Incomitant Strabismus/Module 4: Fourth (IVth) Cranial Nerve Palsy

Characteristics of IVth CN Palsy

As with recently acquired VI N palsy, when considering the characteristics of an individual cyclovertical muscle palsy, the following areas need to be considered:

• ocular posture
• head posture
• cover test
• ocular movements
• torsion
• Hess chart
• diplopia chart
• area of BSV
• past pointing

Torsion has been added to this list since it is also an integral part of the examination of incomitant strabismus, its presence and extent being an important factor in planning both surgical and orthoptic treatment.

Orthoptic Investigation

For the proper management of a patient with a neurogenic palsy, it is necessary to establish the most probable onset of the condition. This involves differential diagnosis between:

• a recently acquired neurogenic palsy
• an acquired palsy of long standing, and
• a congenital palsy

The characteristics of a congenital palsy and a longstanding acquired palsy are usually very similar, perhaps the only difference being the time of onset as determined in the history. Since the management of these two conditions is the same, as compared to that of a recently acquired palsy, the importance of differential diagnosis is not so great. Therefore the aim of the orthoptic investigation is to differentiate between recently acquired and congenital/longstanding cases so that the correct plan of management can be pursued. The following are important in the differential diagnosis:

• history
• and old photos
• head posture
• extent of concomitance: cover test, ocular movements, Hess chart
• presence of (subjective) torsion
• fusion amplitude: horizontal, vertical
• presence of sensory adaptations: suppression, amblyopia, ARC
• past pointing

Non-surgical Management

The principles of treatment using non-surgical options when treating recently acquired IV N palsy are similar to those for recently acquired VI N palsy. Prisms, either Fresnel or ground, are positioned vertically, but also obliquely if there is an associated horizontal component to the diplopia or deviation. Occlusion methods are also useful. A CHP to obtain BSV or eliminate the diplopia are only encouraged if it is comfortable and inconspicuous.

Surgical Management

Knapp's classification of IV N palsy is useful in terms of planning surgical treatment since the presentation of IV N palsy has a variety of clinical manifestations. These may be determined by such factors as anatomy and the development of muscle sequelae. Note that IV N palsy has also been classified in terms of being either acquired or congenital etc., however, this should not be confused with the present classification.

Management of Longstanding/Congenital IVth CN Palsy

The management considerations for congenital and acquired neurogenic palsies of longstanding are different and depend on whether the patient is a child or an adult.

Children: Children may require optical correction and occlusion for amblyopia. Surgical intervention may be functional to maintain or restore BSV, or cosmetic if no binocular functions are demonstrable.

Adults: Adult presentation with a congenital or longstanding neurogenic palsy is usually for cosmetic reasons or because the deviation is decompensating and causing diplopia or asthenopic symptoms.

The pre-op assessment in those presenting for cosmetic reasons should include careful investigation of suppression areas so as to avoid the possibility of post-op diplopia.

The following are guidelines in cases of decompensation:

• prisms or occlusion should be used as a temporary relief of diplopia, while reviewing the condition to assess stability
• any precipitating factor should receive the appropriate attention
• moderate to large deviations should be treated surgically, and an undercorrection is advisable because a CHP is likely to persist to some extent post-op and the fusion range is likely to be large due to long term compensation
• small deviations can be successfully treated with minimum prismatic correction to achieve comfortable BSV, if the patient is so willing or surgery is not an option
• orthoptic exercises and prism bar training can be used (and in combination with prisms) to extend fusional ability

Further considerations include 'congenital' IV N palsies that have an anatomical basis, that is, an abnormal SO tendon. These cases may require a 'strengthening' procedure on the tendon.