The article discusses the issues of the normal anatomy of the autonomic innervation of the eye muscles, mainly its sympathetic part. The symptoms included in the Claude Bernard Horner's syndrome, which occurs when the sympathetic pupillomotor pathway is affected, are considered. The causes of this syndrome are numerous. The classification of acquired conditions the clinical picture of which involves Horner syndrome is given. Three classes of etiological processes are distinguished: central (suprasegmental and segmental), peripheral, and idiopathic. Nosological components are listed for each of these. The complexity of the topographic anatomy of the sympathetic pathways innervating the eye muscles leads to significant difficulties in diagnosing the level of their lesion. Pathological processes affecting the peripheral part of the sympathetic chain are detected much faster, as the causes that provoked the development of Horner syndrome are often more obvious and diagnostically accessible. Cases of Horner syndrome of central genesis are significantly more difficult in diagnostic terms. The ar ticle discusses additional symptoms that form the clinical picture for stem strokes, tumors of the hypothalamic-pituitary region and brain stem, multiple sclerosis, and various diseases of the spinal cord, where the Claude Bernard Horner's syndrome is part of the clinical picture. In such cases, the analysis of neurological symptoms provides the basis for niveau diagnosis. The most difficult situations are when the only reason for the patient's visit and the objective component of the neurological status is just a triad of symptoms: miosis, partial ptosis, and enophthalmos. The paper presents an example of a similar case from clinical practice, when a patient with an acutely developed isolated left-sided symptom complex, including miosis, ptosis, and enophthalmos, had a focus of lacunar strok e in the lef t thalamo-hypothalamic region.