Scaphoid fracture non-union: a systematic review of the arthroscopic management

BACKGROUND AND AIM

There is no consensus regarding the most appropriate treatment of scaphoid nonunion. This systematic review aimed to investigate whether wrist arthroscopy exerts a positive influence on bone union and clinical outcomes.

METHODS

We searched the literature on Medline (PubMed), Web of Science, Embase and Scopus databases using the combined keywords "scaphoid" AND "arthroscopy" AND "pseudoarthrosis" OR "nonunion". Eighteen studies were finally included in our review. The quality of the studies was assessed using the Coleman Methodological Score.

RESULTS

Our systematic review has shown that arthroscopic management of scaphoid nonunion achieves a high rate of union and satisfactory clinical outcomes with minimal complications.

CONCLUSIONS

There is need to perform randomized controlled trials reporting on the use of arthroscopy. In addition, the different pattern of pseudoarthrosis should be better classified to manage the patients who will benefit after the management.

Role of lateral soft tissue release in percutaneous hallux valgus surgery: a systematic review and meta-analysis of the literature

BACKGROUND

It is unclear whether lateral soft tissue release (LSTR) is required as part of percutaneous hallux valgus (PHV) surgery. The primary aim of this systematic review was to assess whether LSTR reduces the risk of recurrence of hallux valgus deformity. The secondary aims were to assess if LSTR increases the risk of complications, improves the clinical outcome and leads to a greater radiographic correction.

METHODS

We performed a PRISMA-compliant PROSPERO-registered systematic review, pooling clinical papers reporting results after PHV surgery into two categories (PHV with (Group 1, G1) and without LSTR (Group 2, G2)) and comparing them. Data regarding the study design, demographics, the surgical procedure and the clinical and radiological outcome were extracted and compared. Risk of bias was assessed using the modified Coleman Methodology Score (mCMS).

RESULTS

Sixteen studies were selected (G1:594 feet; G2:553 feet). The pooled proportion of recurrence at a minimum 21-month follow-up (2%, 95%CI 0-3 vs 2%, 95%CI 0-5; p = 0.70) did not differ in the two groups. Similarly, the pooled proportion of complications (27%, 95%CI 17-38 vs 25%, 95%CI 12-37; p = 0.79) was similar. The pre- (p = 0.23) and post-operative AOFAS scores (p = 0.16), the pre-(HVA: p = 0.23) (IMA: p = 0.94) and post-operative radiological angles (HVA: p = 0.47) (IMA: p = 0.2) and the methodological quality of studies (p = 0.2) did not differ either between G1 and G2.

CONCLUSION

There is no evidence that LSTR performed during percutaneous HV surgery reduces the risk of recurrence of the deformity at a mean 4-year follow-up nor improves the clinical and radiological outcome.

LEVEL OF EVIDENCE

Level IV systematic review of Level I to IV studies.

CLINICAL RESULTS OF COLLAGENASE TREATMENT FOR DUPUYTREN'S DISEASE: A CASE SERIES STUDY WITH 2-YEARS FOLLOW-UP

Objectives

This study aims to report our experience with Clostridium Histolyticum collagenase (CCH) to support the importance of its clinical use and assess its clinical efficacy, complications, and recurrences.

Methods

This prospective observational study of 66 patients with a 2-year follow-up. Patients with an extension lag major of 20° at the metacarpophalangeal joint (MPJ) and/or proximal interphalangeal joint (PIPJ) were included. We collected data on demographic and anamnestic details, MPJ and PIPJ contracture degrees, DASH score, complications, and recurrences.

Results

The mean pre-injection contracture was 34° for MPJ and 31° for PIPJ. At the 2-year follow-up, the mean contracture for the MPJ and PIPJ were respectively 3° and 14.5°. The mean DASH score decreased from 21.8 before injection to 10,4 after 2 years. The disease recurrence occurred in 34.8% of the patients, all with PIPJ contracture. The main complication was skin breakage (25.7%).

Conclusion

The CCH injections remain a consistent option in treating DD; withdrawal from the European market deprives surgeons and patients of low invasiveness and safe tool for treating DD. Level of evidence IV, Therapeutic study investigating treatment results, Case series .

