Ciaran Bolger

Dominant frequency content of ocular microtremor from normal subjects

Ocular microtremor (OMT) is a high frequency tremor of the eyes present during fixation and probably related to brainstem activity (Coakley, D. (1983). Minute eye movement and brain stem function. CRC Press, FL.). Published observations on the frequency of OMT have varied widely. Ocular microtremor was recorded in 105 normal healthy subjects using the Piezoelectric strain gauge technique. The dominant frequency content of a signal was determined using the peak counting method. Values recorded ranged from 70 to 103 Hz, the mean frequency being 83.68 Hz (S.D. +/- 5.78 Hz).

Frameless stereotaxy and anterior cervical surgery

OBJECTIVE The role of a frameless image guidance system for anterior cervical surgery was evaluated in 40 patients with cervical spondylosis. An assessment was made of the challenges posed in achieving accurate three-dimensional image registration in the absence of multiple fixed anatomical landmarks, and a solution to this problem is outlined using a novel registration tracking device. MATERIALS AND METHODS Patients underwent preoperative CT scanning of the surgical field of interest, and the image data was reconstructed on a StealthStation (Sofamor Danek, Memphis, Tennessee, USA). An image guidance tracking device was attached to a modified Caspar retractor. Point matching and surface mapping were used for registration. RESULTS It was not possible to achieve accurate registration in the first 7 patients. In the next 5 patients, the time taken to achieve registration was prolonged. For 28 patients, registration was achieved in an acceptable time with an accuracy of 0.74 +/- 0.4 mm. Anatomical resection margins were accurately identified and 36 anterior cervical screws were precisely placed. CONCLUSIONS Frameless stereotaxy can now be applied to procedures involving the anterior cervical spine. Potential benefits include accurate definition of the anatomy, demonstration of resection margins, and intraoperative guidance. The technique may prove particularly useful in cases of complex or disrupted anatomy.

Ocular microtremor in patients with idiopathic Parkinson’s disease

Abnormalities in the oculomotor control mechanism of patients with idiopathic Parkinson’s disease are well recognised. In this study the effect of Parkinson’s disease on tonic output from oculomotor nuclei was studied by using oculomicrotremor as an index of such output. Oculomicrotremor readings were taken from 22 parkinsonian patients and 22 normal healthy volunteers using the piezoelectric strain gauge technique. There was a slower overall tremor frequency, baseline, and burst frequency in the parkinsonian group. There was also a significant increase in the duration of baseline, with a decrease in the number of bursts a second and a decrease in average duration of bursts in the patient group compared with the normal group. One patient, whose medication was withdrawn, showed a marked decrease in mean frequency and baseline frequency with a decrease in number of bursts and increase in baseline duration compared with readings taken when treatment recommenced. These results suggest that variables measured in oculomicrotremor are altered compared with normal subjects, reflecting altered tonic output from oculomotor nuclei in patients with idiopathic Parkinson’s disease.

A technique for frameless stereotaxy and placement of transarticular screws for atlanto-axial instability in rheumatoid arthritis

Abstract The aim of the present study was to outline a new surgical technique and describe how, in a clinical setting, computer-generated image-guidance can assist in the planning and accurate placement of transarticular C1/C2 screws inserted using a minimally invasive exposure. Forty-six patients with atlanto-axial instability due to rheumatoid arthritis underwent posterior stabilisation with transarticular screws. This was achieved with a minimal posterior exposure limited to C1 and C2 and percutaneous screw insertions via minor stab incisions. The Stealth Station (Medtronic Sofamor Danek, Memphis, Tenn., USA) was used for image guidance to navigate safely through C2. Reconstructed computed tomographic (CT) scans of the atlanto-axial complex were used for image guidance. It was possible to perform preoperative planning of the screw trajectory taking into account the position of the intraosseous portion of the vertebral arteries, the size of the pars interarticularis and the quality of bone in C2. Screws could be inserted percutaneously over K-wires using a drill guide linked to the image-guidance system. Preoperative planning was performed in all 46 patients and accurate registration allowed proposed screw trajectories to be identified. Thirty-eight patients had bilateral screws inserted and eight had a unilateral screw. A total of 84 screws were inserted using the Stealth Station. There were no neurovascular injuries. This technique for placing transarticular screws is accurate and safe. It allows a minimally invasive approach to be followed. Image guidance is a useful adjunct for the surgeon undertaking complex spinal procedures.

Ocular microtremor in brain stem death

OBJECTIVE: This study was undertaken to establish whether measurement of ocular microtremor (OMT) activity could be used as a method to establish brain stem death. Presently, the diagnosis of brain stem death can be made using clinical criteria alone. OMT is a high-frequency, low-amplitude physiological tremor of the eye caused by impulses emanating from the brain stem. There have been a number of reports indicating that the recording of OMT may be useful in the assessment of comatose states and in establishing brain stem viability or death. METHODS: We obtained the OMT recordings of 32 patients suspected of having brain stem death using the piezoelectric strain gauge technique. This method involves mounting the piezoelectric probe in a headset and lowering the rubber-tipped end piece onto the anesthetized scleral surface of the subject. The signal produced is recorded on audiomagnetic tape and later played back and analyzed on an electrocardiographic tape analyzer. RESULTS: In 28 patients, initial clinical assessment confirmed the diagnosis of brain stem death and no OMT activity was recorded from these subjects. In three patients in whom initial clinical assessment demonstrated brain stem function, OMT activity was present; when brain stem death was subsequently diagnosed in these three patients, no OMT activity could be demonstrated. In the remaining patient, two of three OMT recordings demonstrated activity in spite of the absence of clinical evidence of brain stem function. A post mortem revealed bacterial cerebritis in this subject. CONCLUSION: The results suggest that OMT is a sensitive method of detecting brain stem life and that it could play an important role in the assessment of brain stem death.

Ocular Microtremor in Oculomotor Palsy

OBJECTIVES Ocular microtremor (OMT) is a high frequency tremor of the eyes present in all individuals. Recent reports suggest that OMT may be a useful indicator of brainstem function. However, the actual origin of ocular microtremor remains controversial. This study aims to provide evidence that OMT has a neurogenic origin. MATERIALS AND METHODS The OMT activity of five subjects with unilateral oculomotor nerve palsy and one subject with complete unilateral internal and external ophthalmoplegia were recorded from both eyes of each subject using the piezoelectric strain gauge technique, with the normal eye acting as a control. Five parameters of OMT activity were studied in each subject: the peak count, the power of the high frequency peak, the percentage power between 60 and 100 Hz, the percentage power between 70 and 80 Hz, and the 10 dB cut-off point. RESULTS In the five subjects with oculomotor nerve palsy, the mean peak count in the normal eye was 88.4 Hz (SD+/-16.9) and in the affected eye was 59 Hz (SD+/-8.6), P < 0.0096. There was also a fall in the peak power, the power between 60 and 100 Hz, and the power between 70 and 80 Hz. In subject six, who had complete opthalmoplegia, there was no evidence of OMT activity in the denervated eye. CONCLUSIONS These results suggest that innervation of the extraocular muscles is necessary for normal OMT activity, and OMT therefore has a neurogenic origin.