While levels and severity (AIS) are basically applicable in all injuries, the ZPPs are not. In most of the incomplete injuries, sensorimotor functions extend to the lowest sacral segments. Therefore, in previous ISNCSCI revisions, ZPPs were only applicable in complete (AIS grade A) injuries defined by the total absence of sensory and motor functions in the lowest sacral segments [15]. However, the ASIA International Standards Committee agreed that this scope had limited face validity: (1) It did not match the concept of providing spinal segmental levels separately for each assessment modality, namely the sensory and motor levels, and (2) it did not allow the use of ZPPs for clinical characterization of patients with incomplete injuries where only motor or sensory function is absent in the lowest sacral segments. With the current 2019 ISNCSCI revision, the scope of the ZPPs was redefined so that the applicability of motor and sensory ZPPs is checked separately.

The committee considers the basic definition of the ZPP, which “refers to those dermatomes and myotomes caudal to the sensory and motor levels that remain partially innervated” [15, 16] to have high face validity. Therefore, neither this basic ZPP definition nor the wording was changed for the 2019 ISNCSCI revision. The committee explicitly wanted to emphasize that the ZPP starts rostrally at the sensory/motor level and ends at the most caudal segment with preserved sensory/motor function on that side [10]. This means that four independent zones of partial preservation, the right and left motor ZPPs as well as the right and left sensory ZPPs, together with their lengths are defined. In respect to clinical meaningfulness, however, it makes sense to restrict the application of the ZPP to those situations where the caudal extent of that ZPP does not reach to the lowest sacral segments. In the previous ISNCSCI revision [15], all four ZPPs were defined to be only applicable in AIS A injuries with fully absent motor (VAC) and sensory (LT and PP sensation in S4-5, DAP) functions in the lowest sacral segments [15].

The recording of motor ZPPs in cases with absent VAC is of substantial clinical value, because they provide complementary information to the Upper and Lower Extremity Motor Scores regarding the extent of the preserved motor functions below the motor level.

With the revised ZPP, motor ZPPs are applicable in approximately 1/3 of all datasets with incomplete injuries in a benchmark dataset of individuals with traumatic SCI queried from the EMSCI database. We found a slightly higher applicability in the early phase (43.5%) as compared to late phase after SCI (28%) due to the recovery of voluntary anal contraction over time.

In incomplete injuries, motor ZPPs are much more frequent than sensory ZPPs. A sensory ZPP is only defined in incomplete injuries with present VAC, but no sensory function in the lowest sacral segments on a given side of the body. This constellation was found to be rather rare in the analyzed EMSCI dataset, but is in line with other data sources, e.g. the Spinal Cord Injury Model Systems (SCIMS) database (e.g. Table 3 of [12]). A SCIMS analysis of the initial AIS grades reveals that present VAC and absence of any sensation in S4-5 (LT, PP and DAP) is found in 3.2% (EMSCI: 3.4%) of all AIS C patients and in 0.1% (EMSCI: 0%) of all AIS D patients. Sparing of sensory, but not motor (VAC) function in the lowest sacral segments is found in 40.6% (EMSCI: 51.7%) of all AIS C patients and in 12% (EMSCI: 12.5%) of all AIS D patients. These numbers obtained independently in two large, representative cohorts of individuals with SCI underline the rationale and usefulness of the refined ZPP scope with which motor ZPPs can be reported in all AIS B injuries, approx. 50% of all AIS C and approx. 10% of all AIS D injuries.

An important question is the significance of the revised ZPP definition for prediction of neurological recovery. This question will be analyzed in detail in a follow-up publication. It is anticipated that prediction models using ZPP variables from within the first week after injury will even more accurately predict the outcome 1 year after injury than from the average assessment time point of 11.8 ± 7.6 days after injury applied in this study. A recent review [17] concludes that examinations conducted within the first 24 h to one week after injury are highly reliable and predictive of the late neurological outcome.

This revision implies some noteworthy features and characteristics, which are discussed in the following ”frequently asked question”-alike subchapters.

Is the revised ZPP compatible with previous revisions?

The ASIA International Standards Committee placed a strong focus on backward compatibility. In particular, the revised ZPP definition is compatible in patients with complete SCI, for which all ZPPs determined according to former revisions are identical to ZPPs determined with the 2019 revision. However, with the 2019 revision, more cases will have applicable ZPPs, predominantly motor ZPPs.

This feature will help to compare populations of existing studies with those of future studies. In future studies, authors are strongly advised to report the ZPPs distributions grouped by AIS. The ZPP distribution of the AIS A group equals to the distribution of the ZPPs according to the 2015 ISNCSCI update [15].

Can a sensory ZPP be present only on one side of the body?