Diagnostic Performance of Neutrophil to Lymphocyte Ratio, Monocyte to Lymphocyte Ratio, Platelet to Lymphocyte Ratio, and Platelet to Mean Platelet Volume Ratio in Periprosthetic Hip and Knee Infections: A Systematic Review and Meta-Analysis

The current literature on the diagnosis of periprosthetic joint infection provides controversial evidence on the diagnostic accuracy of MLR, NLR, PVR, and PLR. Therefore, this critical literature search and meta-analysis was aimed to summarize the diagnostic accuracy of these biomarkers for the diagnosis of hip and knee prosthetic infection. According to the PRISMA flowchart, we searched MEDLINE, Scopus, and Web of Science, for studies on these ratios for diagnosing PJI. Sensitivity, specificity, positive and negative likelihood ratio, diagnostic odds ratio, and AUC were analyzed. We included 11 articles in our meta-analysis, including 7537 patients who underwent total hip and knee arthroplasties; among these, 1974 (26%) patients reported a joint infection. The pooled sensitivity and specificity were 0.72 and 0.74, respectively, for NLR, 0.72 and 0.77 for PVR, and 0.77 and 0.75 for PLR. The sensitivity of MLR ranges from 0.54 to 0.81, while the specificity ranges from 0.78 to 0.81. Regarding the evaluation of AUCs, the best diagnostic performance was achieved by MLR (AUC = 0.77) followed by PLR (AUC = 0.75), NLR (AUC = 0.73), and PVR (AUC = 0.70). This meta-analysis demonstrates a fair diagnostic accuracy of these ratios, thus not being useful as a screening tool.

Pyrocardan® interpositional arthroplasty for trapeziometacarpal osteoarthritis: a minimum four year follow-up

BACKGROUND

Pyrocardan® (Wright Medical-Tornier) is a pyrocarbon implant proposed in the treatment of trapeziometacarpal joint (TMCJ) osteoarthritis. Our aim was to assess the clinical and radiographic results after Pyrocardan® arthroplasty at midterm follow-up.

METHODS

In this prospective monocentric study, we enrolled 119 patients treated with Pyrocardan® for TMCJ osteoarthritis and followed up at a minimum of four years. The clinical outcome was assessed through the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire, the Visual Analog Score (VAS) for pain and the Kapandji score collected pre-operatively, at three, six and 12 months, then yearly. Hand radiographs were taken before surgery, at three months and every year. Complications and re-operations were also recorded.

RESULTS

The mean follow-up was 5.2 years (range, 4-9). DASH, VAS and Kapandji scores significantly improved at three (p < 0.001 in all cases) and six months (p < 0.001, p = 0.01 and p < 0.001, respectively), remaining stable over time. The dislocation and subluxation rates were 3.3% (4 cases) and 16.8% (20 patients), respectively. The two year, four year and seven year survivorship of the implant was 99%, 98% and 95%, respectively.

CONCLUSION

Pyrocardan® arthroplasty provides a satisfactory clinical and radiographic outcome for treating TMCJ osteoarthritis, with a 97% survival rate at four years. We advocate comparative studies with more common techniques (i.e., trapeziectomy) to verify its cost-effectiveness.

Adaptive proximal scaphoid implant (APSI): a systematic review of the literature

Scapholunate advanced collapse collapse (SLAC) is a challenging topic for hand surgeons. The adaptative proximal scaphoid implant (APSI) (Bioprofile-Tornier) is a pyrocarbon ovoid shaped interpositional implant, that allows adaptive mobility during motion. The aim of this systematic review is to analyze the clinical and radiological outcomes of APSI implants and possible complications. We performed a literature search combining the following key-words: "APSI", "Scaphoid's proximal pole", "implant", "scaphoid avascular necrosis", "SLAC", "SNAC", "pyrocarbon", "prosthesis", and "spacer" with no limitations for year of publication. We selected seven studies considered relevant to our systematic review. All studies described an improvement in the grip strength and the flexion extension arch compared to pre-operative values. The percentage of patients who reported progression of osteoarthritis (OA) with APSI was 17.3%, and implant's mobilization has a rate 5.1% (8/156). In conclusion the APSI implant is a reliable alternative for the treatment of SNAC wrist and SLAC wrist.

