Microtropia

"A misalignment of the eyes with an angular deviation so small (less than 5 degrees) that it would usually be controlled except on dissociation of the eyes, in which case it becomes a phoria."

• Cosmetically fine and eyes work 'almost binocularly' (see later)
• Often no movement seen on cover test
• You could call it a strab that is 'perfectly adapted' - the EF and ARC are at the same point

Characteristic Features

• Freq presents betw 2-3 yrs but may be found later in life when VA in one eye slightly low
• Often made evident by crowding phenomenon
• Presence of HARC in small angle strabismus is assoc w/EF and amblyopia
• Normally ESO, EXO is rare

General Characteristics

• Small angle (less that 6 prism dioptres)
• Usually 1.50D anisotropia or more
• Amblyopia but usually reduced VA to 6/9 or 6/12 only
• EF always occurs. ANGLE OF ECCENTRICITY = ANGLE OF STRABISMUS thus no movement on cover test. The area on which the image falls in binocular conditions is the same as the eccentrically fixing area
• Harmonious ARC - retinal area where img falls in px's habitual vision = anomalously corresponding area = area used for monocular fixation = "Microtropia w/identity"
• Peripheral fusion which helps eyes to maintain their straight position
• Monofixation Syndrome: in many cases the angle of deviation increases on alternating CT or if one eye is covered longer than usual giving an ESOP superimposed on the microtropia.
  
• Stereopsis - low grade reported

Investigation & Diagnosis

• VA: presence of amblyopia in one eye is usually the first clue. Crowding phenomena present and letters on chart might be missed due to the central scotoma.
• FIXATION: EF (check using ophthalmoscope) present and may be assoc w/ ARC. Will be either (1) EF = angle of anomaly giving no shift on cover test or (2) EF doesn't equal angle of anomaly giving a shift w/CT
• CVR TEST: May find esophoria but not usually a strabismic movement
• 4D PRISM TEST placed before dominant eye - img moves across the retina and the eye moves to take up fixation. The non-dominant eye moves laterally (Hering's) in the same direction as it is not fixing so a VERSIONAL movement is seen and another recovery one when the prism is removed. If you put the same prism in front of the non-dom eye then there won't be any movement at all as the image has been moved across the retina and within the suppression area.
• AMBLYOPIA + NO CT MOVEMENT + POSITIVE 4BASEOUT TEST = MICROTROPIA
• BAGOLINI: Should get HARC - streak passes thru spot with or without a suppression gap.
• AMSLER: Scotoma may be demonstrated due to the EF

Classification of Microtropia

• Primary - remains constant throughout life and is rare
• A primary microtropia which become decompensated particularly between 1-3 years as a result of an accommodative element or superimposed phoria
• Secondary - optical or surgical correction of a concomitant strabismus

Lang's Classification of Microtropia

• Central Fixation
• Eccentric Fixation w/ARC where ang. of anomaly > degree of eccentricity
• Eccentric Fixation w/ARC where angles are the same

First two will show up on cover test. Third one gives a sensory adaptation to the deviation.