The Terminal Latency Index (TLI) compares distal with proximal conduction. Disproportionate distal slowing lowers the index - useful in CTS, anti-MAG, and demyelinating polyneuropathies.
TLI = Distal distance (mm) / [MCV (m/s) × DML (ms)]
| Nerve | Dist. (mm) | DML-D (ms) | MCV-D (m/s) | TLI (D) | DML-E (ms) | MCV-E (m/s) | TLI (E) |
|---|
What is the Terminal Latency Index (TLI) and how to interpret it
The TLI compares distal vs proximal conduction: TLI = VCM/LMD (dimensionless). Values ≥ 0.34 are normal; 0.25–0.33 moderate distal slowing; ≤ 0.25 in two nerves suggests anti-MAG/DADS phenotype.
Reference values
| TLI | Interpretation | Clinical context |
|---|---|---|
| ≥ 0.34 | Normal | Proportional conduction |
| 0.25–0.33 | Moderate distal slowing | CTS, focal demyelination |
| ≤ 0.25 (≥2 nerves) | Severe distal slowing | Anti-MAG/DADS (~90% spec.) |
| < 0.29 (median) | Superimposed CTS | Diabetes + wrist compression |
The Terminal Latency Index (TLI) is an advanced quantitative neurophysiological tool used to assess and compare conduction in the most distal segment of a peripheral nerve relative to its proximal segment. This calculator automatically processes motor nerve conduction data entered by the examiner, supporting clinical interpretation and differential diagnosis of demyelinating polyneuropathies.
1. What Does the Index Reflect?
The TLI is essentially a ratio that predicts what terminal latency should be if conduction velocity were perfectly uniform along the nerve. When demyelination or axonal injury is disproportionately worse in the distal portion (near the muscle) than in the forearm or leg, distal latency increases markedly without a proportional drop in proximal conduction velocity. This “pulls” the index down, revealing disproportionate distal slowing.
2. Formula Used
To quantify this phenomenon, the calculator uses three parameters from routine motor conduction studies:
- Distal distance (mm): skin distance between the distal stimulator cathode and the active recording electrode (E1) on the muscle.
- Motor conduction velocity - MCV (m/s): velocity calculated in the proximal segment (e.g. elbow to wrist).
- Distal motor latency - DML (ms): conduction time in the terminal segment.
TLI = Distal distance (mm) / [MCV (m/s) × DML (ms)]
3. Result Interpretation
Interpretation of TLI results is based on the following cut-offs:
- ≥ 0.34: normal - distal conduction proportional to proximal.
- 0.25 – 0.33: moderate distal slowing (e.g. CTS, demyelination).
- ≤ 0.25: severe distal slowing - markedly prolonged terminal conduction.
4. Differential Diagnosis Utility
IgM Anti-MAG Neuropathy (DADS phenotype)
The TLI is the main electrophysiological tool for this suspicion. Pathology disproportionately affects distal myelin, yielding very low TLI (≤ 0.25 or 0.26). Finding this in two nerves (e.g. median and ulnar) confers ~90% specificity for this paraprotein disorder.
Chronic Inflammatory Demyelinating Polyradiculoneuropathy (CIDP)
In CIDP, slowing is usually more proximal or segmental. TLI tends to be higher, typically 0.35 to 0.45. Only ~16% of CIDP patients will have TLI ≤ 0.25 in two nerves.
Hereditary neuropathies (e.g. CMT1A)
In these genetic disorders, myelin loss is uniform, producing “uniform slowing” along the nerve. Because distal conduction is as slow as proximal, the ratio is preserved and TLI remains normal (≥ 0.25) in classic cases.
Carpal Tunnel Syndrome (CTS) in Diabetics
TLI also falls in CTS due to focal block at the wrist. In patients with underlying diabetic polyneuropathy, median TLI < 0.29 strongly suggests superimposed carpal tunnel compression.
5. Additional Advantage
TLI values do not correlate with patient height - unlike isolated F-wave latency or absolute distal latencies - giving this calculation universal applicability independent of individual anatomy.
- Bhala RP et al. Terminal latency index in carpal tunnel syndrome. Arch Phys Med Rehabil. 1987;68:287–291.
- Kaku DA, England JD, Sumner AJ. Distal accentuation of conduction slowing in polyneuropathy associated with antibodies to myelin-associated glycoprotein. Brain. 1994;117:941–947.
- Pouget J et al. Terminal latency index as a criterion for diagnosis of polyneuropathy associated with IgM anti-MAG. Muscle Nerve. 1996;19:916–917.
- Cappelen-Smith C et al. Activity-dependent hyperpolarization in chronic inflammatory demyelinating polyneuropathy. J Neurol Neurosurg Psychiatry. 2002;73(2):179–184.
Frequently asked questions
How to calculate the terminal latency index of the median nerve?
TLI = motor conduction velocity (MCV) divided by distal motor latency (DML) at the same stimulation distance (e.g. 70 mm wrist–APB).
When is TLI abnormal in carpal tunnel syndrome?
TLI < 0.34 indicates disproportionate distal slowing. In diabetics, TLI < 0.29 in the median nerve suggests superimposed CTS.
Does low TLI indicate anti-MAG neuropathy?
TLI ≤ 0.25 in two or more nerves has ~90% specificity for IgM anti-MAG associated neuropathy (DADS phenotype).