2.2 True and Modified Montages for LTM

2.2 True and Modified Montages for LTM

In Long-Term Monitoring (LTM), raw electrical potentials acquired from scalp electrodes must be organized into logical channel displays known as montages. Montages are spatial and mathematical transformations that determine how cerebral voltage gradients, phase relationships, and localized epileptogenic networks are visualized.

Modern digital LTM systems record all electrode signals referentially against a single hardware acquisition reference and convert them into digital data. Technologists and neurophysiologists can dynamically re-montage the continuous recording into longitudinal bipolar, transverse bipolar, auricular referential, average referential, or Laplacian source derivations without altering the underlying raw data.

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1. Principles of Montage Design & Derivations in LTM

Every EEG channel represents the electrical potential difference between two distinct inputs connected to a differential amplifier:

$\text{Channel Output Voltage} = (V_{\text{Input 1}} - V_{\text{Input 2}}) \times \text{Gain}$

+-----------------------------------------------------------------------------+
|                     BIPOLAR VS REFERENTIAL DERIVATION                       |
|                                                                             |
|   BIPOLAR DERIVATION:                                                       |
|   Input 1 (Active Site A)  -----\                                           |
|                                  (-) ---> Differential Amplifier -> Display |
|   Input 2 (Adjacent Site B) ----/                                           |
|   * Localizes via PHASE REVERSAL at shared electrode                        |
|                                                                             |
|   REFERENTIAL DERIVATION:                                                   |
|   Input 1 (Active Site A)  -----\                                           |
|                                  (-) ---> Differential Amplifier -> Display |
|   Input 2 (Common Reference)---/                                            |
|   * Localizes via HIGHEST AMPLITUDE DEFLECTION                              |
+-----------------------------------------------------------------------------+

Core Montage Rules (ACNS montage and LTM guidance)

1. Standardized Channel Sequencing: Channels should be arranged in continuous, anatomical sequences from anterior to posterior (front to back) or left to right (transverse).
2. Hemispheric Grouping: The left hemisphere should be displayed above the right hemisphere, separated by a blank line or spacer to facilitate rapid inter-hemispheric comparison.
3. Channel Capacity: ACNS standards recommend a minimum of 21 recording channels, with modern EMU clinical systems utilizing 32 to 64 or more channels to accommodate high-density 10-10 arrays and polygraphic inputs.

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2. Bipolar Montages (Longitudinal & Transverse)

Bipolar montages connect adjacent scalp electrodes in linear, continuous chains where each electrode serves sequentially as Input 2 for one channel and Input 1 for the subsequent channel.

+-----------------------------------------------------------------------------+
|            LONGITUDINAL BIPOLAR MONTAGE ("DOUBLE BANANA") ARCS              |
|                                                                             |
|   LEFT HEMISPHERE:                     RIGHT HEMISPHERE:                    |
|   [Temporal Chain]  [Parasagittal]     [Parasagittal]     [Temporal Chain]  |
|     Fp1 - F7          Fp1 - F3           Fp2 - F4           Fp2 - F8        |
|      F7 - T3           F3 - C3            F4 - C4            F8 - T4        |
|      T3 - T5           C3 - P3            C4 - P4            T4 - T6        |
|      T5 - O1           P3 - O1            P4 - O2            T6 - O2        |
|                                                                             |
|   MIDLINE CHAIN: Fz - Cz, Cz - Pz                                           |
+-----------------------------------------------------------------------------+

Longitudinal Bipolar Montage (Anterior-Posterior / "Double Banana")

