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The EEG generally slows

in metabolic encephalopathies, and triphasic waves may

be present.

5-1).

A.

B.

C.

Horizontal calibration: 1 s; verti-

cal calibration: 400 mV in A, 200 mV in B, and 750 mV in C.

(From MJ Aminoff, ed: Electrodiagnosis in Clinical Neurology,

5th ed.

5-1) or subacute scle-

rosing panencephalitis.

Its presence, latency, and symmetry

over the two sides of the scalp are noted.

The VEP findings are therefore

helpful in indicating previous or subclinical optic neuri-

tis.

Normal VEPs

may be elicited by flash stimuli in patients with cortical

blindness.

The EP findings are sometimes of prognostic rel-

evance.

The procedure is painless and apparently

safe.

Nevertheless, it is not used widely for clinical

purposes.

Within each motor unit, all of the

muscle fibers are of the same type.

Slight voluntary contraction of a muscle leads to acti-

vation of a small number of motor units.

5-3).

The number

of units activated depends on the degree of voluntary

activity.

A.

Spontaneous fibrillation

potentials and positive sharp waves.

B.

Complex repetitive

discharges recorded in partially denervated muscle at rest.

C.

Normal triphasic motor unit action potential.

D.

E.

Long-duration polyphasic motor unit action potential such as

may be seen in neuropathic disorders.

SECTION I

Introduction to Neurology

32 of voluntary activity.

5-3).

Such

information is important for prognostic purposes.

Various quantitative EMG approaches have been

developed.

The technique of single-

fiber EMG is discussed separately below.

They may suggest the underlying pathologic basis

in individual cases.

H-REFLEX STUDIES

The H reflex is easily recorded only from the soleus

muscle (S1) in normal adults.

In disorders of neu-

romuscular transmission this safety factor is reduced.

Elizabeth Robbins ■ Stephen L.

Hauser

35

In experienced hands, lumbar puncture (LP) is usually

a safe procedure.

Bleeding com-

plications rarely occur in patients with platelet counts

≥50,000/μL and an INR ≤1.5.

ANALGESIA

Anxiety and pain can be minimized prior to begin-

ning the procedure.

Topi-

cal anesthesia can be achieved by the application of a

lidocaine-based cream.

POSITIONING

Proper positioning of the patient is essential.

6-1 ).

The spinal cord terminates

at approximately the L1 vertebral level in 94% of indi-

viduals.

In the remaining 6%, the conus extends to the

L2-L3 interspace.

LP is therefore performed at or below

the L3-L4 interspace.

An alternative to the lateral recumbent position is the

seated position.

The patient sits at the side of the bed,

with feet supported on a chair.

The patient is instructed

to curl forward, trying to touch the nose to the umbi-

licus.

LP

is sometimes more easily performed in obese patients if

they are sitting.

A pause of ∼15 s

between injections helps to minimize the pain of the