17.2 Age and Height Effects on Nerve Conduction

Why age and height change the same numbers temperature just changed

Outline items VII.A.2 Age and VII.A.3 Height sit beside temperature because they move reference expectations, not because they are diseases. Independent OpenExamPrep teaching for the National Registry Examination for Nerve Conduction Studies is: an 80-year-old's small sural SNAP is not automatically polyneuropathy, and a 198 cm adult's long F wave is not automatically Guillain-Barré. This material is not an AAET normative booklet and does not publish unpublished official age-decade or height-centimeter cut scores.

Temperature still has to be right. Age and height never excuse a 24°C foot. When the limb is warm, the remaining questions are whether you brought the correct age bin and the correct stature table to the tracing.

Infants: slow conduction, then stop duplicating the pediatric chapter

Infants conduct slowly because myelination is incomplete, internodes are short, and myelin is thin. Typical electrodiagnostic teaching, already covered in section 6.3 Pediatric Nerve Conduction Studies, is that velocities rise through infancy and early childhood and that many nerves often approach the adult range by about 3–5 years — a developmental pattern, not an unpublished official cut year. Neonatal motor velocities are often discussed as being on the order of roughly half typical adult values for the same named nerve. Age-matched pediatric norms are required. Adult classroom floors near 50 m/s in the arm and 40 m/s in the leg do not apply to a 10-month-old.

This section recalls that fact so VII.A.2 is complete. It does not repeat the whole pediatric technique lecture. Short-segment millimeter error, infant cooling, co-stimulation in tiny cubital fossae, graded stimulation, and small electrodes live in chapter 6. If a registry item shows a toddler with slow CV against an adult table, the teaching answer is development plus the wrong table, not a canned CIDP protocol and not a second copy of 6.3.

Young children's SNAP amplitudes are often robust once myelination has advanced, because recording distances are short and subcutaneous tissue is thin. That is the opposite of the elderly SNAP problem below. Do not import an adult SNAP floor into a preschooler or an infant velocity floor into an adult.

Older adults: SNAP amplitude is the age-sensitive parameter

After childhood, the high-yield age effect is not a collapse of velocity. It is a decline in SNAP amplitude, especially in the lower limb.

Sural SNAPs shrink across later adulthood. In many laboratories, a very small or even absent sural SNAP can appear in a healthy 80-year-old, particularly with thick subcutaneous tissue, edema, or prior local trauma. Superficial fibular (peroneal) sensory studies follow the same direction. Upper-limb SNAPs (median, ulnar, radial) also fall with age but more often remain obtainable. CMAP amplitudes decline more mildly; a still-generous ADM CMAP in an older adult is not the same statement as a 2 µV sural.

Conduction velocity slows only mildly with age — textbook discussions often describe on the order of about 0.5–2 m/s per decade in later adulthood, as a teaching range, not as an unpublished official AAET slope. Distal latencies lengthen slightly. F-wave and H-reflex latencies lengthen modestly with age even after you account for height, but height still dominates those late responses (next heading).

Physiologic contributors taught in electrodiagnostic texts include loss of large sensory axons, increased temporal dispersion over long-used nerves, thicker skin and more tissue between nerve and recording electrodes, and milder motor-unit remodeling. You do not need a histopathology exam. You need the operational rule: do not diagnose length-dependent polyneuropathy from an isolated low sural SNAP in an 80-year-old without the rest of the picture.

The rest of the picture, when polyneuropathy is the question, still matters: symptoms, distal CMAPs, upper-limb SNAPs, velocities, late responses, and side-to-side comparison. Age explains a small sural. It does not erase a 1 mV fibular CMAP, a 28 m/s tibial motor CV, and an absent median SNAP in a 45-year-old.

Height: longer limbs, longer late responses, slightly slower CV

Taller people have longer nerves. A tibial F wave that travels to the sacral cord and back has more internodes to cross in a 200 cm adult than in a 150 cm adult. F-wave minimum latency (Fmin) and H-reflex latency therefore increase with height (and with limb length). Many laboratories' F-wave and H-reflex normal values are height-adjusted — a table or regression by centimeters, not a single universal upper limit.

