SHORT INCREMENT SENSITIVITY INDEX (SISI) TEST

A Comprehensive Review — 50 Marks

1. INTRODUCTION AND HISTORICAL BACKGROUND

"Related tests, such as threshold tone decay and the short increment sensitivity index, were formerly used to a greater extent than they are currently." — Adams and Victor's Principles of Neurology, 12th Ed.

2. PHYSIOLOGICAL BASIS

2a. The Concept of Recruitment

2b. Neural Basis of Recruitment

• The outer hair cells (OHCs) of the organ of Corti act as cochlear amplifiers
• When OHCs are damaged (cochlear lesion), their amplifying function is lost, raising thresholds
• However, inner hair cells (IHCs) remain functional and respond normally at higher intensities
• This leads to a compressed dynamic range — loudness grows from threshold to maximum in a shorter dB range
• The result: a hypersensitivity to small intensity changes — exactly what SISI measures

3. PURPOSE AND PRINCIPLE

• A patient with cochlear damage (recruitment) detects even tiny 1 dB increments → HIGH SISI score (70–100%) → Positive SISI
• A patient with a normal ear or retrocochlear lesion (no recruitment) cannot detect 1 dB increments → LOW SISI score (0–30%) → Negative SISI

4. EQUIPMENT REQUIRED

• Calibrated pure-tone audiometer with ability to generate brief intensity increments (1 dB)
• The standard audiometer must be capable of:
  • Producing a continuous tone at a fixed level
  • Superimposing brief intensity increments of varying dB
  • Automatic or manual increment delivery
• The original Jerger et al. procedure used a specially modified audiometer; modern digital audiometers have SISI modes built in

5. TEST PROCEDURE — STEP BY STEP

PRE-TEST REQUIREMENT

• Pure-tone audiogram must be completed before SISI testing
• The test is performed for each ear separately (monaural test)
• Typically performed for frequencies: 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz (most informative at 1000 Hz and above)

FLOWCHART: SISI TEST PROCEDURE

┌─────────────────────────────────────────────────────┐
│           PURE TONE AUDIOGRAM COMPLETED              │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│    Set continuous tone at 20 dB SL (20 dB above     │
│    patient's pure-tone threshold at test frequency)  │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│     TRAINING / PRACTICE PHASE                       │
│  Present large increments: 5 dB increments first   │
│  (to familiarize patient with the sensation)        │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│   Patient is instructed to raise finger/press button│
│   every time they perceive a "momentary blip" or    │
│   brief increase in loudness                         │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│   MAIN TEST: Present 20 increments of 1 dB each    │
│   • Duration of each increment: 200 ms (0.2 sec)   │
│   • Rise/fall time: 50 ms                          │
│   • Inter-increment interval: 5 seconds             │
│   • All at 20 dB SL continuous carrier tone        │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│   SCORING                                           │
│   SISI Score = (No. of increments detected / 20)   │
│               × 100 = % score                       │
└──────────────────────┬──────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│              INTERPRET RESULTS                       │
└─────────────────────────────────────────────────────┘

6. TECHNICAL PARAMETERS (Standard Jerger Method)

7. SCORING AND INTERPRETATION

Score Calculation

Interpretation Table

INTERPRETATION FLOWCHART

          SISI Score Result
                │
    ┌───────────┼───────────┐
    ▼           ▼           ▼
  0–30%      31–69%      70–100%
    │           │           │
SISI          Equi-      SISI
Negative      vocal      Positive
    │           │           │
    ▼           ▼           ▼
Normal ear  Repeat      COCHLEAR
OR retro-   test at     LESION
cochlear    other       (Recruitment
(no         freqs /     present)
recruitment) batteries

8. HIGH-SISI vs. LOW-SISI: KEY DIFFERENTIATING FEATURES

HIGH SISI (Positive, Score 70–100%)

