Nerve palsy

• Paralysis of the lateral rectus muscle
• Cranial Nerve Paralysis VI

Dysfunction of the nerve is caused by esotropia , a convergent strabismus at a distance of fixation. On fixation close to the affected person can only have a latent deviation and be able to maintain binocularity or have a smaller esotropy. Patients sometimes take a turn of the face towards the affected eye, moving the eye from the field of action of the affected lateral rectus muscle, in order to control diplopia and maintain binocular vision.

Diplopia is usually experienced by adults with abdominal nerve palsy, but in children in this condition, diplopia may not occur due to suppression. ^[4] Neuroplasticity is present in childhood and allows the child to "turn off" information coming from one eye, thereby reducing any symptoms of diplopia. Although this is a positive adaptation in the short term, in the long term it can lead to a lack of proper development of the visual cortex, resulting in permanent loss of vision in the suppressed eye; a condition known as amblyopia .

As the nerve exits in the lower part of the brain , it often contracts first with increased intracranial pressure . Different representations of the condition or association with other conditions can help localize the site of damage along the path of the abduction nerve.

General etiology

The most common causes of abdominal nerve palsy in adults are:

• More common: vasculopathy ( diabetes mellitus , arterial hypertension , atherosclerosis ), trauma , idiopathy.
• Less common: Increased intracranial pressure , giant cell arteritis, cavernous sinus (e.g., meningioma , glioblastoma, brain stem aneurysm , metastases ), multiple sclerosis , sarcoidosis / vasculitis , lumbar puncture , stroke (usually not isolated), cari, hydrocephalus, intracranial hypertension

In children, Harley ^[5] reports typical etiologies of traumatic, neoplastic (most often glioma brain stem), as well as idiopathic nature. Paralysis of the abducent nerve causes deviation of the eyes inward (see the pathophysiology of strabismus). Valle et al. ^[6] report that benign and rapidly recovering, isolated abdominal nerve palsy can occur in childhood, sometimes from infections of the ear, nose, and throat. ^[7]

Pathophysiology

The pathophysiological mechanism of paralysis of the abducent nerve with increased intracranial pressure has traditionally been considered to be a stretching of the nerve in its long intracranial path, or pressing by a ligament of the temporal bone or rib of the temporal bone. Collier, however, “could not accept this explanation”, in his opinion, since the abducent nerve exits directly from the front of the brain stem, while other cranial nerves exit at an angle or across, it is more prone to mechanical effects of crowding out of the intracranial space engaged in damage to the back of the brain stem. (J Neurol Neurosurg Psychiatry 2003; 74: 415-418)

Signs of localization

1. The brain stem

Isolated lesions of the nucleus of the abduction nerve will not lead to isolated paralysis of the abduction nerve, since the formation of paramedian bridge reticular fibers passes through the nucleus of the opposite oculomotor nerve. Thus, nuclear damage will lead to paralysis of the ipsilateral gaze. In addition, the fibers of the facial cranial nerve are wrapped around the nucleus, and if they are also affected, paralysis of the abduction nerve will lead to ipsilateral paralysis of the facial nerve. In Millard Gubler's syndrome, unilateral softening of brain tissue resulting from obstruction of the blood vessels of the bridge involving the abdominal and facial cranial nerves and the corticospinal tract, abdominal paralysis and paresis of the ipsilateral facial nerve occur with contralateral hemiparesis. ^[8] Foville syndrome can also result from stem lesions that affect the trigeminal, facial, and abducent cranial nerves.

2. Subarachnoid space

Since the abduction nerve passes through the subarachnoid space , it is located next to the anterior lower and posterior lower cerebellar and basilar arteries and, therefore, is vulnerable to compression opposite the slope of the skull. Typically, paralysis in this case will be associated with signs and symptoms of headache and / or increased intracranial pressure.

3. Rocky top

The nerve passes near the mastoid sinus and is vulnerable to mastoiditis , which leads to inflammation of the meninges, which can lead to Gradenigo syndrome. This condition leads to paralysis of the abducent nerve with corresponding ipsilateral hearing loss, as well as paralyzing facial pain and photophobia . Similar symptoms can occur a second time due to fractures of the stony part or tumors of the nasopharynx.

4. Cavernous sinus

The nerve extends into the sinuses of the body adjacent to the internal carotid artery and the oculosympathetic fibers responsible for controlling the pupil , so lesions here may be associated with pupil dysfunctions, such as Horner's syndrome . In addition, the participation of the oculomotor, block, V1 and V2 trigeminal nerve may also indicate a sinus lesion, since all go to the sinuses of the orbit wall. Lesions in this area can occur as a result of vascular problems, inflammation, metastases and primary meningiomas .

5. Orbit

Of course, the abducent nerve is short and lesions in the orbit rarely lead to isolated paralysis of the abducent nerve, but it is more typical to include one or more of the other groups of intraocular muscles in the symptoms.

Differential diagnosis is not difficult in adults. The onset of the disease is usually sudden with symptoms of horizontal diplopia . Restrictions on eye movement are associated with abduction of the injured eye (or abduction of both eyes, if bilateral) and the size of the resulting esotropy is always greater on remote fixation - where the lateral rectus muscle is more active - than on close fixation - where the medial rectus dominates. Limitations of abduction that mimic abdominal nerve palsy may be secondary to surgery, trauma, or the result of other conditions, such as myasthenia gravis or thyroid disease.