• Structure: Arranged in four parallel longitudinal anteroposterior chains plus a midline chain:
  1. Left Temporal Chain (Fp1-F7, F7-T3, T3-T5, T5-O1)
  2. Left Parasagittal Chain (Fp1-F3, F3-C3, C3-P3, P3-O1)
  3. Midline Chain (Fz-Cz, Cz-Pz)
  4. Right Parasagittal Chain (Fp2-F4, F4-C4, C4-P4, P4-O2)
  5. Right Temporal Chain (Fp2-F8, F8-T4, T4-T6, T6-O2)
• Localization Principle (Phase Reversal): When a focal negative spike occurs beneath an electrode shared by two adjacent channels (e.g., F7), Input 2 of channel 1 (Fp1-F7) is negative (causing a downward deflection), while Input 1 of channel 2 (F7-T3) is negative (causing an upward deflection). The two channels deflect toward each other, forming a pointing phase reversal that identifies F7 as the electrical focus.
• End-of-Chain Phenomenon: If a focus resides at the end of a chain (such as Fp1 or O1), no subsequent adjacent channel exists to generate an opposing deflection. A phase reversal cannot form, necessitating referential montage confirmation.

Transverse Bipolar Montage (Coronal)

• Structure: Arranged in lateral coronal chains moving strictly from left to right across the skull:
  1. Frontopolar Chain (Fp1-Fpz, Fpz-Fp2)
  2. Anterior Frontal Chain (F7-F3, F3-Fz, Fz-F4, F4-F8)
  3. Central / Bipreauricular Chain (T3-C3, C3-Cz, Cz-C4, C4-T4)
  4. Parietal Chain (T5-P3, P3-Pz, Pz-P4, P4-T6)
  5. Occipital Chain (O1-Oz, Oz-O2)
• Clinical Utility: Essential for distinguishing midline vertex spikes (Cz, Fz) from parasagittal discharges (C3, C4), and for mapping the lateral extent of widespread frontal epileptogenic zones.

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3. Referential Montages & Mathematical Derivations

Referential montages connect every exploratory scalp electrode (Input 1) to a shared reference electrode (Input 2). In referential displays, the location of a focal discharge is identified by the channel exhibiting the highest amplitude deflection (assuming an electrically inactive reference).

+-----------------------------------------------------------------------------+
|                      REFERENTIAL MONTAGE CONFIGURATIONS                     |
|                                                                             |
|   1. IPSILATERAL EAR (A1 / A2):                                             |
|      Left scalp -> Ref A1 (Left ear)   | Right scalp -> Ref A2 (Right ear)  |
|      * Excellent for temporal spikes; preserves inter-hemispheric balance   |
|                                                                             |
|   2. COMMON AVERAGE REFERENCE (CAR):                                        |
|      Ref = (V1 + V2 + V3 + ... + Vn) / n                                    |
|      * PITFALL: Massive focal spike at F3 injects inverted "ghost" spikes   |
|        into normal channels (O1, Cz, P4, etc.)                              |
|                                                                             |
|   3. LAPLACIAN / SOURCE DERIVATION:                                         |
|      Ref_i = Weighted average of nearest 4 orthogonal neighbors             |
|      * Acts as high-pass spatial filter; maximizes local cortex resolution  |
+-----------------------------------------------------------------------------+

Ipsilateral Auricular / Mastoid Reference (A1 / A2)

• Configuration: All left-hemisphere electrodes are referenced to A1 (left earlobe/mastoid); all right-hemisphere electrodes are referenced to A2 (right earlobe/mastoid).
• Advantages: Prevents cross-contamination across the midline; maintains true amplitude asymmetries between hemispheres; highly effective for temporal lobe recordings.
• Pitfall (Active Reference): If a broad epileptogenic field extends into the earlobe/mastoid (A1), the reference becomes electrically active. This leads to amplitude cancellation in adjacent temporal channels and paradoxical inverted deflections across distant frontal/occipital leads.

Linked Ears / Mastoids Reference ((A1 + A2) / 2)

• Configuration: A1 and A2 are physically or mathematically linked together to create a unified reference.
• Pitfall: Physical linking creates a low-resistance electrical bridge across the base of the skull, shunting current, distorting temporal field gradients, and artificially dampening genuine hemispheric amplitude asymmetries.