While motor ZPPs are always applicable bilaterally in cases with absent VAC, the revised ZPP definition implies the possibility of a unilateral sensory ZPP. A unilateral sensory ZPP occurs, when DAP is absent, but sensory function in S4-5 is preserved only on one side of the body. Figure 3 exemplarily depicts such a case with absent DAP and a unilateral sensory ZPP of T6 on the right side, but preserved sensory function in S4-5 on the left and therefore a non-applicable left sensory ZPP. However, such a constellation occurs very rarely, which is supported by our analysis of the EMSCI datasets with only 5 of 1330 ISNCSCI datasets having a unilateral sensory ZPP.

Are non-key muscle functions incorporated in the determination of ZPPs?

The ASIA International Standards Committee agreed many years ago that preserved functions of non-key muscles are NOT relevant for AIS classification except for the differentiation between AIS grade B and grades C/D: In an individual with an apparent AIS B classification, non-key muscle functions more than 3 levels below the motor level on each side should be tested to most accurately classify the injury, i.e. to differentiate between AIS grade B and grades C/D. In ISNCSCI instructional courses [18], it is typically trained to ask sensory incomplete but motor complete patients as last step in the motor examination: “Can you move anything else [below the motor level], which I have not tested yet?”

ISNCSCI worksheet (backside) and booklet (page 35) [16] contain a reference table of important muscle functions with an assignment to a corresponding spinal root level. A case, which would be classified as AIS grade B based on the examination results of the key muscles only, is classified as motor incomplete and therefore AIS grade C, if the most caudal root level of preserved non-key muscle functions is located more than three segments below the ipsilateral motor level. Figure 4 depicts such a case, which is classified as AIS grade C due to a sensory and motor level at T6 and preserved hip adduction associated with segment L2 on the left side. As this non-key muscle function more than 3 segments below the motor level is decisive for the AIS C classification, the corresponding caudal extent of the left motor ZPP is also set to L2.

Fig. 4: International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) case with the motor zone of partial preservation (ZPP) determined by a preserved non-key muscle function.

The left motor level in this case is T5 with preserved left hip adduction associated to the spinal segment L2. Due to the preservation of sensory function in the lowest sacral segment S4-5, preserved motor function more than three segments below the motor level, but absent lower extremity motor functions, this individual is classified as AIS grade C. Because of the impact of the preserved non-key muscle function in the left L2 segment on the American Spinal Injury Association Impairment Scale classification, L2 is recorded as left motor ZPP.

When to use ‘not applicable (NA)’ or ‘not determinable (ND)’?

The ZPP boxes in the right lower corner of the worksheet should always be completed. Empty boxes should be avoided to clearly indicate that the examiner did not simply forget to fill in the box. The committee decided that NA (‘not applicable’) should be recorded, when a particular ZPP is not applicable versus leaving the box blank.

Motor ZPPs are not applicable, if VAC is present. Sensory ZPPs are not applicable, if DAP is present. If DAP is absent, a sensory ZPP is not applicable when LT or PP sensation is preserved (scored one or two) in S4-5 on the respective body side.

In other words, the sensory / motor ZPPs are only applicable when sensory / motor function is absent in the lowest sacral segments. Anecdotally, beginners of ISNCSCI often record S4-5 as sensory ZPPs in incomplete injuries (unpublished results from the ISNCSCI instructional courses conducted in the EMSCI network [19]). Although, S4-5 and NA basically convey the same information, the recording of NA is recommended to clearly indicate that function is preserved at the lowest sacral segments and there is no absent function caudal to the ZPP.

ZPPs might be not determinable (ND), if the ISNCSCI dataset contains not testable (NT) score(s) [13]. For example, a motor ZPP is not determinable, if a motor score below the motor level is NT and motor function is totally absent in all key muscles caudal to this NT myotome. In this situation, the motor ZPP is not uniquely determinable as it depends on the “real” motor score of the not testable myotome. A motor score graded 1 or better would lead to a motor ZPP at that segment. A motor score of 0 would result in a more rostral motor ZPP. As the motor ZPP starts at the motor level and ends at the most caudal segment with preserved motor function, the motor level has to be considered in the decision whether a motor ZPP is defined or not. If the motor level is ND and no motor functions are preserved caudal to the not-determinable motor level, the motor ZPP equals the motor level and is therefore also not determinable.

Please see Schuld et al. [13] for a comprehensive discussion of this determinability problem not only for ZPPs but for all ISNCSCI classification variables.

Not testable (NT) is only used for grading of the sensory and motor examination (including DAP and VAC), and does not apply to any classification variable like the levels, the AIS or the ZPPs.

When to tag ZPPs with an asterisk?

The 2019 ISNCSCI revision introduced a taxonomy for documentation of non-SCI conditions which are impacting the examination results and the classification. There might be cases, when ZPPs need to be tagged with an asterisk to indicate that they are based on clinical assumptions. More details can be found in [20].