Biomedical subjects

G A Horstmann

Publications and source records attributed to G A Horstmann.

At least 19 recordsLinked to original sources

PubMed1997

[Profile of ambulatory radiosurgery with the gamma knife system. 1: Method and multicenter irradiation concept].

Radiosurgery represents a minimally invasive radiologic method for the treatment of intracranial tumours and arteriovenous malformations. In 1994 the radiosurgical device 'Leksell Gamma Knife' (LGK) was installed in a dedicated logistic environment for outpatient treatments. High quality requirements have to be met for radiosurgery. The target point accuracy taking into account the whole system was shown to be reliably below 0.5 mm whereas the spatial therapeutic resolution was 0.035 cm3. Quality parameters of the dose plan were evaluated for the first 500 consecutive treatments. These values and examples of dose plans were used to emphasize the advantages of the treatment principle with multiple isocenters. An analysis of data in the literature revealed that there is no uniform standard of treatment available in radiosurgery. A highly significant correlation between a risk prediction model for the stereotactic linear accelerator on the one hand and a different model for the LGK on the other could be shown. This result could be helpful in order to proceed towards a more uniform treatment standard in radiosurgery and to improve overall treatment results.

Ambulatory Surgical Procedures↗

PubMed1997

[Profile of ambulatory radiosurgery with the gamma knife system. 2: Report of clinical experiences].

Gamma Knife radiosurgery (GKRS) was applied in 500 consecutive treatments for 445 patients within 2 years. Indications were arterio-venous malformations (93 patients), schwannomas of cranial nerves (75 patients), meningiomas (79 patients; 73 of the tumors involving the skull base), pituitary adenomas (40 patients), craniopharyngiomas (13 cases), gliomas (13 cases), rare indications (12 cases), and brain metastases (126 patients). In arterio-venous malformations two complications were observed whereas two other patients underwent surgery due to intracranial hemorrhage in the latent period after GKRS. In all cases follow-up with MRI showed evidence of an active obliteration process. Out of 24 patients with a follow-up over 1 year, angiography revealed complete obliteration in 9 patients so far. A partial obliteration was evidenced by MRI in 15 cases. In benign tumors (meningiomas and vestibular schwannomas) tumor control rates of 88% and 89% were achieved, respectively. Treatment related side effects were mild and rare; no facial palsy occured after primary Gamma Knife treatment. GKRS was particularly effective in inoperable skull base meningiomas. Cerebral metastases were controlled in 89.5% by a single Gamma Knife treatment. The mean survival period was 11.8 months. In patients receiving a single Gamma Knife treatment the mean survival time was 9.1 months. For patients undergoing multiple (up to 5) sessions of GKRS (because of new tumors) the mean survival period was 17.2 months. MRI showed evidence of adverse radiation reactions in 10/124 patients (8.1%) which were symptomatic in 3 patients (0.8%). The results obtained in patients with cerebral metastases emphasize that GKRS alone is as effective as the combined treatment of these lesions by surgery and fractionated radiotherapy. Our results demonstrated an attractively high therapeutic gain factor of Gamma Knife treatment in key indications of radiosurgery.

Ambulatory Surgical Procedures↗

PubMed1996

Computer assisted brain surgery for small lesions in the central sensorimotor region.

The capacity of a new optical navigation device is demonstrated by six microsurgical procedures for small subcortical lesions within the central sensorimotor strip. This small series is aimed at less invasive resection in this functionally critical region, independently of primary diagnosis and outcome. Guided by high resolution CT imaging data five brain tumours and one cavernous angioma was selectively located and most sparingly removed without additional sensorimotor deficit. In two cases improvement of a pre-operative paresis was observed immediately after surgery. Thanks to light-weight freehand pointing instruments and a ranging accuracy of +/- 1 mm, damage to functionally important brain areas and vessels was avoided by using uncommonly oblique, e.g., transsulcal ways of access which would hardly have been possible even with guidance by conventional stereotaxy. The demanding systematic cortical stimulation of the precentral gyrus applied in three cases was only sensitive in infiltrating tumours-e.g., low grade astrocytomas-where for want of adjuvant therapy it was essential to proceed to the extreme limits of resection. In general, precise anatomical localisation by computer aided surgery (CAS) is sufficient in small central lesions which guarantees minimally invasive surgery. The potential of this new, soon commercially available optical navigation system in (neuro) surgery, quality control and teaching is discussed.

