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In preganglionic Horner's syndrome, the anhidrosis—loss of sweating—affects the ipsilateral side of the face. This occurs due to the disruption of sympathetic nerve pathways that typically supply the sweat glands. Since the condition results from an interruption of the sympathetic fibers before they synapse in the sympathetic ganglion, the entire face on the same side (ipsilateral) experiences loss of sweating, not just a certain area.

In this context, while other areas may assist in identifying different aspects of Horner's syndrome, the critical aspect of this condition is that the sympathetic supply, which is responsible for sweat production, is affected on the same side of the lesion. This results in anhidrosis on the ipsilateral portion of the face, leading to the patient experiencing reduced or absent sweating in that entire area. Understanding this characteristic helps differentiate Horner's syndrome based on whether it’s preganglionic or postganglionic involvement, but regardless, the implications around sweating remain consistent in the context of the syndrome's presentation.