intro to incom

what is incom strab

An eye misalignment where the degree of deviation varies depending on the direction of gaze or which eye is used for fixation. This contrasts with concomitant strabismus, where the angle of deviation remains constant across all positions of gaze.

Front: What are the three main types of incomitant strabismus?

1. Neurogenic: Issues with nerve supply to extraocular muscles

2. Myogenic: Weakness or dysfunction of the muscle itself

3. Mechanical: Physical obstruction or limitation affecting globe or muscle movement

what does a hess chart describe

-            Smaller eye = affected eye

-            Larger field = larger deviation – caused by fixing with paretic eye

-            Inwards displacement – u/a – limitation

-            Squashed field = mechanical

What is a key feature of incomitant strabismus?

The deviation increases in the direction of the affected muscle.

Front: What is the relationship between primary and secondary deviation in incomitant strabismus?

Secondary deviation (when the patient fixes with the paretic eye) is greater than the primary deviation.

What is Hering's Law of Equal Innervation?

Yoke muscles of both eyes (muscles that move the eyes in the same direction) receive equal neural innervation. When the paretic eye is used for fixation, the yoke muscle in the non-paretic eye overacts, leading to a larger deviation

muscle sequale stages

 

1. Primary underaction of a muscle

2. Overaction of the contralateral synergist (Hering's law)

3. Contracture of the ipsilateral antagonist (Sherrington's law)

4. Secondary inhibition of the contralateral antagonist

Front: What should be included in the case history for incomitant strabismus?

 

• Determine if congenital or acquired

• Assess for diplopia (type, frequency, alleviating factors)

• Check for compensatory head posture (CHP)

• Look for systemic causes or trauma history

Front: What are the key clinical tests for assessing incomitant strabismus?

• Smooth Pursuits

• Versions and Ductions

• Secondary Deviation Measurement

• Binocular Single Vision (BSV)

• Hess and Lees Charting

• Field of BSV and Uniocular Fixation

• Forced duction test

Front: How do versions and ductions help differentiate between neurogenic and mechanical causes?

Versions assess eye movement in both eyes together, while ductions evaluate movement in each eye individually. Differences in these movements can indicate whether the cause is neurogenic (nervous system) or mechanical (muscle or connective tissue).

• Neurogenic: Maximum deviation occurs in the direction of the affected muscle's action - duction exceed versions

• mechanical: deviation is worse in the direction opposite to the affected muscle action and may show globe retraction - duction = version

congenital incom

neurogenic

• isolated feature

form

• aplasia/ hypolplasia or of nrve/ nucleus

• abnornal innervation

• trauma during delievry

• inflammation: neonatal. antenatal

mechanical

• Duanes

• Browns

Myogenic

• developmental abnormality - hypoplasia/ hyperplasia of eom

• Abnormal attachments - e.g muslce insertions to eyeball or orbital contents

• fibrosis

• adhesions - muscle to orbit

acuired

neurogenic

• trauma

• inflammation e.g. ms

• vascular - hypertension

• SOL

• metabolic disorder e.g diabetes

Mechanical

• trauma - fractures, development of tethors

• SOL

• iatorgenic adhesion

• secondary to myogenic inflammation - e.g TED

myogenic

• lesions at

  • neuromuscular junctions e.g. MG

  • muscle fibre membranes - myotonia

  • muscle fibre content - dystrophies - CPEO,

    • endocrine - dysthyroid eye disease

    • inflammatory - myositis

differences between acquired & congential

acquired

presentation

• diplopia & occasionally pain

• aware of AHP

OM

• muscle seq not fully developed

BV

• normal fusion range

Congenital

presentation

• symptoms of decompensation

• unware of AHP

• concern of cosmtic apparence

OM

• often full muscle seq

BV

• extended vertical fusion range

differences between neurogenic & myogenic

CT

• Neurogenic

  • see dev in pp

• Mechanical

  • no/sm dev in pp

OM

• neurogenic

  • ductions > versions

  • no retraction of globe

• mechanical

  • ductions = versions

  • retraction of globe

Hess chart

• Neurogenic

  • space between inner & outer fields are =

  • fields are displaced from position of most limitation

• Mechanical

  • squashed field

  • 2 stages of muscle seq

FDT

• neurogenic

  • -ve

• mechanical

  • +Ve

IOP

• neurogenic

  • same position of gaze

• Mechanical

  • increases when looking away from position of limitation