Using non-invasive neuroimaging to enhance the care, well-being and experimental outcomes of laboratory non-human primates (monkeys)

Over the past 10-20 years, neuroscience witnessed an explosion in the use of non-invasive imaging methods, particularly magnetic resonance imaging (MRI), to study brain structure and function. Simultaneously, with access to MRI in many research institutions, MRI has become an indispensable tool for researchers and veterinarians to guide improvements in surgical procedures and implants and thus, experimental as well as clinical outcomes, given that access to MRI also allows for improved diagnosis and monitoring for brain disease. As part of the PRIMEatE Data Exchange, we gathered expert scientists, veterinarians, and clinicians who treat humans, to provide an overview of the use of non-invasive imaging tools, primarily MRI, to enhance experimental and welfare outcomes for laboratory non-human primates engaged in neuroscientific experiments. We aimed to provide guidance for other researchers, scientists and veterinarians in the use of this powerful imaging technology as well as to foster a larger conversation and community of scientists and veterinarians with a shared goal of improving the well-being and experimental outcomes for laboratory animals.

Thumb basal joint interpositional arthroplasty: a systematic review

The aim of this systematic review was to analyze the failure rates among different trapeziometacarpal interposition implants used to treat thumb basal joint osteoarthritis. We searched Medline (PubMed), Web of Science and Scopus databases, to identify articles reporting on thumb interpositional arthroplasty, in English literature. We excluded studies with less than 35 cases and with a follow-up shorter than 24 months. Twenty-one studies were included. We assessed the quality of the studies using the Coleman Methodological Score. The mean quality of the studies was moderate. The total number of procedures included in this review was 1205. The failure rate for interposition implants was 11%. The main longterm complication was dislocation, which is also the major reason for revision.

Volar distal radius vascularized bone graft vs non-vascularized bone graft: a prospective comparative study

The pseudoarthrosis (PSA) of scaphoid leads to alteration in load transfer in the wrist joint. Its treatment aims to achieve consolidation to improve clinical complaints and prevent post-traumatic arthritis. The indication for using vascularized bone grafts is still controversial. This prospective comparative study aimed to compare consolidation rate and time to healing of scaphoid PSA treated by volar distal radius vascularized bone graft vs non-vascularized iliac bone graft. Nine patients underwent vascularized grafting of scaphoid PSA. These patients were compared to a control group consisting of twelve patients treated with iliac crest-free bone graft. PSA consolidation was obtained in 8 of 9 patients (88%) and 9 of 12 patients (75%) in the study and control group, respectively. The difference in consolidation rate was not significant. Two of three patients with AVN of the proximal pole in the study group (66%) went to consolidation. In the control group no patient with AVN obtained bone consolidation. This difference almost reached statistical significance (p = 0.083). The mean time to consolidation was 8.6 weeks (range 8-11) and 11.7 weeks (range 10-16), respectively, in the study and control group. This difference was significant (p < 0.05). In conclusion, the distal radius vascularized graft led to satisfactory consolidation rate of PSA in the current study, even in cases of AVN of the proximal pole. Moreover, the vascularized bone graft resulted in shorter healing time compared to the non-vascularized graft.

Exploring the role of the substantia nigra pars reticulata in eye movements

Experiments that demonstrated a role for the substantia nigra in eye movements have played an important role in our understanding of the function of the basal ganglia in behavior more broadly. In this review we explore more recent experiments that extend the role of the substantia nigra pars reticulata from a simple gate for eye movements to include a role in cognitive processes for eye movements. We review recent evidence suggesting that basal ganglia nuclei beyond the substantia nigra may also play a role in eye movements and the cognitive events leading up to the production of eye movements. We close by pointing out some unresolved questions in our understanding of the relationship of basal ganglia nuclei and eye movements.