In children, differential diagnosis is more difficult due to problems associated with the fact that it is difficult to work with children to fully investigate eye movement. As an alternative, diagnosis of abduction deficiency is possible, which will include:

. 1 Mobius syndrome is a rare congenital disorder in which both the abdominal and facial nerves are bilaterally affected, leading to a typically “inexpressive” face.

2. Dwayne syndrome - a condition in which both abduction and adduction suffer as a result of the partial innervation of the lateral rectus muscle from the oculomotor cranial nerve.

3. Cross-fixation, which develops in the presence of infantile esotropia or nystagmus block syndrome and leads to weakness of the lateral rectus muscle.

4. Iatrogenic injuries. The paralysis of the abducent nerve is known to occur from the placement of the halo of the orthoses . As a result, paralysis is identified through a loss of lateral gaze after installation of the orthosis and is the most common traumatic brain injury of a nerve associated with this device. ^[9]

The first goals of management should be to identify the cause of the disease and treatment, where possible, or to alleviate the symptoms of the patient, where they are present. In children who rarely pay attention to diplopia , the goal will be to maintain binocularity of vision and thus promote proper visual development.

After this, a follow-up period of 9 to 12 months is needed without further intervention, as some paralysis can be restored without surgery.

Relief of symptoms and / or maintenance of binocular vision

This is usually achieved through the use of Fresnel prisms . These thin flexible plastic prisms can be attached to the patient’s glasses, or glasses with simple glasses, if the patient does not have refractive errors, and serve to compensate for misalignment inside the affected eye. Unfortunately, the prism is correct within a certain degree of bias, and since the degree of misalignment of the affected individual will vary depending on the direction of the gaze, they can still experience diplopia when looking at the affected side. Prisms are available with different angles and must be selected for a particular patient. However, in patients with large deviations, the thickness of the required prism can reduce vision so that binocularity is unattainable. In such cases, it may be more appropriate to simply temporarily close one eye. Occlusion should never be used for children, firstly because of the risk of inducing the amblyopia stimulus and secondly because they do not experience diplopia.

Other management options at this initial stage include the use of botulinum toxin , which is injected into the ipsilateral medial rectus muscle. Using BT serves a number of purposes. Firstly, it helps to prevent contractures of the medial rectus muscle, which can arise as a result of resistance to its action for a long period. Secondly, by reducing the size of the deviation, it is possible to temporarily use a prismatic correction, the use of which was impossible earlier and, thirdly, to remove the medial rectus muscle traction, can be used to determine partial paralysis or complete, by allowing any movement of the lateral rectus muscle. Thus, the toxin works both therapeutically, helping to reduce symptoms and improve prospects for more complete eye movements after surgery, and diagnostically, helping to determine the type of surgery that is most suitable for each patient.

Long-Term Management

Where complete recovery has not occurred within 9 to 12 months of follow-up, management will be either a “conservative” or a course of surgery.

1. Conservative treatment If residual esotropia is small and there is a risk of surgical hypercorrection or when the patient is unsuitable or unwilling to have surgery, prisms can be mounted in his glasses to provide more stable symptom relief. Where the deviation is too large for an effective prismatic correction, permanent occlusion may be the only option for unsuitable or unwilling patients.

2. Surgical

The choice of procedure will depend on the degree of residual function in the affected lateral rectus muscle. In the case of complete paralysis, the preferred option is to perform a transposition of the vertical muscle, such as the Jensen, Hummelheim or full transposition of all muscles, in order to use the functioning of the lower and upper rectus muscles to achieve at least some degree of abduction. ^{[10] [11] [12]} As an alternative, and less satisfactory, approach, it is possible to operate both the lateral and medial rectus muscles of the affected eye, in order to stabilize it in the midline, thereby giving a single vision right in front, but with diplopia at the left and right glance. This procedure is rarely used, but may be appropriate for people with complete paralysis who, due to other health problems, are at increased risk for anterior segment ischemia associated with complex multimuscular transposition procedures.

If some functions remain in the damaged eye, the preferred procedure depends on the degree of development of muscle complications. In case of abnormal nerve palsy, one would expect during the observation period from 9 to 12 months that most patients will show the following picture of changes in their ocular muscular actions: firstly, the overactivity of the medial rectus muscle of the affected eye, then the overactivity of the medial rectus muscle of the opposite eye and, finally, the weak activity of the lateral rectus muscle of the unaffected eye is what is known as delayed paralysis. These changes serve to reduce variations in the distortion of the eyes in different positions of the gaze. Where this process is fully carried out, the preferred option is a simple recession, or weakening of the medial rectus muscle of the affected eye in combination with resection of the lateral rectus muscle of the same eye. However, where delayed paralysis of the opposite lateral rectus muscle has not developed, there will still be a mismatch between the positions of the eyes, more noticeable in the field of action of the affected muscle. In such cases, the recession of the medial rectus muscle of the affected eye is accompanied by recession and / or withering of the opposite medial rectus muscle.

The same approaches are suitable for bilateral paralysis when both eyes are affected.