Unifying Macular Ischemic Syndromes: Pathophysiology, Multimodal Imaging, and Clinical Management of Coincident PAMM and AMN

1. Introduction & Clinical Context

Advances in spectral-domain optical coherence tomography (SD-OCT) and optical coherence tomography angiography (OCTA) have fundamentally transformed our understanding of deep retinal vascular microarchitecture and macular ischemic pathobiology. High-resolution, depth-resolved structural and angiographic modalities now enable vitreoretinal specialists to isolate distinct capillary plexuses with exquisite precision, directly mapping microvascular insults to specific cellular strata within the central macula.

Among these depth-resolved ischemic manifestations, Paracentral Acute Middle Maculopathy (PAMM) and Acute Macular Neuroretinopathy (AMN) represent distinct yet intimately linked manifestations within the macular capillary network:

• PAMM reflects acute middle retinal infarction involving the Inner Nuclear Layer (INL), with variable extension into the Inner Plexiform Layer (IPL) and Outer Plexiform Layer (OPL), driven secondary to hypoperfusion within the Deep Capillary Plexus (DCP).
• AMN presents as an acute outer retinal insult primarily originating at the level of the OPL and Henle Fiber Layer (HFL), extending retrogradely into the Outer Nuclear Layer (ONL) with secondary impairment of the Ellipsoid Zone (EZ) and Interdigitation Zone (IZ).

While historically categorized as separate clinical entities, accumulating multimodal imaging evidence demonstrates that PAMM and AMN frequently co-occur within the same eye. Pathophysiologically, coincident PAMM and AMN represent a unified spectrum of a common ischemic process. This spectrum is governed by DVC/DCP hypoperfusion, altered venous outflow dynamics, and secondary cytotoxic and mechanical disruption propagating through the HFL and Müller glia.

2. Retinal Microvasculature and the Hybrid Perfusion Model

Trilaminar Vascular Architecture

Within the central macula, the retinal capillary network is organized into three distinct, depth-resolved horizontal layers:

1. Superficial Capillary Plexus (SCP): Resides within the Ganglion Cell Layer (GCL) and Nerve Fiber Layer (RNFL).
2. Intermediate Capillary Plexus (ICP): Situated along the inner border of the INL, adjacent to the IPL.
3. Deep Capillary Plexus (DCP): Positioned along the outer border of the INL, directly bordering the OPL.

In current anatomical and angiographic nomenclature, the radial peripapillary capillary plexus (RPCP) and SCP are designated as the Superficial Vascular Complex (SVC), whereas the ICP and DCP are collectively classified as the Deep Vascular Complex (DVC).

Retinal Microvascular Stratification

• Superficial Vascular Complex (SVC)
  • Radial Peripapillary Capillary Plexus (RPCP) /RNFL ]
  • Superficial Capillary Plexus (SCP)/RNFL / GCL} ]
• Deep Vascular Complex (DVC)
  • Intermediate Capillary Plexus (ICP) /IPL / Inner INL border]
  • Deep Capillary Plexus (DCP)/Outer INL / OPL border} ]

The Hybrid Blood Flow Model

Three-dimensional confocal laser microscopy and functional OCTA studies support a hybrid blood flow model characterized by combined parallel and in-series perfusion dynamics:

• Arterial Inflow (Parallel Dynamics): Primary retinal arteries supply the capillary beds of the SCP and ICP directly. The ICP receives additional blood flow via short, connecting arteriolar branches arising from the SCP. Crucially, the DCP lacks direct arterial input from retinal arteries and receives no descending arterioles directly from the SCP. Instead, the DCP is supplied exclusively via anastomotic capillary connections originating from the ICP.
• Venous Outflow (In-Series Drainage): While arterial inflow enters at the superficial plane, venous outflow drains in-series downward into the deeper layers. The DCP serves as the primary drainage floor for retinal venous outflow. Draining blood within the DCP converges centripetally through horizontal capillaries toward central vortex venules, which feed ascending venules that traverse vertically upward through the retinal layers to enter the superficial retinal veins.

