Mechanical vs neurogenic deviations differential diagnosis

Question 1

Deviation

Answer 1

Neurogenic:- Dependant on the extent of the palsy, the deviationin the primary position can be marked.

Mechanical:- In many cases with mechanical restriction there is a little deviation in the primary position e.g. orbita floor fractures.

Question 2

Diplopia

Answer 2

Neurogenic:- Exept for third nerve paslies, the nature of diploipa remians the same in different gaze positions E.g. Right image remians above the left image.

Mechanical:- diplopia can reverse i.e. between upgaze and downgaze.

Question 3

Head posture

Answer 3

neurogenic:- a head tilt is usual in vertical muscle palsies

Mechanical:- Head elevation or depression compensates for vertical deviations of a mechanical type: therefore head tilt is rare.

Question 4

Ocular movements

Answer 4

Neurogenic:- the amount of movement elicited is often greater on duction than on version. movement ceases gradually in the directin of limitation. maximal limitation is in the position of main action of the muscle.

Mechanical:- Duction and version movements are equally limited. movememnt ceases suddenly when mechanical factors intervene. Movement is usually limited on depression than on laevodepression.

Question 5

HESS chart

Answer 5

Neurogenic:- although the feild of the affected eye will be smaller, there is proportional spacing between the inner and outer fields. Both feilds are disolaced due to the deviation.

Mechanical:- the outer field of the chart will be very close to the inner feild imn the direction of maximum limitation of movement. The feild of the affected eye may be very narrow, either horozontally or vertically. Typically in the field there is limited upgaze rather than dextroelevation. There is no contracture of the ipsilateral antagonist.

Question 6

Globe Retraction

Answer 6

Neurogenic:- None

Mechanical:- the globe often retracts when turned in the direction opposite to the side of the mechanical lesion.

Question 7

Pain on movement

Answer 7

Neurogenic:- None

Mechanical:- Found in acquired cases and in some cases Browns syndrome

Question 8

Forced duction

Answer 8

Neurogenic:- fully passive movement unless secondary muscle contracture hs occured over a long peroid.

Mechanical:- Limited passive movement, generally in the direction opposite to the site of the lesion, sometimes in the same direction or in both directions.

Question 9

Forced generation test

Answer 9

Neurogenic:- Reduced, dependant on the extent of the palsy

Mechanical:- Normal muscle function

Question 10

Saccadic eye movements

Answer 10

Neurogenic:- Reduced peak saccadic velocities

Mechanical:- Normal

Question 11

IOP

Answer 11

Neurogenic:- remains unchanged in different gaze positions.

Mechanical:- Will be riased wehn looking in away from the site of the mechanical lesion, a difference of more than 55mmHg between two opposite directions of gaze is significant.