Adult↗

PubMed1995

Microstereometrically guided cortical stimulation for the intraoperative identification of the central motor strip.

A rangable stimulation tool based on the sonar microstereometric system has been developed for the intraoperative identification of the central motor strip. The elicited motor evoked potentials (MEPs) were recorded with surface electrodes and displayed together with the corresponding stereometric coordinates of the stimulation site. MEP data were overlaid in 'real time' on preoperatively scanned CT images on a computer screen. The system has been used during open microsurgical procedures in 6 patients with small tumors near the central motor strip or related 'eloquent' areas. Three operations have been performed under local anesthesia, 3 under general anesthesia. It was possible to operate on tumors located closely to functionalal relevant areas of the brain not only stereotactically, but also with function guidance.

Adult↗

PubMed1994

Ranging accuracy test of the sonic microstereometric system.

We have recently introduced a new type of ultrasonic-based, three-dimensional ranging system for the localization of small handheld surgical tools to be used during operation in open stereotactic brain surgery. This targeting system has now been extensively laboratory tested during thousands of measuring cycles with a CNC-based measuring table. The typical accuracy found in a single direction was 0.4 mm and better. The accuracy vector in space was 0.897 mm (n = 2312). The test-retest accuracy was typically 0.4 mm. With the given accuracy, the system is suitable for all ranging tasks in open stereotactically guided microsurgery.

Brain Diseases↗

PubMed1993

Sonic stereometry in microsurgical procedures for deep-seated brain tumors and vascular malformations.

A frameless computerized navigating system was successfully employed in 20 cases of open microsurgical operations on deep-seated brain tumors and vascular malformations. Localization in space was made by measuring the traveling time of sonic waves (24 kHz) in air. This allowed the construction of mechanically simple, lightweight freehand targeting instruments. The tips can be localized with a measuring accuracy of +/- 1 mm intracranially on the basis of computed tomographic scans. This thoroughly redesigned sonic stereometrical device is being optimized for everyday use; magnetic resonance imaging, digital substraction angiography, and other diagnostic modalities will be implemented. Some perspectives on "computer-aided (neuro)surgery" are discussed.

Adolescent↗

PubMed1993

Stereo-microvision. Development of an opto-electronic operating microscope.

A novel opto-electronic operating microscope has been designed and clinically tested. It consists of a small camera microscope, a central electronic unit, and a stereoscopic video monitor. Advanced miniaturization permitted ergonomics superior to those of conventional optomechanical microscopes. Electronic imaging facilitates coupling to an ultrasound navigation system which enables the neurosurgeon to verify the location of the focus in real time, correlated with CT and MRI pictures. A fully computerized, digital operating microscope will now be developed based on this prototype.

Brain Diseases↗

PubMed1991

Compensation of human stance perturbations: selection of the appropriate electromyographic pattern.

Perturbations of stance evoke purposive EMG patterns which are directed to hold the body's centre of gravity over the feet. Dorsiflexing rotation of the feet is followed by a monosynaptic stretch reflex response in the gastrocnemius muscle, succeeded by a late compensatory tibialis anterior activation. Backward translation of the feet elicits only a compensatory polysynaptic EMG response in the gastrocnemius muscle, while an early gastrocnemius response is absent. The amplitude modulation of the gastrocnemius H-reflex has been investigated during the early part of the two modes of perturbation. Only during translational perturbation a progressive decrease in gastrocnemius H-reflex amplitude started within 5 ms after onset of displacement. The degree of the reduction in amplitude in the former perturbation was dependent on the displacement velocity. Only the contact forces (torques) differed between the two modes of perturbations within the first 10 ms after onset of perturbations. It is suggested that signals from pressure receptors within the body are responsible for the early change in H-reflex amplitude during translational perturbations and it is concluded that the simplest spinal reflex is under very rapid and powerful moment-to-moment control by changes in peripheral feedback. In view of a strong reciprocal modulation of monosynaptic and polysynaptic reflex responses, the later purposive EMG responses may be determined by early changes in presynaptic inhibition of group I afferents.

