What is heterophoria and heterotropia squint

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heterophoria vs heterotropia squint classification types

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Heterophoria and Heterotropia (Squint / Strabismus)

Both terms describe misalignment of the visual axes, but they differ fundamentally in whether the deviation is latent or manifest.

The Core Distinction

Feature	Heterophoria	Heterotropia
Other name	Latent squint / phoria	Manifest squint / tropia
Visibility	Hidden - only detected when fusion is disrupted	Visible at all times (or intermittently)
Fusion status	Kept in check by the fusion reflex	Fusion has broken down
Binocular single vision	Maintained normally	Lost (or partially maintained in small deviations)

As Kanski's puts it: "Heterophoria ('phoria') implies a tendency of the eyes to deviate when fusion is blocked (latent squint). Slight phoria is present in most normal individuals and is overcome by the fusion reflex." - Kanski's Clinical Ophthalmology, 10th ed.

HETEROPHORIA (Latent Squint)

Definition

A tendency for the eyes to deviate from perfect alignment that is kept latent (hidden) by the active fusional mechanism. When fusion is disrupted - by covering one eye, fatigue, illness, alcohol, or certain drugs (anticonvulsants, sedatives) - the eye drifts to reveal the underlying deviation.

Orthophoria = perfect ocular balance with no tendency to deviate (rare in absolute form; most people have trace phoria).

Types by Direction

Type	Deviation
Esophoria	Latent inward (convergent) deviation
Exophoria	Latent outward (divergent) deviation
Hyperphoria	Latent upward deviation
Hypophoria	Latent downward deviation
Cyclophoria	Latent torsional deviation

Clinical Subtypes (by distance)

Convergence excess - esophoria greater at near
Convergence weakness - esophoria greater at distance
Divergence weakness - exophoria greater at distance
Divergence excess - exophoria greater at near
Mixed - equal at near and distance

When Does It Become a Problem?

Heterophoria is normally well-compensated. It becomes symptomatic (decompensating phoria) when fusional amplitudes are insufficient, causing asthenopia (eye strain, headache, blurring) and possibly diplopia, especially under stress or poor health. It can also decompensate into a frank heterotropia.

Tests to Detect Heterophoria

Cover-uncover test (uncover component detects phoria)
Alternate cover test (dissociates fusion; reveals total deviation)
Maddox rod test - dissociates images for subjective measurement
Maddox wing - measures heterophoria specifically at near (1/3 m)
Prism bar cover test - quantifies the angle

Treatment

Correct any significant refractive error
Orthoptic exercises (especially for convergence weakness exophoria)
Prisms - Fresnel prisms temporarily, then incorporated into spectacles (maximum ~10-12 prism dioptres split between both eyes)
Surgery - for larger deviations unresponsive to other measures

HETEROTROPIA (Manifest Squint / Strabismus)

Definition

A manifest deviation in which the visual axes do not intersect at the fixation point. The images from the two eyes are misaligned. As stated in Kanski's: "Heterotropia ('tropia') implies a manifest deviation in which the visual axes do not intersect at the point of fixation."

Types by Direction

Type	Deviation
Esotropia	Convergent squint (eye turns inward)
Exotropia	Divergent squint (eye turns outward)
Hypertropia	Eye displaced upward
Hypotropia	Eye displaced downward
Cyclotropia	Torsional manifest deviation

Major Classification: Concomitant vs. Incomitant

1. Concomitant (Comitant) Strabismus

The angle of deviation is the same in all directions of gaze
Usually no underlying neurological or muscular cause
Typically develops in childhood
Examples: infantile esotropia, accommodative esotropia

2. Incomitant (Paralytic/Non-comitant) Strabismus

The angle of deviation varies with the direction of gaze
Caused by weakness or restriction of one or more extraocular muscles
Examples: cranial nerve palsies (CN III, IV, VI), thyroid eye disease, Duane syndrome

By Frequency

Constant - deviation present at all times
Intermittent - deviation present only sometimes (some binocularity retained; exotropia is commonly intermittent)

By Eye Fixing

Unilateral (monocular) - the same eye always deviates; risk of amblyopia is high
Alternating - either eye may deviate; amblyopia less likely because both foveae are used