Hemodynamic & Metabolic Vulnerability: The DCP as a Perivenular Bottleneck

The middle retina (INL) and outer retina (OPL/HFL) possess heightened anatomical and metabolic vulnerability to ischemic degradation during states of arterial insufficiency or elevated venous impedance:

• In-Series Oxygen Depletion: Because blood flows in-series from the ICP down to the DCP, oxygen tension progressively declines along the capillary path. The terminal perivenular loops of the DCP operate at the lowest intraluminal hydrostatic pressure and lowest oxygen saturation across the entire retinal circulation.
• High Metabolic Demand: The interneurons of the INL and the photoreceptor synapses within the OPL maintain exceptional mitochondrial density and metabolic activity, making them exquisite sensors of minor decrements in oxygen tension.
• Venous Hydrostatic Bottlenecks: In retinal vein occlusion (RVO), elevated systemic venous pressure produces immediate, selective hydrostatic stagnation at the central vortex venules of the DCP before back-pressure affects the superficial plexuses. Conversely, during systemic arterial insufficiency (e.g., evolving retinal artery occlusion), the distal perivenular poles of the DCP experience the most severe tissue hypoxia and anoxia, initiating focal microvascular infarction.

3. Diagnostic Hallmarks and Comparative Analysis: PAMM vs. AMN

Paracentral Acute Middle Maculopathy (PAMM)

• SD-OCT Presentation: Characterized by an acute, hyperreflective band-like lesion confined to the middle retina at the level of the INL, with variable extension into the adjacent IPL or OPL.
• En Face OCT Patterns: En face OCT reveals distinct spatial configurations corresponding to the pattern of DCP hypoperfusion, including perivenular “fern-like” hyperreflectivity, skip lesions, or diffuse “globular” hyperreflectivity.
• Progression & Legacy: Acute PAMM hyperreflective bands spare the outer photoreceptor layers. Over weeks to months, acute hyperreflectivity fades, leaving permanent, localized INL thinning (“resolved PAMM”).

Acute Macular Neuroretinopathy (AMN)

• SD-OCT Presentation: Classically originates at the OPL level and extends retrogradely through the HFL into the ONL, accompanied by attenuation and disruption of the underlying EZ and IZ.
• En Face / NIR Presentation: Well-demarcated, dark, wedge-shaped or hyporeflective paracentral lesions pointing toward the fovea, corresponding precisely to patient-reported paracentral scotomas.
• Progression & Legacy: Acute lesions exhibit angular or Z-shaped hyperreflectivity along HFL striations. Chronic stages manifest as localized ONL thinning and variable, persistent disruption of the EZ/IZ interface.

Comparative Diagnostic Matrix

Feature	Paracentral Acute Middle Maculopathy (PAMM)	Acute Macular Neuroretinopathy (AMN)
Primary Retinal Layer Invalidation	Inner Nuclear Layer (INL)/ IPL/OPL	Outer Plexiform Layer (OPL), Henle Fiber Layer (HFL), Outer Nuclear Layer (ONL)
Direction of Lesion Progression	Anteriorly toward middle/inner retina (spares outer photoreceptors)	Retrogradely toward outer retina, EZ, and IZ
Near-Infrared Reflectance (NIR)	Perivenular fern-like or hyporeflective patches	Dark, wedge-shaped or hyporeflective paracentral lesions
OCT Angiography (OCTA) Findings	Flow signal deficits in the DCP and ICP (occasionally SCP in severe cases)	Flow signal deficits isolated to deep/distal DCP capillary vortices
Long-Term Structural Sequelae	Permanent INL thinning/atrophy (“resolved PAMM”)	Permanent ONL thinning with localized EZ/IZ disruption

4. Coincident PAMM and AMN: Clinical Evidence and The “Missing Link”

Clinical Cohort Findings

In a foundational observational study, Iovino et al. evaluated 15 consecutive patients (17 eyes) presenting with coincident PAMM and AMN within the same eye:

• Demographics: Mean age of 44.4 \pm 15.3 years (range: 26–73 years; 58.8% male); follow-up ranged from 1 to 32 weeks (mean: 11.9 \pm 11.4 weeks).
• Visual Acuity: Baseline visual acuity averaged 20/126 (0.8 \pm 0.6 logMAR), which improved significantly to 20/40 (0.3 \pm 0.4 logMAR) at final evaluation.
• Etiological Distribution:
  • Retinal Vein Occlusion (RVO): 7 eyes (4 CRVO, 3 Hemiretinal Vein Occlusion [HRVO])
  • Purtscher’s / Purtscher-like Retinopathy: 4 eyes (3 patients; secondary to motor vehicle trauma, acute pancreatitis, or severe chest compression)
  • Central Retinal Artery Occlusion (CRAO): 1 eye
  • Idiopathic Retinal Vasculitis: 1 eye
  • Idiopathic / Unassociated: 4 eyes (3 patients; including post-pacemaker placement and chest trauma)

Vertical Colocalization & Radial HFL Topography

• Vertical Colocalization: In 64.7% (11/17) of eyes, cross-sectional SD-OCT B-scans demonstrated direct vertical colocalization of PAMM and AMN lesions along the identical vertical B-scan axis.
• Z-Shaped Extension: In 64.7% (11/17) of eyes, the hyperreflective AMN bands displayed a distinct Z-shaped morphology tracking along the HFL .
• Anatomical Radial Gradient of the HFL: The characteristic Z-profile is explained by the radial microanatomy of Henle’s Fiber Layer across the macula :
  1. Foveal Center: HFL fibers are short and oriented vertically.
  2. Perifovea: HFL photoreceptor axons and intermingled Müller processes lengthen and assume an oblique, almost horizontal trajectory, yielding the classic Z-shaped profile on cross-sectional OCT.
  3. Extra-Perifovea: HFL obliqueness diminishes, reverting to a vertical orientation outside the perifoveal boundary.

Pathophysiological “Missing Link”: Cytotoxic Müller Glial Edema vs. Vasogenic Edema

The architectural bridge unifying middle retinal PAMM and outer retinal AMN lies within the cellular composition of the HFL and DVC:

1. Structural and Metabolic Coupling: The HFL consists of unmyelinated photoreceptor axons intimately paired in a 1:1 structural ratio with outer Müller cell processes .
2. Cytotoxic vs. Vasogenic Edema: Müller cell bodies reside within the INL and derive their primary oxygen supply from the DVC/DCP. Early histopathological investigations by Fine & Brucker (1981) demonstrated that acute ischemia to Müller cells causes selective cytotoxic edema—characterized by intracellular swelling of Müller cell cytoplasm without early expansion of intercellular spaces. This contrasts sharply with vasogenic edema, which involves breakdown of the blood-retinal barrier leading to extracellular fluid accumulation.
3. Retrograde Cascade: Severe DCP hypoperfusion causes cytotoxic swelling of Müller cell bodies in the INL (manifesting as PAMM). Because the DCP supplies both the INL and the synaptic layer of the OPL, severe deep capillary ischemia extends from OPL synapses into the outer Müller cell processes and paired photoreceptor axons within the HFL. This cytotoxic insult propagates retrogradely along the oblique trajectory of Henle fibers toward the ONL, EZ, and IZ, manifesting on SD-OCT as an acute AMN lesion vertically colocalized beneath the PAMM band.

The ASHH Sign as a Unifying Directional Biomarker

The Angular Sign of Henle Fiber Layer Hyperreflectivity (ASHH) serves as a unifying structural OCT biomarker indicative of acute HFL disruption:

• Retrograde Propagation (DCP/DVC Origin): Primary ischemic insults in the DCP (such as AMN or PAMM/AMN overlap) propagate downward/retrogradely through the HFL toward the outer photoreceptor segments and EZ/IZ.
• Anterograde Propagation (RPE/Choroid Origin): Outer retinal insults such as blunt ocular trauma, photocoagulation laser, or acute posterior multifocal placoid pigment epitheliopathy (APMPPE) originate at the photoreceptor/RPE complex and propagate upward/anterogradely along the HFL.

5. The Retinal Ischemic Cascade: A Unified Continuum

THE RETINAL ISCHEMIC CASCADE

Step 1: Perivenular DCP Hypoperfusion

• Initial Insult: Hypoxia at distal perivenular poles of the DCP.
• OCT Biomarker: Perivenular / Skip PAMM (“Fern-like” on en face OCT).