Vertex Reference (Cz)

• Configuration: All scalp electrodes are referenced to the midline vertex electrode Cz.
• Clinical Trap in LTM: Highly effective during wakefulness, but severely compromised during sleep. Sleep transients (vertex sharp waves, sleep spindles, and high-voltage K-complexes) originating at Cz are injected into Input 2 of every channel, contaminating the entire montage with inverted sleep transients.

Common Average Reference (CAR)

• Mathematical Formula: The reference potential is the arithmetic mean of all $N$ active electrodes on the scalp:

$V_{\text{Ref}} = \frac{1}{N} \sum_{i=1}^{N} V_i$

• Advantages: Excellent for mapping broad background rhythms, generalized spike-wave discharges, and spatial voltage distributions without favoring any single cranial region.
• The "Ghost Spike" Contamination Trap: If a high-voltage focal spike (e.g., 250 μV at F3) occurs in a 25-channel montage, it contributes $+10\ \mu\text{V}$ to the average reference. Consequently, every normal channel across the brain displays an artificial $-10\ \mu\text{V}$ deflection (an inverted "ghost" spike). The technologist must cross-verify findings on bipolar montages.

Source Derivation (Laplacian / Surface Spline Reference)

• Mechanism: Computes the spatial second derivative of the voltage field. The reference for each electrode is the distance-weighted average of its immediate surrounding orthogonal neighbors.
• Clinical Application: Acts as a powerful high-pass spatial filter that removes diffuse, volume-conducted distant potentials and sharply isolates superficial cortical generators. Ideal for extraoperative functional mapping and precise focal neocortical spike localization.

Comparative Reference Derivation Matrix

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4. Extended 10-10 True Temporal Electrodes (T1/T2, FT9/FT10)

In standard 10-20 montages, temporal electrodes (F7, T3/T7, T5/P7) reside along the superior and lateral temporal neocortex adjacent to the Sylvian fissure. However, the most common form of adult drug-resistant epilepsy—Mesial Temporal Lobe Epilepsy (MTLE)—originates in the hippocampus, amygdala, and parahippocampal gyrus along the inferior-mesial cranial base.

+-----------------------------------------------------------------------------+
|             ANATOMICAL PLACEMENT OF TRUE TEMPORAL SENSORS (T1 / T2)         |
|                                                                             |
|             Outer Canthus of Eye                                            |
|                     \                                                       |
|                      \  (Measure total distance = D)                        |
|                       \                                                     |
|                        x ---- [T1 / T2 Placed 1 cm Superior to this point]  |
|                         \     (Point is 1/3 distance from Meatus to Canthus)|
|                          \                                                  |
|                   External Auditory Meatus                                  |
+-----------------------------------------------------------------------------+

Placement Coordinates for T1 and T2

• Measurement Protocol:
  1. Identify the anatomical line connecting the external auditory meatus to the outer canthus of the eye.
  2. Measure one-third (1/3) the distance along this line moving anteriorly from the external auditory meatus.
  3. Measure 1.0 cm perpendicular/superior to this point on the left (T1) and right (T2) temporal regions.
• 10-10 System Equivalents: In modern digital 10-10 terminology, T1 corresponds closely to FT9 (and T2 to FT10). Additional inferior true temporal sites include F9/F10 and T9/T10.

Clinical Superiority in Temporal Lobe Epilepsy (TLE)

• Up to 30% to 40% of anterior/mesial temporal interictal spikes and initial electrographic seizure onsets are visualized maximally at T1/T2 (FT9/FT10) while appearing attenuated, delayed, or completely absent at standard F7/T3 leads.
• Anterior true temporal electrodes are mandatory in all comprehensive Phase I pre-surgical epilepsy monitoring evaluations.

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5. Polygraphic Channels in LTM

Comprehensive LTM requires continuous acquisition of non-cerebral physiological signals. Polygraphic channels are critical for identifying seizure semiology, evaluating autonomic alterations, scoring sleep architecture, and monitoring life-threatening clinical risks.