A case report on fixation instability in Parkinson's disease with bilateral deep brain stimulation implants

We report on fixation instabilities in a patient diagnosed with Parkinson's disease (PD). This patient underwent deep brain stimulation (DBS) surgery bilaterally in the vicinity of the subthalamic nuclei (STN). Examination of the eye movements of this patient revealed marked fixation instability compared with a healthy age matched control. The eye movements occurring during fixation differed from other reports of fixation instabilities in that they interrupted fixation for only brief durations. These interruptive saccades (IS) had saccade-like amplitude velocity relationships. The frequency of these IS was higher in the patient with PD than in the healthy age matched control. Furthermore, the frequency of the IS in the patient reduced toward control with application of bilateral DBS in the vicinity of the STN. From our observations we conclude that fixation ability may be altered in PD and improved with DBS.

Activation and inactivation of rostral superior colliculus neurons during smooth-pursuit eye movements in monkeys

Neurons in the intermediate and deep layers of the rostral superior colliculus (SC) of monkeys are active during attentive fixation, small saccades, and smooth-pursuit eye movements. Alterations of SC activity have been shown to alter saccades and fixation, but similar manipulations have not been shown to influence smooth-pursuit eye movements. Therefore we both activated (electrical stimulation) and inactivated (reversible chemical injection) rostral SC neurons to establish a causal role for the activity of these neurons in smooth pursuit. First, we stimulated the rostral SC during pursuit initiation as well as pursuit maintenance. For pursuit initiation, stimulation of the rostral SC suppressed pursuit to ipsiversive moving targets primarily and had modest effects on contraversive pursuit. The effect of stimulation on pursuit varied with the location of the stimulation with the most rostral sites producing the most effective inhibition of ipsiversive pursuit. Stimulation was more effective on higher pursuit speeds than on lower and did not evoke smooth-pursuit eye movements during fixation. As with the effects on pursuit initiation, ipsiversive maintained pursuit was suppressed, whereas contraversive pursuit was less affected. The stimulation effect on smooth pursuit did not result from a generalized inhibition because the suppression of smooth pursuit was greater than the suppression of smooth eye movements evoked by head rotations (vestibular-ocular reflex). Nor was the stimulation effect due to the activation of superficial layer visual neurons rather than the intermediate layers of the SC because stimulation of the superficial layers produced effects opposite to those found with intermediate layer stimulation. Second, we inactivated the rostral SC with muscimol and found that contraversive pursuit initiation was reduced and ipsiversive pursuit was increased slightly, changes that were opposite to those resulting from stimulation. The results of both the stimulation and the muscimol injection experiments on pursuit are consistent with the effects of these activation and inactivation experiments on saccades, and the effects on pursuit are consistent with the hypothesis that the SC provides a position signal that is used by the smooth-pursuit eye-movement system.

Discharge properties of neurons in the rostral superior colliculus of the monkey during smooth-pursuit eye movements

The intermediate and deep layers of the monkey superior colliculus (SC) comprise a retinotopically organized map for eye movements. The rostral end of this map, corresponding to the representation of the fovea, contains neurons that have been referred to as "fixation cells" because they discharge tonically during active fixation and pause during the generation of most saccades. These neurons also possess movement fields and are most active for targets close to the fixation point. Because the parafoveal locations encoded by these neurons are also important for guiding pursuit eye movements, we studied these neurons in two monkeys as they generated smooth pursuit. We found that fixation cells exhibit the same directional preferences during pursuit as during small saccades-they increase their discharge during movements toward the contralateral side and decrease their discharge during movements toward the ipsilateral side. This pursuit-related activity could be observed during saccade-free pursuit and was not predictive of small saccades that often accompanied pursuit. When we plotted the discharge rate from individual neurons during pursuit as a function of the position error associated with the moving target, we found tuning curves with peaks within a few degrees contralateral of the fovea. We compared these pursuit-related tuning curves from each neuron to the tuning curves for a saccade task from which we separately measured the visual, delay, and peri-saccadic activity. We found the highest and most consistent correlation with the delay activity recorded while the monkey viewed parafoveal stimuli during fixation. The directional preferences exhibited during pursuit can therefore be attributed to the tuning of these neurons for contralateral locations near the fovea. These results support the idea that fixation cells are the rostral extension of the buildup neurons found in the more caudal colliculus and that their activity conveys information about the size of the mismatch between a parafoveal stimulus and the currently foveated location. Because the generation of pursuit requires a break from fixation, the pursuit-related activity indicates that these neurons are not strictly involved with maintaining fixation. Conversely, because activity during the delay period was found for many neurons even when no eye movement was made, these neurons are also not obligatorily related to the generation of a movement. Thus the tonic activity of these rostral neurons provides a potential position-error signal rather than a motor command-a principle that may be applicable to buildup neurons elsewhere in the SC.