Consequences of Heterotropia

Diplopia - double vision from misaligned visual axes
- Esotropia = uncrossed (homonymous) diplopia
- Exotropia = crossed (heteronymous) diplopia
Suppression - cortical suppression of the deviating eye's image to avoid diplopia
- Central suppression: foveal image suppressed (avoids confusion)
- Peripheral suppression: peripheral image suppressed (avoids diplopia)
Amblyopia - if constant unilateral suppression persists before visual maturity (~6 years), the deviating eye fails to develop normal visual acuity permanently ("lazy eye")
Anomalous Retinal Correspondence (ARC) - non-corresponding retinal points acquire a shared visual direction; allows some rudimentary binocular vision in small-angle squints (microtropia)

Summary Diagram

SQUINT (Strabismus)
├── HETEROPHORIA (Latent)
│   ├── Esophoria (inward tendency)
│   ├── Exophoria (outward tendency)
│   ├── Hyperphoria / Hypophoria (vertical)
│   └── Cyclophoria (torsional)
│
└── HETEROTROPIA (Manifest)
    ├── By direction: Esotropia / Exotropia / Hyper / Hypo
    ├── Concomitant (angle same in all gazes)
    └── Incomitant (angle varies with gaze direction)
        ├── Paralytic (e.g. CN palsy)
        └── Restrictive (e.g. thyroid eye disease)

Key clinical pearl: A heterophoria can decompensate into a heterotropia under physiological stress (fatigue, illness, drugs). In children, new-onset tropia must always prompt refraction and fundus examination to exclude underlying pathology before attributing it to a simple motor imbalance.

Sources: Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., Ch. 18; Bradley and Daroff's Neurology in Clinical Practice, Ch. 18

What is diploplia and types Explain about it to mr

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Diplopia (Double Vision) - A Complete Explanation

What is Diplopia?

Diplopia is the perception of two images of a single object simultaneously - in simple terms, seeing double. It occurs when the visual axes of the two eyes are misaligned so that the same object projects its image onto non-corresponding points on the two retinas, causing the brain to perceive two separate images.

As Adams and Victor's Neurology states: "The term binocular diplopia refers to the symptom of double vision caused by a misalignment of the visual axes of the two eyes. It is only present when both eyes are open and can see. Covering one eye obliterates double vision that stems from misalignment of the eyes."

The Two Fundamental Types

The most important first step in any patient with diplopia is to ask: "Does the double vision go away when you close either eye?"

1. Monocular Diplopia

Definition: Double vision that persists even when one eye is closed - the affected eye alone sees two images
Cause: A problem intrinsic to the eye itself, not an alignment problem
Common causes:
- Corneal irregularities (keratoconus, pterygium, astigmatism)
- Cataract (lens opacities split the image)
- Uncorrected refractive error
- Foveal traction or retinal disease
- Occasionally: malingering or psychiatric/functional disorder
Key point: Monocular diplopia is almost never a neurological emergency. It is a local ocular problem.

2. Binocular Diplopia

Definition: Double vision that is present only with both eyes open and disappears immediately when either eye is covered
Cause: Misalignment of the visual axes (ocular motor problem)
Key point: Binocular diplopia always demands investigation - it can signal serious neurological, vascular, or systemic disease

Diplopia diagram showing crossed and uncrossed images in misaligned eyes

Heteronymous (crossed) diplopia in right exotropia - Kanski's Clinical Ophthalmology, 10th ed.

Classification of Binocular Diplopia

A. By Direction of Image Separation

Type	Description	Seen In
Horizontal diplopia	Two images side by side	Esotropia, exotropia (CN VI or medial rectus palsy)
Vertical diplopia	One image above the other	Hypertropia, CN IV palsy, skew deviation
Torsional (oblique) diplopia	Images tilted relative to each other	CN IV palsy (trochlear nerve), oblique muscle problems
Mixed	Combination of horizontal + vertical	CN III palsy, orbital disease

B. By Position of the False Image

Type	Squint Present	Image Position
Homonymous (uncrossed) diplopia	Esotropia (eye turns in)	False image is on the same side as the squinting eye
Heteronymous (crossed) diplopia	Exotropia (eye turns out)	False image is on the opposite side to the squinting eye

Memory trick: In esotropia (inward turn), the deviating eye's image is projected outward - so it appears on the same side. In exotropia (outward turn), the image is projected inward - appearing on the opposite side.

