Argyll-Robertson sign is a reflex pupillary rigidity with intact near accommodation of the eyes. In this case, a midbrain lesion abolishes the light responsiveness of one or both eyes. This phenomenon plays a role in disorders such as neurolues.
What is the Argyll-Robertson sign?
The Argyll-Robertson sign is an indication of cerebral dysfunction in the midbrain, which manifests as reflex pupillary rigidity. The midbrain is the brainstem portion between the bridge (pons) and the diencephalon. This area of the brain primarily controls the eye muscles. The midbrain belongs to the so-called extrapyramidal system, which cannot always be clearly separated from the pyramidal system of movement control. The extrapyramidal system is a neurophysiological concept for all movement control processes outside the pyramidal tracts in the spinal cord. Excitations of the sensitive midbrain nerves are transmitted from the diencephalon to the cerebrum (telencephalon), where they are switched to motor nerves. The midbrain is divided into three layers. Between the midbrain roof (tectum mesencephali) and the tegmentum lies the so-called cerebrospinal fluid canal, which is filled with cerebrospinal fluid. The Argyll-Robertson sign is an indication of cerebral dysfunction in the midbrain, which manifests as reflex pupillary rigidity. The pathologic phenomenon was named after the Scottish ophthalmologist D. Argyll Robertson, who first described it in the 19th century.
Function and task
The eyes are capable of adaptation to light conditions in the visual field. This adaptation is also called adaptation. The most significant movements in this context are the pupillary light reflexes. The iris bounds the pupil. The pupillary light reflexes result based on a change in tone in the iris smooth muscles. This change in iris tone alters the pupil width, thus adjusting the pupils to the relative amount of incident light. These processes are comparable to the regulation of aperture width on a camera. The iris muscles involved are the dilatator pupillae muscle and the sphincter pupillae muscle. The musculus dilatator pupillae is also called pupil dilator. It is attached to the nervous system by sympathetic nerve fibers originating from the centrum ciliospinale and thus from the spinal cord segments C8 to Th3. If the pupils are dilated unnaturally by this muscle or independently of light stimuli, it is called mydriasis. The sphincter pupillae muscle is also called pupil constrictor. It is innervated not by sympathetic but by parasympathetic nerve fibers from the third cranial nerve (oculomotor nerve). The fibers originate from the nucleus Edinger-Westphal and run via the ciliary ganglion. Activation of these regions occurs during particularly strong light incidence and constricts the pupils. Pathological constriction is called miosis. The incidence of light is reflexively regulated at the pupil by these muscles and nerves. Thus, an external stimulus causes a muscle contraction, adapting the eye to a sudden change in brightness. The reflex chain is subject to a perfectly coordinated circuitry. The afferents of the central nervous system are also called afferents. They are the first point of the eye reflexes. Increased light incidence is registered by the light-sensitive sensory cells of the retina. These photoreceptors conduct the information via the sensitive optic nerve and the optic tract into the epithalamus, where it reaches the nuclei praetectales. Efferents emanate from these nuclei, which conduct information back out of the central nervous system. In this way, information about brightness is conducted via efferent pathways into the Edinger-Westphal nuclei. In the nuclei, the information is switched to the parasympathetic part of the oculomotor nerve. They travel across the ciliary ganglion and thus stimulate the sphincter pupillae muscle to contract. The pupil is constricted as a result. There is a connection from each eye to both pretectal nuclei. Therefore, a pupillary reflex is always performed bilaterally, even when only one side is illuminated.
Diseases and disorders
The Argyll-Robertson sign plays a role especially for the neurologist. It is the loss of the direct and indirect pupillary light response described above. The physician checks for reflex pupillary adaptation using a light as part of the neurologic examination.The Argyll-Robertson sign is a bilateral disorder and manifests itself after light irradiation in laterally constricted, rounded pupils that no longer react or react poorly. Since the convergence response of the eye is intact, the pupils nonetheless constrict during near accommodation. Thus, if only the light pupillary reflexes are abolished, but not the near accommodation processes, the Argyll-Robertson sign is present. The convergence response of the eye is preserved, which means that the eye is still capable of adaptation during fixation of objects. This convergence response is mediated by the oculomotor nerve. This rules out cranial nerve damage as the cause of the Argyll-Robertson phenomenon, and the physician’s suspicion falls on midbrain lesions. Presumably, the connection between the Edinger-Westphal nucleus and the nucleus praetectalis olivaris is affected by damage. Often the causative connections are lesions of a neurolues. This is a progressive form of syphilis. The infectious disease spreads to the central nervous system and can cause paralysis of the cranial nerves and spinal degeneration. The Argyll-Robertson sign is generally associated with a late stage of neurolues and is evaluated as one of the most important indicators of this disease. However, midbrain lesions and the phenomenon of pupillary rigidity do not necessarily have to be associated with syphilis. Multiple sclerosis and other neurological diseases, for example, can also cause midbrain lesions. The further clinical picture can be extremely diverse depending on the overall affected brain region.
