Bulletin 16 - Data issues
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Data issues
The number of newly incident cases of SCI reported by the SUs is likely to
understate the true national incidence figure for the following reasons:
- It excludes cases that died from their SCI or other injuries prior to
hospital admission. Detection of damage to the spinal cord and assessment of
its contribution to death is often very difficult. Cain et al.[19], in an assessment of cervical spine injuries in road
traffic crashes in South Australia from 1981 to 1986 found that post-mortem
radiography of the cervical spine detected almost twice as many cervical
injuries as were identified in routine post-mortem examination. Also,
Leditschke et al.[20] found that damage to the
spine was missed in 8% of radiographic examinations. Whilst these studies
provide information on the extent to which damage to the spinal cord may be
missed, the information is of limited utility for the purposes of estimation of
SCI incidence at national level because there is no routine reporting of
post-mortem results.
- It excludes cases of traumatic SCI managed at other hospitals that meet
the CDC clinical definition. It has not been possible to date to determine the
extent to which other hospitals treat SCI cases due to limitations in the
available hospital inpatient separations data. It is probable that other
hospitals manage cases of SCI especially where neurological deficit was of
short duration or the patient died prior to transfer to a SU. Also, it is has
been suggested that children suffering spinal cord damage, expected to number
less than four per year, are generally managed in paediatric
hospitals.
All-ages rates have been adjusted to overcome the effect of differences in the
proportions of people of different ages (and different injury risks) in the
populations that are compared. Direct standardisation was employed, taking the
Australian population in 1991 as the standard.
All newly incident spinal cord injuries treated at SUs are submitted to ASCIR
for registration, so sampling errors do not apply to these data. However, the
time periods used to group the cases (ie. Fiscal years) are arbitrary. Use of
another period (e.g. Calendar year) can result in different rates. Where case
numbers are small, as they are with spinal cord injury, the effect of chance
variation on the rates can be large. Confidence intervals (95%, based on a
Poisson assumption about the number of cases in a time period) have been placed
around rates as a guide to the size of this variation. Chance variation alone
would be expected to lead to a rate outside the interval only once out of 20
occasions. An extreme rate in a single period of enumeration should not be
ignored simply because of a wide confidence interval--a time series may show
such a rate to be part of a more significant pattern.
[19] Cain, CMJ, Ryan, GA, Fraser, R et al.
Cervical spine injuries in road traffic crashes in South Australia, 1981-86.
Aust. NZ J Surg 1989; 59: 15-19.
[20] Leditschke, J, Anderson, RMD, Hare, WSC.
The cervical spine in fatal motor vehicle accidents. Clinical and Experimental
Neurology 1992; 29: 263-271.
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