Modulation of neuronal activity in superior colliculus by changes in target probability

Complex visual scenes require that a target for an impending saccadic eye movement be selected from a larger number of possible targets. We investigated whether changing the probability that a visual stimulus would be selected as the target for a saccade altered activity of monkey superior colliculus (SC) neurons in two experiments. First, we changed the number of possible targets on each trial. Second, we kept the visual display constant and presented a single saccade target repeatedly so that target probability was established over time. Buildup neurons in the SC, those with delay period activity, showed a consistent reduction in activity as the probability of the saccade decreased, independent of the visual stimulus configuration. Other SC neurons, fixation and burst, were largely unaffected by the changes in saccade target probability. Because we had monkeys making saccades to many locations within the visual field, we could examine activity associated with saccades outside of the movement field of neurons. We found the activity of buildup neurons to be similar across the SC, before the target was identified, and reduced when the number of possible targets increased. The results of our experiments are consistent with a role for this activity in establishing a motor set. We found, consistent with this interpretation, that the activity of these neurons was predictive of the latency of a saccadic eye movement and not other saccade parameters such as end point or peak velocity.

Influence of the superior colliculus on the primate blink reflex

In this study we used microstimulation to investigate the influence of the superior colliculus on the trigeminal blink reflex. We report that stimulation in the intermediate to deep layers of the tectum produced inhibition of reflex blinks at a latency of approximately 26 ms. We considered the hypothesis that the blink inhibition was mediated via the omnipause neurons (OPNs) of the eye movement control system in the brainstem. Our results show that the least effective sites for suppression were in the rostral colliculus. This is inconsistent with the prediction that OPNs should be maximally recruited from the rostral tectum near the "fixation zone." From these points and other considerations, we conclude that the reflex blink suppression from the superior colliculus is not directly mediated by the OPNs or the saccadic eye movement circuits.

Modulation of neuronal activity by target uncertainty

Visual scenes are composed of many elements and although we can appreciate a scene as a whole, we can only move our eyes to one element of the scene at a time. As visual scenes become more complex, the number of potential targets in the scene increases, and the uncertainty that any particular one will be selected for an eye movement also increases. How motor systems accommodate this target uncertainty remains unknown. The activities of neurons in both the cerebral cortex and superior colliculus are modulated by this selection process. We reasoned that activity associated with target uncertainty should be evident in the saccadic motor system at the final stages of neural processing, in the superior colliculus. By systematically changing the number of stimuli from which a selection must be made and recording from superior colliculus neurons, we found that as the target uncertainty increased, the neural activity preceding target selection decreased. These results indicate that neurons within the final common pathway for movement generation are active well in advance of the selection of a particular movement. This early activity varies with the probability that a particular movement will be selected.

Shared motor error for multiple eye movements

Most natural actions are accomplished with a seamless combination of individual movements. Such coordination poses a problem: How does the motor system orchestrate multiple movements to produce a single goal-directed action? The results from current experiments suggest one possible solution. Oculomotor neurons in the superior colliculus of a primate responded to mismatches between eye and target positions, even when the animal made two different types of eye movements. This neuronal activity therefore does not appear to convey a command for a specific type of eye movement but instead encodes an error signal that could be used by multiple movements. The use of shared inputs is one possible strategy for ensuring that different movements share a common goal.

Animal model explains the origins of the cranial dystonia benign essential blepharospasm

The current study demonstrates that combining two mild alterations to the rat trigeminal reflex blink system reproduces the symptoms of benign essential blepharospasm, a cranial dystonia characterized by uncontrollable spasms of blinking. The first modification, a small striatal dopamine depletion, reduces the tonic inhibition of trigeminal reflex blink circuits. The second alteration, a slight weakening of the lid-closing orbicularis oculi muscle, begins an adaptive increase in the drive on trigeminal sensory-motor blink circuits that initiates blepharospasm. By themselves, neither of these modifications causes spasms of lid closure, but combined, they induce bilateral forceful blinking and spasms of lid closure. A two-factor model based on these rodent experiments may explain the development of benign essential blepharospasm in humans. The first factor, a subclinical loss of striatal dopamine, creates a permissive environment within the trigeminal blink circuits. The second factor, an external ophthalmic insult, precipitates benign essential blepharospasm. This two-factor model may also be applicable to the genesis of other cranial dystonias.