Conduction velocity is only slightly slower in longer limbs in typical teaching, more noticeably in the legs. Proposed physiologic reasons include axonal tapering over a longer nerve and the extra internodal time of a longer path. The effect on ordinary forearm median motor CV is small compared with temperature. The effect on F and H latency is not small. That is why the parameter table below assigns SNAP amplitude to age and F latency to height.

Using a short person's F-wave upper limit on a very tall patient is a classic false-positive for proximal slowing. The reverse error exists too: using a tall-person limit on a short patient can hide a genuinely prolonged F. Match the height-stratified (or limb-length-stratified) table the laboratory actually collected. Chapter 8 taught how to record F waves; this section teaches why the normal column must know how tall the patient is.

Height is not a synonym for age. A tall 25-year-old can have a long but normal tibial Fmin and a large sural SNAP. A short 85-year-old can have a modest Fmin and a tiny sural SNAP. Mix those stories and you will stamp the wrong diagnosis on both.

Which parameters are age-sensitive versus height-sensitive

Worked numbers

Scenario A — elderly sural. An 80-year-old has no distal sensory symptoms. Palmar and plantar skin are warm. Sural SNAP is 3 µV (or technically absent at standard gain). Superficial fibular SNAP is similarly small. Median SNAP is 12 µV. Ulnar motor CMAP to ADM is 7.5 mV with CV 52 m/s. The exam trap is the sentence: absent sural proves polyneuropathy. Independent teaching: SNAP amplitude is age-sensitive; an isolated low sural in this age group needs clinical context and the rest of the NCS, not a reflex label.

Scenario B — tall F wave. A 198 cm adult has tibial Fmin 60 ms. A 155 cm adult in the same laboratory has a printed Fmin upper limit of 52 ms on a non-stratified sticky note. Applying 52 ms to the 198 cm patient calls the F wave prolonged. A height-adjusted table that lists an upper limit in the low 60s for that stature can place 60 ms inside expected range. These are illustrative laboratory values, not universal cutoffs. F latency is height-sensitive. The trap is using a short person's upper limit on a very tall patient.

Scenario C — infant recall. A 5-month-old ulnar motor CV is 28 m/s. An adult table floor of 50 m/s would scream demyelination. Section 6.3 already taught that incomplete myelination makes infant CV slow and that age-matched norms are required. VII.A.2 asks you to remember that fact, not to rewrite the pediatric chapter.

Scenario D — mixing drivers. A 160 cm, 82-year-old patient has tibial Fmin 47 ms and an unobtainable sural SNAP. A 190 cm, 30-year-old teammate has tibial Fmin 58 ms and an 18 µV sural SNAP. These example latencies illustrate the direction of the height effect; the laboratory’s height- and age-adjusted reference data determine normality. Age explains the SNAPs. Height explains the F waves. Swapping those attributions is the domain-VII error.

Scenario E — height and ordinary CV. Two warm, young adults differ by 25 cm of stature. Forearm median motor CV is 56 m/s in the shorter person and 53 m/s in the taller person. Both can be normal. Do not spend the case on a 3 m/s height effect while ignoring a 26°C palm. Height moves F and H more than it moves forearm CV.

Traps

• Diagnosing polyneuropathy from an 80-year-old's low or absent sural SNAP with otherwise reassuring studies and no clinical context
• Using a short person's F-wave upper limit on a very tall patient
• Applying adult CV cutoffs to infants (myelination, not mysterious neuropathy)
• Claiming height, not age, is what destroys SNAP amplitude
• Claiming age, not height, is what dominates F-wave latency
• Inventing unpublished official AAET age-decade or height-centimeter formulas
• Ignoring temperature and blaming age for a 25°C foot
• Treating this OpenExamPrep section as a full duplicate of pediatric section 6.3