436
EQUINE VETERINARY EDUCATION / AE / AUGUST 2019
as well to identify vascular structures and is often used in human patients (Marhofer et al. 2005), although blood flow or pulsation can often easily be identified in horses without the use of colour flow Doppler (O’Neill et al. 2014).
Inferior alveolar nerve block In man, the use of ultrasound guidance has been described for the desensitisation of the inferior alveolar nerve block (Hannan et al. 1999; Chanpong et al. 2013). In order to visualise the inferior alveolar nerve that runs medial of the mandibular ramus, the ultrasound probe is placed intraorally medial to the mandibular ramus (Hannan et al. 1999; Chanpong et al. 2013). Introduction of the technique has substantially improved outcome compared to the formerly used blind technique (Chanpong et al. 2013), of which failure rates were as high as 60% (Montagnese et al. 1984) and this position of the ultrasound probe helps to determine the inferior alveolar nerve. In literature, failure rates of up to 62% for the inferior alveolar block have been described in older human studies (Montagnese et al. 1984), greatly caused by anatomical variation and blind needle placement. Ultrasound guidance may improve the outcome of the inferior alveolar nerve blocks (Chanpong et al. 2013) and, at the same time, it could decrease the risk of potential side effects such as vascular punctures. This technique seems rather impractical in the equine head because of the long mouth, but, alternatively, ultrasound guidance could potentially be applicable in the equine head as well with a modified approach from the ventromedial aspect of the mandibular ramus. This technique has not yet been described in the horse and may prove impractical due to the deep location of the nerve.
a)
Retrobulbar block In man, ultrasound-guided retrobulbar blocks have been described by Luyet et al. (2008). They used a cranial approach and ultrasound guidance allowed the needle tip to be advanced up to 2 mm from the optic nerve. No side effects such as contrast injection into the eyeball or into the optic nerve were seen. For ultrasonography of the retrobulbar space by the transbulbar approach, curvilinear array transducers are most suitable at low to intermediate frequencies of 3–8 MHz. Phased array or linear array transducers can be used, but produce a less optimal image for needle guidance. Two indices, thermal index (TI) and mechanical index (MI), are denotive of heat and mechanical agitation that are generated by every ultrasonographic transducer. Low TI- and MI-values are preferred for ultrasonography of the eye (Morath et al. 2013). For the human eye, the maximally allowed TI is 1.0 and the maximally allowed MI is 0.23 (these values are much lower for the eye compared to other tissues). In the horse, the technique of the ultrasound-guided
retrobulbar block has been explored in a cadaver study by Morath et al. (2013). The effect of the volume that was
injected was assessed, as was desensitisation of the orbital fissure after both intra- and extraconal injection of contrast medium. The study used a caudal (supraorbital) approach (comparable to the technique shown in Fig 6) and the spread of contrast medium was evaluated by computed tomography (CT). Needle placement within the cone formed by the retractor bulbi muscles was found to lead to a more
effective spread of the injected fluid towards the orbital fissure and the subjective evaluation of ultrasound performance appeared to correlate well with the results of
b)
c)
d)
A B
Fig 6: Ultrasound-guided retrobulbar nerve block. a) Positioning of the ultrasound probe and the needle on the head. b) Needle placement with respect to the bony landmarks. c) Extrinsic straight eye muscles (A) within the covering fascia (cone) and the optic nerve in the centre of the straight muscles (B). d) Ultrasound image of retrobulbar needle placement and associated structures (white arrows show needle placement with tip at the height of the right arrow, black arrow shows the optic nerve).
© 2017 EVJ Ltd
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88