• Noise-induced hearing loss (NIHL)
• Ménière's disease / endolymphatic hydrops
• Presbycusis (cochlear type)
• Drug-induced ototoxicity (aminoglycosides, cisplatin)
• Hereditary cochlear degeneration (e.g., stria atrophy — positive SISI, flat audiogram, bilateral symmetrical SNHL — per KJ Lee's Essential Otolaryngology)

LOW SISI (Negative, Score 0–30%)

• Acoustic neuroma / vestibular schwannoma
• Lesion of CN VIII
• Central auditory pathway lesions
• Normal hearing individuals (cannot detect 1 dB at 20 dB SL)
• Conductive hearing loss (intermediate scores common)

9. THE SISI TEST IN CONTEXT: THE SITE-OF-LESION BATTERY

SITE-OF-LESION BATTERY
─────────────────────────────────────────────────────
Test                    Cochlear    Retrocochlear
─────────────────────────────────────────────────────
SISI                    HIGH        LOW
ABLB (Fowler test)      Recruitment  No recruitment
Tone Decay Test         Mild (<25dB) Marked (>25dB)
Békésy Audiometry       Type II      Type III / IV
Acoustic Reflex Decay   Absent       Present (rapid decay)
ABR                     Normal       Prolonged I–V
─────────────────────────────────────────────────────

"In cases of sensorineural hearing loss, there are tests that can help identify whether the site of lesion is cochlear or retrocochlear. The tests are both behavioral and physiologic, the latter being far more efficient. Objective physiologic procedures along with improved radiologic imaging have rendered the behavioral approaches virtually obsolete." — Cummings Otolaryngology, 6th Ed.

10. MODIFIED SISI TESTS

10a. High-Level SISI (HL-SISI)

• Test performed at 75 dB HL (rather than 20 dB SL)
• Differentiates sensorineural from conductive loss
• Normal ears and conductive losses show high scores (70–100%) at 75 dB HL
• Cochlear losses maintain high scores; retrocochlear lesions remain low

10b. SISI at Different Intensity Levels

• Threshold SISI: At very low sensation levels
• Suprathreshold SISI: At higher sensation levels
• Varying the level helps plot the recruitment function

10c. Low Frequency SISI

• SISI scores at 250 Hz and 500 Hz are less reliable
• Recruitment is more commonly detected at higher frequencies (2000–4000 Hz)
• More diagnostically useful for cochlear losses with high-frequency configuration

10d. Modified Increment Sizes

• 5 dB practice: all patients should score ~100%
• 2 dB intermediate
• 1 dB diagnostic

11. COMPARISON WITH OTHER RECRUITMENT TESTS

SISI vs. ABLB (Alternate Binaural Loudness Balance / Fowler Test)

SISI vs. Tone Decay Test

12. DIAGRAM: SISI TEST STIMULUS PATTERN

INTENSITY
(dB HL)
   │
25─┤                     ┌──┐           ┌──┐
   │                     │  │           │  │
24─┤                     │  │           │  │
   │                     │  │           │  │
23─┤                     │  │           │  │        ← 1 dB increment
   │                     │  │           │  │           (200 ms duration)
22─┤─────────────────────┘  └───────────┘  └───────  ← Carrier tone
   │                                                    at 20 dB SL
21─┤
   │
   └────────────────────────────────────────────── TIME
        ◄── 5 sec ──►    ◄── 5 sec ──►
        inter-increment  inter-increment
        interval         interval

13. FACTORS AFFECTING SISI SCORES

1. Hearing threshold level — The test must be done at the correct sensation level (20 dB SL)
2. Frequency tested — High frequencies (2000–4000 Hz) give more reliable results
3. Patient attentiveness and cooperation — False positives if over-responsive; false negatives if inattentive
4. Type of audiometer — Must be properly calibrated
5. Background noise — Can mask the 1 dB increment
6. Degree of hearing loss — Patients with very steep audiograms may give atypical results
7. Conductive component — Middle ear pathology can affect scores (usually intermediate/low)
8. Age — Elderly patients may be less reliable responders