Adult↗

PubMed1991

[Microsurgical removal of deep vascular malformations using sonar-stereometry].

The advantages of a new, dynamic sonar technology in open stereotactic microsurgery are demonstrated by means of three surgical procedures for deep-seated vascular malformations that are not easy of access. With this method, targets can be aimed at without using rigid, obstructive pointing devices as in conventional stereotaxy. On the contrary, it is possible to take advantage of preformed anatomical spaces reaching the lesion most carefully by means of light-weight, free-hand on-target instruments. The spatial information, correlated with a CT data set, is displayed in real time with an accuracy of +/- 1 mm. Additional image data from MR and digital angiography can be used interactively.

Adult↗

PubMed1990

Interlimb coordination of stance in children: divergent modulation of spinal reflex responses and cerebral evoked potentials in terms of age.

EMG responses in the gastrocnemius (GM) and tibialis anterior muscles (TA) of both legs together with cerebral evoked potentials (CP), were recorded following perturbations of stance on a treadmill with split belts, in two age groups of children. Unilateral displacements were followed by ipsilateral short latency and bilateral long latency EMG responses. The CP was similar in both tasks. When displacements were simultaneously induced in opposite directions, a significant reduction in the long latency components of EMG responses occurred, while the amplitude of the CP was maximal in this condition. In the older children the CP and long latency EMG responses were larger and the short latency reflex potentials smaller in all conditions compared to the younger children. It is concluded that (1) CP and EMG responses reflect a divergent modulation of a given somatosensory input; (2) developmental changes are reflected in alterations in the amplitude of CP and EMG responses; (3) there is no evidence of transcortically mediated muscle responses.

Aging↗

PubMed1990

Gating and reversal of reflexes in ankle muscles during human walking.

Phase-dependent reflex modulation was studied by recording the electromyographic (EMG) responses in ankle flexors (Tibialis Anterior, TA) and extensors (Gastrocnemius Medialis, GM and Soleus, SOL) to a 20 ms train of electrical pulses, applied to the tibial or sural nerve at the ankle, in human volunteers walking on a treadmill at 4 km/h. For low intensity stimuli (i.e. 1.6 times perception threshold), given during the swing phase, the most common response was a suppression of the TA activity with a latency of 67 to 118 ms. With high intensity of stimulation (i.e. 2.8 x T), a facilitatory response appeared in TA with a latency of 74 ms. This latter response was largest during the middle of the swing phase, when it was correlated with exaggerated ankle dorsiflexion. The TA reflex amplitude was not a simple function of the level of spontaneous ongoing activity. During stance, TA responses were small or absent and accompanied by a suppression of the GM activity with a latency ranging from 62 to 101 ms. A few subjects showed an early facilitatory, instead of a suppressive, GM response (88 to 136 ms latency). They showed a phase-dependent reflex reversal from a dominant TA response during swing to a facilitatory GM response with an equivalent latency during stance. The GM facilitation occurred exclusively during the early stance phase and habituated more than the TA responses. It is concluded that phase-dependent gating of reflexes occurs in ankle muscles of man, but only when vigorous extensor reflexes are present. More commonly, a phase-dependent modulation is seen, both of facilitatory and suppressive responses.

Adolescent↗

PubMed1990

Reproducibility of electromyographic patterns in stretch-shortening type contractions.