+-----------------------------------------------------------------------------+
|                        POLYGRAPHIC RECORDING CHANNELS IN LTM                |
|                                                                             |
|   1. ECG (Lead I / Lead II):                                                |
|      - Ictal Tachycardia (80% TLE), Ictal Bradycardia / Asystole, SUDEP     |
|                                                                             |
|   2. SURFACE EMG (Bilateral Deltoids / Tibialis / Submental):               |
|      - Tonic Posturing (Figure-of-4), Clonic Jerks, Epileptic Spasms, REM   |
|                                                                             |
|   3. RESPIRATORY EFFORT (Thoracic & Abdominal Inductance Bands):            |
|      - Central vs Obstructive Ictal Apnea, Postictal Hypoventilation        |
|                                                                             |
|   4. PULSE OXIMETRY (Continuous SpO2):                                      |
|      - Peri-ictal Hypoxemia (<90%), Airway Obstruction Risk Stratification  |
+-----------------------------------------------------------------------------+

Electrocardiogram (ECG)

• Electrode Placement: Modified Lead I or Lead II configuration. Standard placement: negative electrode below right clavicle; positive electrode over left lower rib cage (precordium).
• Diagnostic Imperatives:
  • Ictal Tachycardia: Occurs in >80% of temporal lobe seizures, often preceding electrographic scalp onset.
  • Ictal Bradycardia & Asystole: Severe parasympathetic activation (most common in left temporal lobe seizures) can induce sudden ictal asystole, causing syncopal collapse and requiring emergency pacing.
  • Differentiating ECG Artifact: ECG channels allow instant cross-correlation to confirm whether rhythmic temporal sharp waves are genuine cerebral spikes or volume-conducted cardiac QRS complexes.

Basic Cardiac Rhythm Recognition and EEG Consequences

The ECG polygraphic channel is not merely an artifact reference. It helps the technologist connect cerebral change with circulation and recognize a possible emergency. Describe the observed rhythm and correlate it with pulse oximetry, video, symptoms, and bedside monitors; formal cardiac diagnosis belongs to qualified clinical staff.

When an apparent arrhythmia appears, first confirm that the ECG lead is connected and compare the tracing with video, pulse/oximetry, and the bedside monitor. Annotate onset and symptoms, immediately use the unit’s critical-value or emergency pathway for sustained bradycardia, tachyarrhythmia, pause/asystole, loss of pulse, or clinical deterioration, and preserve the EEG evolution. Do not delay escalation while trying to name a rhythm precisely.

Surface Electromyography (EMG)

• Target Muscle Groups:
  • Bilateral Deltoids / Biceps: Evaluates upper extremity asymmetric tonic posturing (e.g., the "fencing posture" or "Figure-of-4 sign" localizing contralateral to the extended arm).
  • Anterior Tibialis: Detects lower extremity myoclonic jerking and periodic limb movements of sleep (PLMS).
  • Submental (Chin) EMG: Mandatory for sleep staging; demonstrates loss of muscle tone during REM sleep and assists in diagnosing REM Sleep Behavior Disorder (RBD).

Respiratory Effort & Airflow

• Sensors: Piezoelectric or Respiratory Inductance Plethysmography (RIP) dual thoracic and abdominal bands, combined with nasal pressure cannula or thermistors.
• Clinical Value: Detects central ictal apnea (cessation of respiratory drive triggered by seizure propagation to the amygdala/brainstem), obstructive airway collapse during hypermotor seizures, and Postictal Generalized EEG Suppression (PGES) hypoventilation.

Continuous Pulse Oximetry (SpO2)

• Integration: Real-time plethysmographic waveform and oxygen saturation percentage time-locked with EEG.
• SUDEP Risk Mitigation: Severe postictal hypoxemia (SpO2 < 85% or <90%) is a major biomarker for Sudden Unexpected Death in Epilepsy (SUDEP), prompting immediate nursing/technologist bedside intervention.