To blink or not to blink: inhibition and facilitation of reflex blinks

A reflex blink typically inhibits subsequent blinks. In this study, we investigated whether the nature and time course of this inhibition vary when different combinations of blink-evoking stimuli are used. We used the paired stimulus paradigm, in which two blink-evoking stimuli-a conditioning stimulus followed by a test stimulus-are presented with a variety of interstimulus intervals, to examine the interactions between blinks evoked by trigeminal and acoustic stimuli in rats and humans. In addition, we studied the effect of a blink-evoking trigeminal stimulus on subsequent gaze-evoked blinks in humans. The results revealed that long-lasting inhibition occurred when the conditioning and test stimuli were within the same modality. A shorter period of inhibition followed by facilitation occurred when the stimuli were in different modalities. The data demonstrate that a blink-evoking stimulus initiates a lengthy period of inhibition in its own sensory pathway and a shorter period of inhibition in the reticular formation and/or in blink motoneurons. In addition, the results show that the blink-evoking stimulus also initiates a facilitatory process. Thus, the magnitude of a blink reflects a balance between inhibitory and facilitatory processes.

An explanation for reflex blink hyperexcitability in Parkinson's disease. I. Superior colliculus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. We investigated the neural circuit through which a loss of dopamine in the substantia nigra pars compacta (SNc) leads to increased reflex blink excitability. Through its inhibitory inputs to the thalamus, the basal ganglia could modulate the brainstem reflex blink circuits via descending cortical projections. Alternatively, with its inhibitory input to the superior colliculus, the basal ganglia could regulate brainstem reflex blink circuits via tecto-reticular projections. Our study demonstrated that the basal ganglia utilizes its GABAergic input to the superior colliculus to modulate reflex blinks. In rats with previous unilateral 6-hydroxydopamine (6-OHDA) lesions of the dopamine neurons of the SNc, we found that microinjections of bicuculline, a GABA antagonist, into the superior colliculus of both alert and anesthetized rats eliminated the reflex blink hyperexcitability associated with dopamine depletion. In normal, alert rats, decreasing the basal ganglia output to the superior colliculus by injecting muscimol, a GABA agonist, into the substantia nigra pars reticulata (SNr) markedly reduced blink amplitude. Finally, brief trains of microstimulation to the superior colliculus reduced blink amplitude. Histological analysis revealed that effective muscimol microinjection and microstimulation sites in the superior colliculus overlapped the nigrotectal projection from the basal ganglia. These data support models of Parkinsonian symtomatology that rely on changes in the inhibitory drive from basal ganglia output structures. Moreover, they support a model of Parkinsonian reflex blink hyper-excitability in which the SNr-SC target projection is critical.

An explanation for reflex blink hyperexcitability in Parkinson's disease. II. Nucleus raphe magnus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. The first step in the circuit linking the basal ganglia and brainstem reflex blink circuits is the inhibitory nigrostriatal pathway (Basso et al., 1996). The current study reports the circuits linking the superior colliculus (SC) to trigeminal reflex blink circuits. Microstimulation of the deep layers of the SC suppresses subsequent reflex blinks at a latency of 5.4 msec. This microstimulation does not activate periaqueductal gray antinociceptive circuits. The brainstem structure linking SC to reflex blink circuits must suppress reflex blinks at a shorter latency than the SC and produce the same effect on reflex blink circuits as SC stimulation, and removal of the structure must block SC modulation of reflex blinks. Only the nucleus raphe magnus (NRM) meets these requirements. NRM microstimulation suppresses reflex blinks with a latency of 4.4 msec. Like SC stimulation, NRM microstimulation reduces the responsiveness of the spinal trigeminal nucleus. Finally, blocking the receptors for the NRM transmitter serotonin eliminates SC modulation of reflex blinks, and muscimol inactivation of the NRM transiently prevents SC modulation of reflex blinks. Thus, the circuit through which the basal ganglia modulates reflex blinking is (1) the substantia nigra pars reticulata inhibits SC neurons, (2) the SC excites tonically active NRM neurons, and (3) NRM neurons inhibit spinal trigeminal neurons involved in reflex blink circuits.

