Argyll-Robertson Sign – Function, Task & Diseases
Argyll-Robertson sign
The Argyll-Robertson sign is a reflex pupillary rigidity with intact near accommodation of the eyes . A midbrain lesion eliminates the ability to react to light in one or both eyes. This phenomenon plays a role in diseases such as neurosyphilis .
What is the Argyll-Robertson sign?
The part of the brainstem between the pons (pons) and the diencephalon (diencephalon) is called the midbrain . In this part of the brain, the eye muscles in particular are controlled.
The midbrain is part of the so-called extrapyramidal system , which is not always 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 . The 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 structured in three layers. Between the mesencephalic roof (tectum mesencephali) and the tegmentum is the so-called cerebrospinal fluid canal, which is filled with liquor.
The Argyll-Robertson sign is an indication of a cerebral disorder in the midbrain, which manifests itself in a reflex pupillary rigidity. The pathological phenomenon was named after the Scottish ophthalmologist D. Argyll Robertson, who first described it in the 19th century.
Function & task
The eyes are able to adapt to light conditions in the visual field. This adaptation is also called adaptation. The most important movements in this context are the pupillary light reflexes . The iris delimits the pupil. The pupillary light reflexes result from a change in tone in the smooth muscles of the iris. This change in the iris changes the pupil size and thus adjusts the pupil to the relative amount of light coming in. These processes can be compared to adjusting the aperture on a camera.The iris muscles involved are the dilatator pupillae muscle and the sphincter pupillae muscle. The dilatator pupillae muscle is also called the pupil dilator. It is communicated to the nervous system by sympathetic nerve fibers that originate from the ciliospinal center and thus from the C8 to Th3 spinal cord segments. When the pupils are unnaturally dilated by this muscle or independently of light stimuli, this is referred to as mydriasis .
The sphincter pupillae muscle is also called the pupil constrictor. It is not innervated by sympathetic, but by parasympathetic nerve fibers from the third cranial nerve (Nervus oculomotorius). The fibers originate from the Edinger-Westphal nucleus and run via the ciliary ganglion. These regions are activated when there is particularly strong incidence of light and constricts the pupils. A pathological narrowing is called miosis . The incidence of light is regulated by these muscles and nerves at the pupil. An external stimulus thus causes a muscle contraction and thus adapts the eye to a sudden change in brightness.
The reflex chain is subject to a well-coordinated interconnection. The afferent nerve tracts of the central nervous system are also called afferents. They are the first place of the eye reflexes. Increased incidence of light is registered by the light-sensitive sensory cells of the retina . These photoreceptors conduct the information via the sensory optic nerve (Nervus opticus) and the Tractus opticus to the epithalamus , where they reach the nuclei pretectales. From these nuclei emanate efferents that conduct information back out of the central nervous system.
In this way, information about the brightness is conveyed to the Edinger-Westphal nuclei via efferent pathways. In the nuclei, the information is switched to the parasympathetic part of the oculomotor nerve. They migrate via the ciliary ganglion and thus stimulate the sphincter pupillae muscle to contract. This narrows the pupil.
Each eye has a connection to both pretectal nuclei. Therefore, a pupillary reflex is always performed bilaterally, even when only one side is illuminated.
Diseases & Ailments
The Argyll-Robertson sign plays a role above all for the neurologist . This is the loss of the described direct and indirect pupillary light response. The doctor checks the reflex pupil adaptation using a lamp as part of the neurological examination.The Argyll-Robertson sign is a bilateral disorder and, after exposure to light, manifests itself in narrow, unrounded pupils on both sides, which no longer react or only react poorly. Since the convergence response of the eye is intact, the pupils nonetheless constrict during near accommodation. The Argyll-Robertson sign is present when only the light pupil reflexes but not the near-accommodation processes are eliminated. The convergence reaction of the eye is preserved, which means that the eye is still able to adapt when fixing objects.
This convergence response is mediated via the oculomotor nerve. This excludes cranial nerve damage as the cause of the Argyll-Robertson phenomenon and the doctor suspects midbrain lesions. The connection between the Edinger-Westphal nucleus and the nucleus preectalis olivaris is probably affected by damage.
The causal relationships are often lesions of a neurosyphilis. This is a progressive form of syphilis . The infectious disease spreads to the central nervous system and, in addition to paralysis , can cause failures of the cranial nerves and spinal degeneration. The Argyll-Robertson sign is usually associated with a late stage of neurosyphilis and is considered one of the most important indicators of this disease.
However, midbrain lesions and the phenomenon of rigid pupils do not necessarily have to be associated with syphilis. Multiple sclerosis and other neurological diseases, for example, can also cause brain lesions in the midbrain. The further clinical picture can be extremely diverse, depending on the overall affected brain region.
Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.