Within a set of repeated measurements, 12 male subjects performed the following types of muscle contraction: (1) running at constant velocity (12 km.h-1); (2) one-leg hopping at the frequency preferred; (3) both-legs hopping at the frequency preferred; (4) drop jumps from a height of 0.40 m. The surface electromyographs (EMG) from the left and right legs were recorded, together with the averaged signals from vertical ground reaction forces and angular displacement of the ankle joint. Reliability coefficients for the day to day and the week to week comparison were calculated. In addition, on any one test day the positions of the electrodes were systematically changed. The analysis of various EMG parameters (integrated-EMG) revealed high coefficients for both comparisons as well as for the total set of measurements. Qualitative comparisons of the EMG showed a high reproducibility of the shape of the patterns. If the relative position of the electrodes with respect to the belly of the muscle is changed, the amplitudes of the EMG recordings are markedly reduced. It is concluded that surface EMG is a reliable method for studies of the neuromuscular system. On the basis of the data presented, it is suggested that the term "reliability" should be replaced by the term "reproducibility" in order to cover both the aspect of the relative stability plus the aspects of linear changes and the scattering of the data in repeated measurements.

Adult↗

PubMed1990

A basic posture control mechanism: the stabilization of the centre of gravity.

Holding the body's centre of gravity steady represents the crucial variable for the stabilization of posture in upright stance in man. The visual, vestibular and muscle proprioceptive systems have all been shown to contribute to sway stabilization. Nevertheless, earlier work has suggested that an additional receptor system is needed to signal the position of the body's centre of gravity relative to the support surface, i.e., the feet. This proposed receptor system should be 'gravity' dependent. To evaluate the properties of this receptor system, an experiment was designed under simulated 'microgravity', in water immersion. An approximately linear relationship was found between contact force and impulse directed EMG response amplitudes in the leg muscles. Out of water loading of the subjects resulted in no further increase of the response amplitude. A gain control mechanism for postural reflexes which is dependent on body weight was demonstrated. In a further experiment it could be shown that the receptors for this mechanism are distributed along the vertical axis of the body: it is suggested that these force-dependent receptors are pressure receptors within the joints and the vertebral column.

Adult↗

PubMed1990

[CT/MRI correlated stereometry with ultrasound in brain surgery].

Today the accuracy of medical imaging systems lies between +/- 0.5 mm (CT) and +/- 2.5 mm (MRI). In neurosurgery the overall uncertainty may reach the dimension centimetres. A non-tactile ultrasonic measurement system is described that allows three-dimensional measurement with a theoretical accuracy of +/- 0.1 mm in a frequency range between 50 and 70 kHz. In practice the accuracy is better than +/- 1 mm with a repetition rate of 10 per second for the x,y,z-data sets.

Angiography, Digital Subtraction↗

PubMed1989

Human postural reflexes and gravity--an under water simulation.

This study represents the first attempt to investigate the influence of gravity on postural adjustments. Subjects were displaced while standing under water on a movable platform, while the buoyancy of the body was adjusted by using a variety of lead vests. Under water, an approximately linear relationship was found between body weight and impulse directed electromyographic response amplitudes in the leg and thigh muscles. Loading of the subjects out of water resulted in a saturation of the response amplitude. The biomechanical signals recorded during the displacements indicated that neither vestibulospinal nor muscle proprioceptive reflex mechanisms can account for the effect observed under water. It is suggested that the EMG responses are mediated by reflexes which are activated by pressure receptors within the body in order to hold the centre of gravity over the feet.

Adult↗

PubMed1989

Compensation of translational and rotational perturbations in human posture: stabilization of the centre of gravity.

EMG responses in the leg muscles, head acceleration and joint movements induced by dorsiflexing rotation of the ankle have been analysed. Perturbations were induced while subjects were standing on a platform with the ankle joints co-linear with the axis of rotation or while standing above the axis (between 5 and 25 cm), which introduced an additional translational component to the displacement. The former condition was followed by a monosynaptic gastrocnemius reflex potential and a late tibialis anterior activation (latency about 100 ms); in the latter condition the monosynaptic reflex response became smaller and a longer latency gastrocnemius response (latency about 70 ms) appeared, the strength and duration of which increased in parallel with the translational component, while the amplitude of the tibialis anterior activity decreased. Neither vestibulospinal nor muscle proprioceptive mechanisms can solely account for this effect as the biomechanical parameters were little changed in the different conditions. It is suggested that the controlled variable in the investigated task is to hold the bodies' centre of gravity over the feet which makes the change in the pattern meaningful: pure dorsiflexion of the feet is followed by a backwards sway of the body, thus a tibialis anterior activation is functionally essential to hold the centre of gravity over the feet. A backwards translation of the feet withdraws the supporting surface, thus a gastrocnemius activation is needed to restore the centre of gravity over the feet. The inhibition of the monosynaptic reflex with increasing translation is suggested to arise from a reciprocal modulation of mono- and polysynaptic gastrocnemius reflex responses.