Not looking while leaping: the linkage of blinking and saccadic gaze shifts

Many vertebrates generate blinks as a component of saccadic gaze shifts. We investigated the nature of this linkage between saccades and blinking in normal humans. Activation of the orbicularis oculi, the lid closing muscle, EMG occurred with 97% of saccadic gaze shifts larger than 33 degrees. The blinks typically began simultaneously with the initiation of head and/or eye movement. To minimize the possibility that the blinks accompanying saccadic gaze shifts were reflex blinks evoked by the wind rushing across the cornea and eye-lashes as the head and eyes turned, the subjects made saccadic head turns with their eyes closed. In this condition, orbicularis oculi EMG activity occurred with all head turns greater than 17 degrees in amplitude and the EMG activity began an average of 39.3 ms before the start of the head movement. Thus, one component of the command for large saccadic gaze shifts appears to be a blink. We call these blinks gaze-evoked blinks. The linkage between saccadic gaze shifts and blinking is reciprocal. Evoking a reflex blink prior to initiating a voluntary saccadic gaze shift dramatically reduces the latency of the initiation of the head movement.

Midbrain 6-hydroxydopamine lesions modulate blink reflex excitability

The blink reflex abnormalities present in the 6 hydroxydopamine (6-OHDA) lesioned rat model of parkinsonism mimicked those of the human with Parkinson's disease. In alert rats, we monitored the long and short latency components of the orbicularis oculi electromyographic (OOemg) response evoked by electrical stimulation of the supraorbital branch of the trigeminal nerve (SO). Two paradigms, habituation and double pulse, provided a measure of blink reflex excitability. In normal rats, repeated stimulation of the SO produced habituation of the R2 component of the blink. In the double pulse paradigm, presentation of two identical SO stimuli resulted in a reduced or suppressed OOemg response to the second stimulus relative to the first. In rats with complete, unilateral lesions of midbrain dopamine neurons, repeated SO stimulation produced facilitation rather than habituation of the R2 component of the blink reflex. This facilitation occurred only with the eyelid contralateral to the lesion. In the double pulse paradigm, the lesioned rats showed increased excitability rather than suppression. This effect occurred bilaterally, although the increased excitability was strongest contralateral to the lesion. Rats with partial lesions of midbrain dopamine neurons exhibited qualitatively similar, but less pronounced blink reflex abnormalities. The R1 component of the blink reflex was unaffected by either the complete or partial lesions. Thus, modification of the blink reflex by 6-OHDA lesions provides a reproducible parkinsonian-like symptom which is amenable to investigations of increases in reflex excitability.

A role for the basal ganglia in nicotinic modulation of the blink reflex

In humans and rats we found that nicotine transiently modifies the blink reflex. For blinks elicited by stimulation of the supraorbital branch of the trigeminal nerve, nicotine decreased the magnitude of the orbicularis oculi electromyogram (OOemg) and increased the latency of only the long-latency (R2) component. For blinks elicited by electrical stimulation of the cornea, nicotine decreased the magnitude and increased the latency of the single component of OOemg response. Since nicotine modified only one component of the supraorbitally elicited blink reflex, nicotine must act primarily on the central nervous system rather than at the muscle. The effects of nicotine could be caused by direct action on lower brainstem interneurons or indirectly by modulating descending systems impinging on blink interneurons. Since precollicular decerebration eliminated nicotine's effects on the blink reflex, nicotine must act through descending systems. Three lines of evidence suggest that nicotine affects the blink reflex through the basal ganglia by causing dopamine release in the striatum. First, stimulation of the substantia nigra mimicked the effects of nicotine on the blink reflex. Second, haloperidol, a dopamine (D2) receptor antagonist, blocked the effect of nicotine on the blink reflex. Third, apomorphine, a D2 receptor agonist, mimicked the effects of nicotine on the blink reflex.