Dipopia refers to a clinical situation where the visual axis of both eyes are not in alignment, the patient experiences double vision (diplopia).
While disorders of ocular movement include Nuclear and infranuclear lesions causing isolated paralysis of one or more extraocular muscles.
Diplopia When the visual axes of both the eyes are not in alignment, the patient experiences double vision (diplopia).
Study of the relative position of the images from the two eyes helps to determine the paretic muscle.
The image arising from the normal eye is bright (true image) and that from the paralyzed eye is faint (false image).
Ask the patient to look at the examiners finger which is moved in all directions.
He is then asked to indicate the position of the finger when there is maximum separation of the two images and also when the double vision disappears.
Diplopia is maximum when the eye is moved in the direction of action of the paretic muscle.
Thus in a right lateral rectus palsy, there is maximum separation of the images on looking to the right, while the diplopia disappears on looking to the left.
By placing a red glass in front of one eye, the source of the false image could be identified.
Disorders of ocular movements Nuclear and infra-nuclear lesions cause isolated paralysis of one or move extra-ocular muscles.
Involvement of extra-ocular muscles sparing the pupillary sphincters is called external Ophthalmoplegia.
In total Ophthalmoplegia all the muscles are paralyzed.
Weakness of ocular muscles due to a nuclear lesion is identified by the associated brain stem signs like involvement of other cranisl nerve nuclei and he long tracts.
Infra-nuclear ocular palsy may or may not be solitary.
The common causes include tumors of the base of the brain, injuries to the base of the skull, anurysms of circle of Willis, ischemic infarction of the nerve as in diabetes and basal meningitis, especially tubercular.
LEss common causes include Ophthalmoplegic migraine, herpes Zoster, subdural hematoma, tempral arteritis and sarcoidosis.
Myasthenia gravis has to be excluded in all cases of acute ocular palsy.
In about 20-30% of cases, the exact cause cannot be determined and usually the palsy disappears in a few weeks (idiopathic Ophthalmoplegia).
In the third nerve lesions caused by external compression by aneuryms, tumor or temporal lobe hernation, pupillary dilatation is an early sign since the pupillary fibers are peripheral.
In contrast, in cases of infarction of the third nerve, that may occur in diabetes, the pupil is spared since infarction involves more of the central portion of the nerve.
Supranuclear and internuclear lesions Cerebral lesions involving the frontal eye fields may be irritative as in seizures or paralytic as in an infarct or tumor.
Irritative lesions produce conjugate deviation of both eyes to the opposite side.
In paralytic lesions, there is paresis of conjugate deviation to the opposite side and hence the eyes will be pointing to the side of lesion.
However, this is a transitory phenomenon.
In lesions in the brain stem involving the parapontine gaze center, the eyes will be deviated to opposite side.
A lesion of the medial longitudinal fasciculus that interrupts the impulses from the center for lateral gaze in the lower pons to the third nerve nucleus in the mid-brain causes a dissociation of conjugate eye movements.
This is called internuclear ophthalmoplegia.
Presence of internuclear Ophthalmoplegia indicates usually an intrinsic brain stem Lesion-like multiple sclerosis, glioma or infarct of the brain stem.
Nystagmus This refers to involuntary rhythmic movements of the eyes.
This may be jerky or pendular.
Jerky nystagmus is characterized by alternate quick and slow movements.
In pendular nystagmus, the oscillations are almost equal in rate in both directions.
While testing for nystagmus, the eyes should be examined first int he central position, and then during upward, downward and lateral movements.
A complete description of nystagmus should include the type, direction, rate, amplitude, duration, intensity, and relationship of the response to movements of the eyes, head and body.
Sometimes, only a few irregular jerks without any sustained rhythmic movements may be observed.
These are nystagmoid movements and they may not indicate any pathology.
Also, while testing, extreme lateral gaze beyond the field of vision should be avoided.
Jerky nystagmus may be horizontal, vertical or rotary.
The direction of nystagmus is named according to the direction of the fast component.
In cerebellar lesions, the nystagmus is openly transient and may have a rotary component, as seen in brain stem lesions.
Pendular nystagmus is usually seen in congenital blindness or people working in darkness as in mines (miner's nystagmus).