Thursday, 5 December 2013

Disorder of eye

Macular Degenerations
Gradual, Painless bilateral central visual loss in elderly
* Two forms
- exudative
- non exudative
* Non exudative – Drusen.
* Exudative – Metamorphopsia.
* Associated with gene for complement factor H, an inhibitor of the alternative complement pathway.
*Treatment with vitamins C and E, beta carotene, and zinc.
*Age-related macular degeneration-yellow subretinal deposits.
* Optical coherence tomography, a new technique for acquiring images of the retina in cross-section.
* Treatment- Either bevacizumab or ranibizumab is administered into the vitreous cavity- vascular endothelial growth factor antagonist.
* Leber's congenital amaurosis, a rare cone dystrophy, has been treated by replacement of the missing RPE65 protein through gene therapy.
A "One-and-a-Half" syndrome
*Characterized by a lateral gaze palsy in one direction with an internuclear ophthalmoplegia (INO) in the other direction.
* Due to a unilateral lesion in the lower part of the dorsal pontine tegmentum affecting the ipsilateral paramedian pontine reticular formation (PPRF), the abducens nucleus, and internuclear fibers of the ipsilateral medial longitudinal fasciculus (MLF).
*Brainstem infarction was the most common cause in elderly adults.
*MS was the most common cause in 14 young adults.
*The most common associated cranial nerve lesions - ipsilateral trigeminal sensory loss and a peripheral-type facial palsy.
*The one-had-a-half syndrome is characterized by, on horizontal gaze:
1. An ipsilateral gaze paresis or palsy;
2. An INO on contralateral gaze; and
3. At rest, an exotropia of the eye contralateral to the lesion in the acute phase
*The association of exotropia in the one-and-a-half syndrome - termed "paralytic pontine exotropia"-acute ipsilateral involvement of the PPRF(Paramedial Pontine Reticular Formation)
The Clinical Features of an INO are:
1. Medial rectus muscle weakness ipsilateral to the side of the lesion with paresis of adduction or adduction lag
2. Abducting nystagmus of the eye contralateral to the lesion—dissociated nystag-mus
3. Normal convergence
4. Skew deviation—hypertropia on the side of the lesion
5. Dissociated vertical nystagmus—downbeat with greater torsional component in the contralateral eye.
*Oscillopsia, an illusion of movement of the visual world, is a common presenting symptom of INO.
*Horizontal oscillopsia - from either the adduction lag or the abducting nystagmus.
*Vertical oscillopsia -during head movements and is caused by a deficient vertical VOR.
The medial longitudinal fasciculus (MLF)
*Major pathway in the brainstem extending from the pons up to the midbrain. The MLF carries signals for the control of horizontal eye movements.
For horizontal gaze:
1. The MLF contains axons from the abducens internuclear neurons and carries signals for
horizontal saccades, the vestibulo-ocular reflex (VOR), and smooth pursuit.
For vertical gaze:
1. The MLF contains axons from the rostral interstitial MLF (riMLF), which carry vertical saccadic signals.
2. The MLF also contains ascending axons from the vestibular nuclei, which carry signals for the vertical VOR, smooth pursuit, gaze holding, and otolith-ocular reflex.
3. Axons project to the oculomotor and trochlear (fourth nerve) nuclei, as well as to the interstitial nucleus of Cajal.
Ocular Flutter
*Oscillations is usually high- 10–25 cycles per second
1. Paraneoplastic opsoclonus/flutter due to an occult neoplasm
2. MS
3. Drugs: lithium, amitriptyline, cocaine, and phenytoin with diazepam
4. Toxins: chlordecone, thallium, strychnine, toluene, and organophosphates
5. Complication of pregnancy
*Caused by loss of "pause" neuronal inhibition of burst neuron function in the paramedian pontine reticular formation (PPRF).
*Glycine channelopathy affecting the membrane of omnipause cells - the underlying mechanism for ocular flutter.
*Opsoclonus-myoclonus syndrome (OMS) - trunkal ataxia, and myoclonic jerks of the neck and limbs-middle to late stages of pregnancy.
*Antineural antibodies associated with opsoclonus -Anti-Ri, Anti-Hu, Anti-Yo, Anti-Mal, and Anti-amphiphysin antibodies.
Periodic Alternating Nystagmus
*Partway through the period of downbeat nystagmus, the direction of the nystagmus changes to a spontaneous horizontal jerk nystagmus present in central gaze, which reverses direction approximately every 2 min; this is diagnostic of PAN.
*Due to a lesion of the cerebellar nodulus and uvula.
*Cause-Multiple sclerosis.
*An effective treatment - drug baclofen.
*Thymic hyperplasia - 65–70%.
*Thymoma - 5–20% of myasthenic patients. Patients with thymoma have more severe disease, higher serum titers of AChR antibodies, and more severe abnormalities on EMG.
*The number of quanta of acetylcholine released by each nerve stimulus is normal, but the effect of each quantum on its receptor is reduced.
Electrophysiological studies
Stimulated Single-Fiber EMG (SF-EMG)
*Performed in the right digitorum communis.
*More sensitive electrophysiologic method for the diagnosis of myasthenia.
*Jitter is the variability of the interpotential interval of successive discharges of two single muscle fibers belonging to the same motor unit.
Repetitive stimulation
*Repetitive stimuli are delivered at a rate of 3 stimuli per second, with 6–10 stimuli in a train.
* The compound muscle action potential of the first response is compared with the fifth response and the percentage decrement measured.
*A decrement of greater than 10% - positive test for myasthenia gravis (MG).
Hering's Law of Equal Eyelid Innervation:
Unilateral ptosis and contralateral lid retraction demonstrate Hering's law of equal eyelid innervation. Thus, when the ptotic lid is manually raised, the contralateral lid falls to a normal position since a large innervation is no longer required.
Ptosis is defined as the lid covering more than 2 mm of the cornea.

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