Effects of Botulinum Toxin on Muscle and Brain Activity

Primary Cervical Dystonia (PCD) is the most common type of focal dystonia. In addition to
pain, PCD is associated with disability in many activities of daily living; social stigma and
embarrassment; and decreased quality of life. Botulinum toxin (BoNT) therapy is the "gold
standard" for treatment of PCD. Although effective in improving dystonia symptoms, BoNT
injections have been associated with suboptimal improvements and the benefits of BoNT may
last shorter than the expected time frame of 12 weeks. PCD subjects are referred for deep
brain stimulation surgery if there is poor or inconsistent response to medical treatment. In
addition to the need for repetitive injections, subjects may suffer from side effects such as
neck pain, muscle weakness, head drop, breathing difficulty, and swallowing issues. BoNT
therapy outcomes are not likely to improve until and unless the investigators understand the
underlying mechanisms of action.

The primary goal of this study is to examine the physiological effects of BoNT therapy and to
advance the understanding of the pathophysiology of dystonia. BoNT therapy is commonly
perceived to induce peripheral muscle weakness through inhibition of acetylcholine release at
the neuromuscular junction. However many argue that this is not likely the only or primary
mechanism of action, as many subjects have improvement in dystonia without discernible muscle
weakness and others have significant weakness and no improvement in their dystonia. Indeed,
BoNT has been proposed to induce central effects possibly related to modulation of the muscle
spindle afferent feedback or a retrograde transport of toxin to the central nervous system. A
leading theory underpinning the pathophysiology of dystonia is loss of motor inhibition (or
increased excitability) at the level of the spinal cord, brainstem and the motor cortex.
Thus, modulation of pathology in these central pathways is critical for control of dystonia.
Transcranial magnetic stimulation (TMS) is a noninvasive physiological technique for
assessment of motor cortex excitability. Paired-pulse TMS paradigms, such as short-interval
intracortical inhibition (SICI) and intracortical facilitation (ICF) are well established
paradigms for evaluation of motor cortex excitability.

SICI is measured by delivering a subthreshold conditioning pulse prior to the suprathreshold
test pulse at short interstimulus intervals (ISI) of 1-5 milliseconds (ms) resulting in a
lower motor evoked potential (MEP) response to the test pulse. SICI is regarded as a
gamma-aminobutyric acid A (GABA-A) receptor-mediated inhibition that involves activation of
the cortical inhibitory interneurons. ICF is measured using a paradigm similar to SICI but
with a longer ISI of 8-30 ms resulting in increase in MEP response. Glutamate is probably
involved in producing ICF through cortical facilitation.

In focal dystonia, including PCD, there is failure of SICI recorded from hand muscles, and
conversely, there is enhanced ICF recorded from hand muscles. These paradigms were not
recorded from neck muscles as they are technically challenging. Nevertheless an important
finding was noted that in PCD, the motor cortical inhibition is widespread and extends beyond
the area of symptomatic muscles.

TMS was used to assess the effects of BoNT on SICI in subjects with arm dystonia. SICI in
distal hand muscle increases at one month after BoNT injections and returns to the previously
abnormal levels of excitability at three months. It can be speculated that the BoNT therapy
to arm muscles modulates the afferent input from muscles, which probably results in
reorganization of the motor cortex. It is not clear if the physiological change induced by
BoNT therapy had any correlation with the clinical improvement. In addition, it is not clear
if the change in motor cortex excitability ultimately affects the corticospinal drive to the
dystonic muscles.

In this study, the investigators will focus on the physiological effects of BoNT using
broader TMS measures of motor cortex excitability. The central hypothesis is that BoNT
modulates the motor cortex excitability and the corticospinal drive to the muscles and that
these physiological effects of BoNT will have a clear correlation with the clinical response.
To test this hypothesis, the investigators plan to measure the corticospinal drive to
dystonic muscles using electromyographic (EMG) spectral analysis. They will record the
clinical outcome with the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) which
is a standardized validated rating scale for PCD.

The first and second aim will focus on the physiological aspects of BoNT therapy in PCD. The
investigators plan to determine the cortical and corticospinal physiologic changes at the
time of peak BoNT effects (BoNT ON) which are typically seen around 4-8 weeks after the
injections and at the time of wearing off related to BoNT therapy which will correspond to
the time of the next injection cycle (BoNT OFF). The third aim will help the investigators
understand the physiological differences between clinical responders and non-responders.
Healthy controls will be enrolled for normative physiological data. The main significance of
this study is advancement of physiological knowledge related to BoNT therapy in subjects with
PCD.

Aim 1:

To determine the effect of BoNT therapy on the motor cortex excitability in PCD.

TMS measures (such as SICI, ICF,...) will be collected using standardized protocols at the
time of peak BoNT effects (BoNT ON) and at the time of trough BoNT effects (BoNT OFF).

Hypothesis 1:

The TMS measures will be normalized to healthy controls at the time of peak BoNT effects and
these effects will reverse once the BoNT effects wear off.

Aim 2:

To determine the effects of BoNT therapy on the corticospinal drive to the PCD muscles.

EMG spectral analysis for the auto-spectral peak of 4-7 Hertz (Hz) at the sternocleidomastoid
(SCM) and the 10-12 Hz coherence between the SCM and the splenius capitis (SPL) will be used
at the time of peak (BoNT ON) and trough BoNT effects (BoNT OFF).

Hypothesis 2:

The coherence between SPL and SCM muscles will be lost at the time of peak BoNT effects.
There will be a re-appearance of the auto-spectral peak in SPL muscle as seen in healthy
controls. These spectral analysis changes will reverse as the BoNT effects wear off during
trough.

Aim 3:

To determine the correlation between the physiological measures (TMS and EMG measures) during
peak BoNT effects and the clinical scores.

Hypothesis 3:

The change in TMS measures and EMG spectral findings at the time of peak BoNT effects will
correlate with the change in clinical score on the Toronto Western Spasmodic Torticollis
Rating Scale (TWSTRS) scale.

Subject inclusion criteria:

- Diagnosis: PCD

- Receiving BoNT at the University of Florida (UF)

Subject exclusion criteria:

- Secondary torticollis

- Pregnancy

- Active seizure disorder

- Presence of metallic body such as pacemaker, implants, metal rods and hearing aid

Control inclusion criteria:

- Age 21-80 years

Control exclusion criteria:

- Any form of torticollis

- Pregnancy

- Active seizure disorder

- Presence of metallic body such as pacemaker, implants, metal rods and hearing aid