14. CLINICAL APPLICATIONS

When to Order SISI:

• Unilateral or asymmetric SNHL to differentiate cochlear vs. retrocochlear cause
• Suspected acoustic neuroma (vestibular schwannoma)
• Ménière's disease evaluation
• NIHL assessment
• Pre-surgical audiologic workup
• Medicolegal evaluation of hearing

Conditions and Expected SISI Results:

15. LIMITATIONS OF THE SISI TEST

1. Overlap zone (30–70%) — Wide equivocal range reduces diagnostic certainty
2. Cannot differentiate conductive from other losses — Both may show low/intermediate scores
3. Not pathognomonic — A single test result is insufficient without a test battery
4. Patient cooperation essential — Purely behavioral, subjective test
5. Less sensitive and specific than modern electrophysiological tests (ABR, OAE, acoustic reflexes)
6. Not useful in profound hearing loss — Patients cannot perform the test
7. False negatives in cochlear lesions with very high thresholds (difficult to achieve adequate 20 dB SL level)
8. Frequency-dependent — Less reliable at 250 and 500 Hz
9. Replaced by ABR and MRI for retrocochlear evaluation in current clinical practice

16. RECENT ADVANCES AND CURRENT STATUS

Current Position of SISI in Modern Audiology

1. Auditory Brainstem Response (ABR) — The gold standard for retrocochlear assessment. Prolonged I–V interpeak latency or absent waves indicate retrocochlear pathology with high sensitivity
2. Otoacoustic Emissions (OAEs) — Directly assess outer hair cell function; absent in cochlear lesions, present in retrocochlear lesions
3. Acoustic Reflex Testing — Acoustic reflex decay is a superior test of auditory adaptation and is less time-consuming than tone decay or SISI
4. MRI with gadolinium — Definitive investigation for acoustic neuroma/vestibular schwannoma
5. ASSR (Auditory Steady State Response) — For threshold estimation

"As in routine audiologic evaluations, site-of-lesion testing involves a battery approach... Objective physiologic procedures along with improved radiologic imaging have rendered the behavioral approaches virtually obsolete." — Cummings Otolaryngology Head and Neck Surgery

SISI Still Valuable In:

• Low-resource settings without electrophysiology equipment
• Cross-checking and corroborating other audiologic findings
• Historical/academic understanding of recruitment
• Medicolegal examinations in certain jurisdictions
• Teaching audiologic principles of recruitment

17. SUMMARY TABLE: SISI AT A GLANCE

18. IMPORTANT QUESTIONS FROM RGUHS PERSPECTIVE

Q: What is the SISI test? What does a high score indicate?

Q: What is the principle of recruitment used in SISI?

Q: How does SISI differentiate cochlear from retrocochlear lesion?

REFERENCES (Textbook Sources)

1. Jerger JF, Shedd JL, Harford E (1959) — Original description of SISI test
2. KJ Lee's Essential Otolaryngology, 12th Ed — Behavioral Tests for Site of Lesion; SISI under Stria Atrophy
3. Cummings Otolaryngology Head and Neck Surgery, 6th Ed — Audiologic Test Battery, Summary of Diagnostic Audiology Tests and Applications (Table 134.1)
4. Adams and Victor's Principles of Neurology, 12th Ed — Special Audiologic Procedures; SISI noted as formerly used test now superseded
5. Scott Brown's Otorhinolaryngology — SISI in site-of-lesion battery context
6. Dhingra PL — Diseases of Ear, Nose and Throat: SISI as recruitment test
7. Hazarika P — Textbook of Ear, Nose and Throat: SISI in cochlear vs. retrocochlear differentiation
8. Zakir Hussain — Otolaryngology Review: SISI scoring and interpretation
9. Stell & Maran — Head and Neck Surgery: Audiologic assessment battery