Step 2: Horizontal Propagation

• Mechanism: Persistent venous impedance or arterial capillary nonperfusion.
• OCT Biomarker: Diffuse / Globular PAMM across the INL.

Step 3: Vertical Propagation

• Anterograde (Upward): Transmural Inner Retinal Infarction (GCL / RNFL).
• Retrograde (Downward): AMN via Müller Glia & HFL Disruption (ONL / EZ / IZ loss).

Step 4: Atrophic Resolution & Biomarkers (“Forme Fruste”)

• Chronic Sequelae: “Resolved PAMM” / Retinal Ischemic Perivascular Lesions (RIPLs).
• OCT Biomarker: Focal INL thinning; “Black holes” on OPL en face slab.

Spatial and Temporal Progression

The Retinal Ischemic Cascade conceptual framework integrates diverse macular ischemic phenotypes into a continuous spatial and temporal spectrum :

1. Initial Perivenular Insult: Mild or hyperacute hypoperfusion selectively impacts the most distal, oxygen-depleted perivenular poles of the DCP, manifesting as perivenular “skip” PAMM or “fern-like” perivenular whitening on en face imaging.
2. Horizontal Extension: If capillary nonperfusion or elevated venous impedance persists, isolated perivenular ischemic foci merge horizontally, creating diffuse “globular” PAMM across the entire INL .
3. Vertical Propagation:
   • Anterograde (Upward): Severe vascular occlusion propagates vertically toward the superficial retina, resulting in transmural middle and inner retinal infarction.
   • Retrograde (Downward): Ischemia extending through the OPL and DVC damages Müller glia and photoreceptor axons, propagating retrogradely along Henle’s fibers to manifest as coincident AMN with outer retinal (EZ/IZ) loss.

Chronic Biomarkers (“Forme Fruste” Ischemia)

As acute PAMM lesions resolve, ischemic necrosis of INL interneurons results in localized middle retinal atrophy, termed resolved PAMM or Retinal Ischemic Perivascular Lesions (RIPLs).

• Cross-Sectional OCT: Characterized by focal thinning of the INL with compensatory elevation/wavy contour of the underlying OPL and ONL.
• En Face OCT: Appears as localized hyporeflective “black holes” at the level of the OPL slab.

Systemic Cardiovascular Implications

Resolved PAMM and RIPLs represent structural “memories” of past subclinical ischemic events. In eyes without overt clinical vascular occlusions, detecting these atrophic loci serves as a highly sensitive biomarker for underlying systemic vascular disease, strongly correlating with systemic hypertension, carotid artery stenosis, cardiovascular disease, prior stroke, and hypercoagulable states.

7. Key Clinical Takeaways and Practice Recommendations

• Execute High-Resolution, Tracked Multimodal Protocols: Always pair cross-sectional SD-OCT B-scans with Near-Infrared Reflectance (NIR) and depth-resolved en face OCT/OCTA when evaluating macular ischemia. Subtle outer retinal AMN lesions can easily be missed due to shadowing artifacts cast by overlying hyperreflective PAMM bands.
• Maintain Vigilance for Coincident PAMM/AMN: When PAMM is identified on SD-OCT, systematically inspect the underlying outer retina and HFL for coincident AMN (Z-shaped hyperreflectivity or the ASHH sign), particularly in eyes presenting with Retinal Vein Occlusion, Purtscher’s/Purtscher-like retinopathy, CRAO, or unexplained paracentral scotomas.
• Account for Fluid Masking in Venous Occlusions: Be aware that early subretinal or intraretinal edema can physically mask outer retinal AMN hyperreflectivity. Re-evaluate structural B-scans following fluid absorption (e.g., post-anti-VEGF therapy) to detect unmasked AMN lesions.
• Mandate Comprehensive Systemic Workup: Identifying PAMM, AMN, or chronic “resolved PAMM” / RIPLs should prompt an immediate cardiovascular and hematologic evaluation. These retinal biomarkers frequently reflect systemic hypertension, vasculitis, hypercoagulability, or occult carotid and cardiac embolic sources.

---