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“This patient has complained of double vision. Please examine their eyes to determine the cause.”
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This patient has a VIth cranial nerve palsy.
A patient with 6th nerve (also called Abducens nerve; VIth cranial nerve) palsy will present paralysis of the lateral rectus muscle. The 6th cranial nerve serves the ipsilateral lateral rectus muscle.
Subsequently esotropia (strasbismus) on primary gaze may be present, with the eye deviated towards the nose. The patient will complain of horizontal diplopia worse in the distance and employ head tilting, a manoeuvre that reduces the doubling of images through alignment of binocular vision.
Initial investigations include a comprehensive ocular exam to assess the basic functions of the muscles, other ocular nerves and pupillary function. Other nerves should be tested for functionality to see assess the level of lesion (for example 5th, 7th and 8th cranial nerves). For example, a lesion of the cavernous sinus which will produce altered sensation of the sensory part of the trigeminal nerve.
The 6th cranial nerve starts its course from the pons, where its nucleus is located, traverses through the subarachnoid area, and it has the longest subarachnoid course and enters the cavernous sinus. From there, it the nerve enters the superior orbital fissure to supply the lateral rectus muscle. An injury at any of these levels can produce a palsy and require different investigations.
It is important to note that the palsy can be either congenital, due to mal-development of the nerves (Duane syndrome is a rare congenital 6th nerve palsy), or acquired form that occurs in the setting of a variety of conditions. Lesions in the posterior fossa may be due to compression (for example a tumour), ischaemic, inflammatory (most commonly multiple sclerosis) or degenerative.
Stretching or compression of the nerve by a mass lesion such as with tumour and vascular growth or in the setting of raised intracranial pressure. In this case a CT scan or an MRI will be the modality of choice. In the case of meningitis, a lumbar puncture should be carried out. Ischaemic injuries, such those occurring within the setting of diabetes mellitus or hypertension will require monitoring of the blood sugar and pressure levels.
There are imitations of 6th nerve palsy that should be considered, and these include thyroid eye diseases, myasthenia gravis and an old blowout fracture of the orbit.
In the adult group, unless the underlying pathology is known, all patients should undergo the following investigations:
– Blood pressure (for hypertension)
– Full blood count (for anaemia)
– HbA1C / oral glucose tolerance test (for diabetes)
– ESR (for temporal arteritis)
– Syphilis serology (for infection with neurosyphilis)
– Thyroid function tests (for thyroid eye disease)
– Imaging with either CT or MRI depending upon the risk factors (for a space occupying lesion)
In children, patches are used to occlude one eye to reduce the chances of development of amblyopia.
In adults, patching is employed to reduce the diplopia. Usually, evaluation of esotropia can be done through prismatic measurements. This is important in the assessment of correction and return of functionality or worsening of condition.
If there is poor, or a complete lack of resolution of symptoms a full further neurological assessment should be carried out.
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