# Incomitant Strabismus/Module 3: Sixth (VIth) Cranial Nerve Palsy
## Characteristics of VIth CN Palsy

Lesions of the final motor pathway cause a deficiency of movement produced by the muscle supplied by that nerve. This may be complete (paralysis) or partial (paresis), the word palsy being all embracing. When considering the characteristics of an individual muscle palsy, the following areas need to be considered:
Ocular posture: As a result of a muscle palsy, the eye with the affected muscle will have a consequent deviation, a heterophoria or heterotropia. The primary and secondary actions of the palsied muscle are all affected, therefore the eye loses these actions and assumes a position determined by the unopposed actions of the other muscles.
Head posture: A CHP may be adopted to compensate for the muscle imbalance and eliminate diplopia. Theoretically, the eyes are turned away from the direction of action of the affected muscle.
Cover test: (i) Primary deviation - the deviation present when the unaffected eye is used for fixation, and (ii) secondary deviation - the deviation present when the affected eye is used for fixation. This deviation is of a larger angle and is one indicator of incomitance.
Ocular movements: As a result of a palsy, changes occur to the muscles known as muscle sequelae. This results in the deviation being different in various positions of gaze and is therefore another indicator of incomitance. The muscle sequelae is as follows:
* overaction of the contralateral synergist (due to Hering’s Law)
* overaction of the ipsilateral antagonist (due to mechanics)
* underaction of the contralateral antagonist (due to a complex interplay affecting both Sherrington’s and Hering’s Laws)
Hess chart: Measures and graphically represents the extent of ocular movement and limitation in all positions of gaze.
Diplopia chart: Generally, the greatest separation of the images is in the direction of the primary action of the palsied muscle, the distal image belonging to the affected eye.
Area of binocular single vision: If the eyes are binocular in any direction, this area will be in the opposite direction to the field of main action of the palsied muscle.
Past pointing (or false orientation): Where the paresis is recent, an individual, with the unaffected eye covered, will 'past point' or over-reach touching an object when asked to do so.

## Aetiologies

A neurogenic palsy of one (or more) of the ocular motor cranial nerves (III, IV and VI) can be congenital or acquired, resulting from various factors including trauma (CHI and orbital/facial injury), vascular (DM, hypertension, atherosclerosis, aneurysm and arteritis), neoplasia, inflammatory disease, MS, and infection.

## Orthoptic Investigation

When confronted with a suspected eye movement disorder, the aims of the orthoptic investigation are:
* to determine the site of the lesion, whether the condition is supranuclear, internuclear, nuclear, or infranuclear, and the likely and possible aetiology;
* if the deviation is nuclear or infranuclear in origin, to determine the palsied muscle(s) and to assess the development of muscle sequelae;
* to determine the onset, whether the condition is recently acquired or longstanding/congenital; and
* to provide a record of the condition for future comparison, and collect sufficient information for the health care team to plan the appropriate management.
Orthoptic assessment will include:
* observation and history taking;
* sight testing (visual acuity testing and determining refraction);
* determining the presence of a deviation (cover testing) and measurement (using PCT, MR, etc.);
* investigation of the ocular motility status and eye movement systems (assessing versions - remember Parks' 3 steps- and ductions, including forced duction testing; measurement in the
* secondary and tertiary positions of gaze; Hess charting or plotting the field of uniocular fixation if this is not possible)
* examination of the near triad (accommodation, convergence and pupillary reactions)
* examination of diplopia (diplopia charting) and areas of BSV (field of BSV)
* investigation of binocular functions

## Non-Surgical Management

A majority of acquired neurogenic palsies show some spontaneous recovery over a period of time, so surgical intervention is usually not considered for at least six months after the onset. During this period there may be spontaneous partial or complete recovery, or the condition may remain unchanged or indeed worsen. Non-surgical methods of treatment must be used temporarily during this period to eliminate the troublesome symptom of diplopia. Further, these non-surgical methods may be used as a permanent treatment if surgery is not an option or the result is unsatisfactory.
General: If there is a systemic condition, which may be a predisposing factor, this should be appropriately treated.
Orthoptic assessment: The condition is regularly monitored over the period when spontaneous recovery or changes can be expected. As diplopia can be a very troublesome symptom to the patient, reassurance and explanation of the progress should be given.
Prisms: Fresnel and/or ground prisms may be used therapeutically on a temporary or permanent basis.
Occlusion: Occlusion to eliminate troublesome diplopia should be only be considered if there is an inability to achieve useful BSV with prisms and/or an inconspicuous and comfortable CHP. Occlusion should be worn only as necessary. There is debate over whether the occlusion should be of the paretic eye, the good eye or alternating. Patients usually choose to occlude the eye with limited ocular motility, the paretic eye, unless the vision is better in this eye. An occluding contact lens is an option for patients who are very troubled by diplopia which cannot be overcome by other means. This, of course, has its risks and complications.
Orthoptics: The aim of orthoptic treatment is to exercise and increase any area of BSV. This can also be done by exploring the use of a CHP and encouraging its use. There is also debate over the value of exercises (active version and duction exercises) aimed at improving the function of the paretic muscle in particular.
Botulinum toxin injection: Botulinum toxin is sometimes used in the non-surgical management of recently acquired neurogenic palsy, particularly VI N palsy, to prevent contracture of the ipsilateral antagonist MR and even allow some more centralised field of BSV. It has also been advocated for treatment of residual and chronic VI N palsy in the hope that it produces a longer term effect.

## Surgical Management

The aim of surgery in neurogenic palsy is to eliminate the deviation in the primary and other positions. It is deemed a functional success, where diplopia is eliminated and BSV is restored, or cosmetic. Surgical intervention is only indicated when the condition has become static. Various options are available for the surgical treatment of neurogenic palsy and following must be considered when planning the operation:
* the size of the deviation for D and N fixation, horizontal and vertical
* the extent of the palsy (paresis or paralysis) or residual muscle function
* the ocular motility pattern and extent of incomitance that is present
* the presence of mechanical restriction which may have developed
* the laterality (unilateral or bilateral)
* the presence and extent of torsion
* the presence of an A/V pattern
Surgical tables indicating numbers in millimetres should be consulted - e.g. refer to Parks' surgical tables.
Specific procedures -
Weakening:
* recession
* augmented recession
* marginal ( or Z) myotomy
* myectomy
* tenotomy
* tenectomy
* faden operation (posterior fixation sutures)
Strengthening:
* resection
* advancement
* plication (tucking)
Transposition procedures (moving whole or part of a muscle):
* whole muscle
* (part-muscle) Hummelsheim
* (part-muscle) Jensen
* Harada-Ito
* desagittalisation (of IO muscle)
Adjustable sutures
* These allow final adjustment of the position of the eye, after the operation.