C. By Cause (Most Clinically Important Classification)

1. Neurogenic Diplopia (Cranial Nerve Palsies)

The most common cause of binocular diplopia requiring urgent workup.

Nerve	Muscle Affected	Pattern of Diplopia	Other Signs
CN III (Oculomotor)	All except LR and SO	Diplopia in all directions except lateral gaze to same side; eye is "down and out"	Ptosis, dilated pupil (if compressive)
CN IV (Trochlear)	Superior oblique	Vertical + torsional diplopia; worse on looking down and inward	Head tilt away from affected side (compensatory)
CN VI (Abducens)	Lateral rectus	Horizontal diplopia; worse on looking to the affected side; esotropia	Most common isolated cranial nerve palsy

2. Neuromuscular Junction Diplopia

Myasthenia Gravis - the classic cause of painless, variable, fatigable diplopia. Not confined to a single nerve distribution. Pupils always normal. Worsens with sustained activity, improves with rest.
Botulism - can mimic ocular myasthenia

3. Restrictive (Mechanical) Diplopia

Caused by physical restriction of globe movement, not nerve weakness:

Thyroid eye disease (Graves') - fibrosis of inferior and medial recti; commonest cause is restriction of elevation (inferior rectus fibrosis)
Orbital blowout fracture - inferior rectus or inferior oblique entrapment; vertical diplopia with upward gaze limitation
Orbital pseudotumor / myositis - painful diplopia with proptosis
Orbital tumors / abscesses

4. Supranuclear / Central Diplopia

Problems in the brainstem pathways:

Internuclear ophthalmoplegia (INO) - lesion of the medial longitudinal fasciculus (MLF); impaired adduction of one eye with nystagmus in the abducting eye; classic in multiple sclerosis
Skew deviation - vertical misalignment from brainstem or cerebellar lesions
Alcohol or anticonvulsant toxicity - can cause transient diplopia by disrupting conjugate gaze

Key Clinical Assessment

Step 1 - Cover Test

Cover one eye: if diplopia disappears = binocular. If persists = monocular.

Step 2 - Direction of maximum diplopia

Ask: "In which direction of gaze is the double vision worst?" The weak muscle acts in the direction where the gap between images is greatest.

Step 3 - Identifying the false image

The image furthest from the midline (most peripheral) always belongs to the abnormal eye. As Neuroanatomy through Clinical Cases explains: "When an extraocular muscle is not working properly, the image further from the midline toward the direction of attempted gaze is always the one seen by the abnormal eye."

Step 4 - Red Glass Test

A red glass is placed over one eye (usually right). The patient looks at a white light and reports the position of the red vs. white images in 9 positions of gaze. This helps identify which muscle is weak.

Step 5 - Head posture

Patients often adopt a compensatory head posture to minimize diplopia (e.g., head tilt in CN IV palsy, face turn in CN VI palsy).

Quick Summary Diagram

DIPLOPIA (Double Vision)
│
├── MONOCULAR (one eye closed = still double)
│   └── Cause: cornea, lens, retina, refractive error
│       → NOT a neurological emergency
│
└── BINOCULAR (one eye closed = resolves)
    └── Cause: ocular misalignment
        │
        ├── By direction: Horizontal / Vertical / Torsional / Mixed
        │
        ├── By image position: Homonymous / Heteronymous
        │
        └── By cause:
            ├── Neurogenic (CN III, IV, VI palsy)
            ├── Neuromuscular (Myasthenia gravis, Botulism)
            ├── Restrictive (Thyroid, Fracture, Tumor)
            └── Central (INO, skew deviation, toxins)

Clinical pearl from Harrison's: "The first point to clarify is whether diplopia persists in either eye after the opposite eye is covered. If it does, the diagnosis is monocular diplopia... Diplopia alleviated by covering one eye is binocular diplopia and is caused by disruption of ocular alignment."

Sources: Adams and Victor's Principles of Neurology, 12th ed.; Harrison's Principles of Internal Medicine, 22nd ed.; Kanski's Clinical Ophthalmology, 10th ed.; Rosen's Emergency Medicine; Neuroanatomy through Clinical Cases, 3rd ed.

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