Causes of Nystagmus – the rhythmic movement of the eyes
Nystagmus is a rhythmic oscillatory movement of the eyeballs , which normally occurs after vestibular stimulation or abnormal in a wide variety of diseases. The blindness caused by earlier diseases of visual pathways produces a complex nystagmus, of search with pendulum oscillations (sinusoidal ) which are irregular and saccadic. This nystagmus is usually called congenital nystagmus, misnomer, since nystagmus is frequently acquired in the early childhood. Sometimes it develops at a perfectly normal child.
Saccadic nystagmus is characterized by a slow removal from the target, followed by a rapidly corrected secusa (fast motion). By convention, nystagmus is named after the rapid phase. Saccadic nystagmus can be oriented superior, inferior, horizontally (left or right) and torsional. The characteristics may vary by eyesight position.
Many patients are not aware of the existence of nystagmus. Others will report a subjective movement of go forth of the environment (oscillopsia), corresponding to nystagmus .
Fine nystagmus may be difficult to identify at normal eyes examination. Observing the nistagmoide movements of the optic disc at ophthalmoscopy is a sensitive method to detect subtle degrees of nystagmus. Slit lamp is also useful.
Nystagmus evoked by sight fixation is the most common form of saccadic nystagmus. When the eyes are held in an eccentric position of the orbits, they have a normal tendency to return to primary position. The subject compensates making a correction secusa to maintain the deviated eye position .
Many normal patients present moderate nystagmus evoked by fixation of the eyesight. Exaggerated nystagmus evoked by fixation of eyesight can be induced by drugs (sedatives, anticonvulsants, alcohol), muscle paresis, myasthenia, demyelinating diseases and injuries of brainstem and cerebellum.
Vestibular nystagmus occurs due to labyrinthine dysfunction (Meniere’s disease), vestibular nerve or vestibular nucleus dysfunction from the brainstem. Peripheral vestibular nystagmus occurs frequently in discrete attacks with symptoms of nausea and vertigo. It can be associated tinnitus and hearing loss. Abrupt change of head position can cause or exacerbate the symptoms.
Nystagmus oriented downwards occurs consecutive to neighboring lesions of craniocervical junction (Chiari malformation, basilar intussusception). It was also reported in the brainstem or cerebellum stroke, lithium or anticonvulsant intoxication and multiple sclerosis. Upward oriented nystagmus is associated with damage of the pontin tegmentum, demyelinating diseases or tumors.