion in this area would involve efferent pupillary fibres on the dorsal aspect of the Edinger-Westphal nucleus (associated with the response to light) while sparing the fibres associated with the response to near, which lie slightly more ventrally. The exact relationship between syphilis and the two types of pupils (AR pupils and tonic pupils) is not known at the present time. The older literature on AR pupils did not report the details of pupillary constriction (brisk vs. tonic) that are necessary to distinguish AR pupils from tonic pupils. Tonic pupils can occur in neurosyphilis. It is not known whether neurosyphilis itself (infection by Treponema pallidum) can cause tonic pupils, or whether tonic pupils in syphilis simply reflect a coexisting peripheral neuropathy. Thompson and Kardon summarize the present view:

The evidence supports a midbrain cause of the AR pupil, provided one follows Loewenfeld’s definition of the AR pupil as small pupils that react very poorly to light and yet seem to retain a normal pupillary near response that is definitely not tonic.To settle the question of whether the AR pupil is of central or peripheral origin, it will be necessary to perform iris transillumination (or a magnified slit-lamp examination) in a substantial  number of patients who have a pupillary light-near dissociation (with and without tonicity of the near reaction), perhaps in many parts of the world. Parinaud syndrome
A third cause of light-near dissociation is Parinaud syndrome, also called dorsal midbrain syndrome.
This uncommon syndrome involves vertical gaze palsy associated with pupils that “accommodate but do not react." The causes of Parinaud syndrome include brain tumors (pinealomas), multiple sclerosis and brainstem infarction. Due to the lack of detail in the older literature and the scarcity of AR pupils at the present time, it is not known whether syphilis can cause Parinaud syndrome. It is not known whether AR pupils are any different from the pupils seen in other dorsal midbrain lesions. The condition is diagnosed clinically by a physician. Treatment
There is no definite treatment, but, because syphilis may be an underlying cause, it should be treated. However, because this sign is associated with neurosyphilis, it should be treated with crystalline penicillin 24 mU intravenous per day for 10 to 14 days. If the patient is allergic to penicillin, they should undergo desensitization and then be treated. History
Argyll Robertson pupils were named after Douglas Argyll Robertson (1837–1909), a Scottish ophthalmologist and surgeon who described the condition in the mid-1860s in the context of neurosyphilis. In the early 20th century, William John Adie described a second type of pupil that could "accommodate but not react". Adies tonic pupil is usually associated with a benign peripheral neuropathy (Adie syndrome), not with syphilis.When penicillin became widely available in the 1940s, the prevalence of AR pupils (which develop only after decades of untreated infection) decreased dramatically. AR pupils are now quite rare. A patient whose pupil “accommodates but does not react” almost always has a tonic pupil, not an AR pupil.