Acceleration↗

PubMed1989

Significance of proprioceptive mechanisms in the regulation of stance.

Compensatory electromyographic (EMG) responses and several biomechanical parameters were studied following impulsive disturbance of the limbs during stance of human volunteers on a treadmill. Treadmill acceleration impulses were backwards or forwards directed, or their initial direction was reversed after 30 ms. Backwards directed impulses were followed by gastrocnemius, forwards directed ones by tibialis anterior EMG responses (latency 65 to 75 ms) whose durations depended on impulse duration. When the direction of the impulse was reversed, the respective antagonistic leg muscles were activated again with a delay of 68 to 75 ms after onset of stretch of these muscles. The behaviour of the EMG responses could best be correlated to the displacement at the ankle joint and may be described in terms of a stretch reflex response. The results indicate that these stretch reflex responses help control of the body's centre of gravity thereby preventing falling. Head movements induced by the impulses showed little correlation with the appearance of the EMG responses, suggesting that the vestibular system is unlikely to be directly involved in the generation of these responses. Vestibular signals may, however, significantly contribute to slow body sway stabilization.

Adult↗

PubMed1989

Interlimb coordination of leg-muscle activation during perturbation of stance in humans.

1. Electromyographic (EMG) responses were recorded in both legs, along with corresponding joint movements, after uni- and bilateral perturbations during stance on a treadmill with split belts. Displacements were directed forward, backward, or in opposing directions. They were induced by randomly timed ramp impulses at one of four different rates of treadmill acceleration. 2. Unilateral perturbations directed backward were followed by a bilateral gastrocnemius-EMG response, forward-directed perturbations by a bilateral tibialis anterior-EMG response. The amplitude of these responses was dependent on the rate of treadmill acceleration. Relative to the response of the displaced leg, the amplitude of the EMG response on the nondisplaced side was smaller when a gastrocnemius EMG response was induced, and about equal when the tibialis anterior muscle was activated. The onset latencies were shorter on the displaced side (displaced leg 75-96 ms, non-displaced leg 93-112 ms). 3. Bilateral perturbations in one direction were followed by larger EMG responses in both legs (in the gastrocnemius for backward-directed impulses, in the tibialis anterior for forward-directed impulses). For a given acceleration rate, their amplitude was about equal to the sum of the EMG amplitude of the displaced leg and that of the nondisplaced leg obtained during unilateral displacement. The inverse result was obtained when the legs were simultaneously displaced in opposite directions: EMG responses in both legs were significantly smaller than those obtained after unilateral displacement. 4. It is concluded that a unilateral displacement evokes reflex EMG responses in the synergistic muscles of both legs, which are graded according to the size of the proprioceptive input from the primarily displaced joint. During bilateral displacements, the activity induced by the respective contralateral leg is linearly summed or subtracted, depending on whether the legs are displaced in the same or in opposite directions. In view of the short latencies of these bilateral responses, it would seem that they are mediated by a spinal mechanism. 5. Distinct differences in the behavior of the antagonistic leg muscles were observed: 1) the coactivation of the contralateral leg muscle was significantly smaller when the gastrocnemius was stretched unilaterally, whereas it was about equal for the tibialis anterior; and 2) the gastrocnemius EMG responses were closely correlated with the displacement velocity, whereas the tibialis anterior response was more closely correlated with acceleration, i.e., the tibialis anterior response was more dynamic in nature.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