A good outcome after absence of bilateral N20 SSEPs post-cardiac arrest

Abstract

A 51-year-old man suffered a cardiac arrest after an attempted hanging. Post-arrest assessment revealed the bilateral absence of negative 20 somatosensory evoked potentials (N20 SSEPs) which is suggestive of a poor neurological outcome. Current evidence recommends its use in prognostication. Our patient made a good recovery which brings into question the value of negative 20 somatosensory evoked potentials in prognostication with concomitant neck injuries and swelling.

Case history

A 51-year-old man presented to the Emergency Department following a failed hanging attempt. Pre-hospital management included cardiopulmonary resuscitation for a pulseless electrical activity rhythm, with a return of output after one cycle. On arrival, he was unconscious with a Glasgow coma scale (GCS) of 3, hypertensive (209/121), hypoxic (saturations of 84% on 15 L oxygen via a non-rebreathe mask), and tachycardic (heart rate 125 beats per minute). Moderate swelling was noted around his neck where the ligature had been.

Pupils were 4 mm bilaterally and sluggishly reactive to light. Blood gases showed a lactic acidosis with pH 7.01, pO₂ 11.8, pCO₂ 6.2, base excess −14 and a lactate of 6.59.

Bloods revealed the absence of paracetamol and salicylate levels. No other toxicology was measured. Immediate management consisted of intubation and ventilation due to his low GCS and hypoxia. Computed tomography (CT) imaging of the head and neck was normal. The patient was cooled to 36℃ for 24 h post-arrest.

On Day 2, he developed generalised tonic clonic seizures whilst on propofol and alfentanil sedation. The seizures appeared to self-resolve after approximately one minute; however an electroencephalogram (EEG) was performed on day 2 which confirmed non-convulsive status. He was treated with phenytoin and lamotrigine. An EEG done on day 4 showed that the non convulsive status had resolved. He remained cardiovascularly stable throughout his admission and did not require any vasopressors. Renal function was normal throughout and he did not require any support. Daily haematology, biochemistry and blood sugars were within normal limits.

Sedation was stopped on Day 3. On Day 5, his GCS remained poor at 4 (Eyes 1, Motor 2, Verbal 1) and so brain magnetic resonance imaging (MRI) and somatosensory evoked potentials (SSEP) investigations were carried out. The MRI was normal. N20 SSEPs conducted on Days 5 and 6 at normothermia were absent bilaterally.

N13 SSEPs were also absent bilaterally on both occasions, whereas N9 SSEPs were present.

On Day 6, clinical assessment revealed the presence of spontaneous eye opening and bilateral pupillary, and corneal responses. Motor assessment revealed an extensor response to pain. Moderate reduction of the neck swelling was observed.

Repeat of the N20 SSEPs on Day 7 showed clear bilateral potentials with normal latency. N13 SSEPs were now also present along with the N9 SSEPs.

On Day 7, the patient had a GCS of 8 (E4, V1, M3) and was extubated successfully. Our patient was discharged from the intensive care unit on Day 12 with a GCS of 8. On Day 20, he was assessed as being Cerebral Performance Category 2 which is classed as a good outcome. ¹ Our patient’s Barthel index score was 80/100 (moderate dependence). ²

Discussion

The task of prognosticating a patient after cardiac arrest is challenging. A multi-disciplinary team and a range of both clinical examination and investigations are required to aid this decision-making process. This is vital to ensure families can be appropriately advised and any futile treatment can be withdrawn.

Clinical examination, electrophysiological studies, neurological biomarkers, and neuroimaging are often used to aid the process, but they all have their limitations.

N20 SSEP monitoring is used to prognosticate coma patients. N20 (negative peak at 20 ms) SSEPs are measured non-invasively by transcutaneous stimulation of the median nerve at the wrist. Electrodes placed over the contralateral skull detect primary cortical somatosensory electrical signals.

N13 SSEPs are recorded by electrodes placed on the dorsal neck, reflecting postsynaptic activity in the cervical cord. N9 SSEPs are recorded from Erb’s point (2–3 cm above the clavicle over the brachial plexus) and detect signals from the peripheral nerve.

The use of N20 SSEPs in this way has been established due to its 100% specificity for patients not recovering from an anoxic coma if bilaterally absent.^3,4 The bilateral absence is caused by widespread cortical necrosis. ⁵

The 2006 American Academy of Neurology used absence of bilateral N20 SSEPs in their post-cardiac arrest prognostication decision algorithm, quoting a false positive rate of 0.7%. ⁶ In spite of the advent of therapeutic hypothermia, the documented reliability of SSEPs has remained high. ⁷

The European Resuscitation Council’s 2014 advisory statement suggests that the bilateral absence of N20 SSEPs in predicting a poor neurological outcome has a false positive rate of 0% (0%–3%). They have recommended its use. ⁸ Three false positive cases were concluded as being undeterminable due to excessive noise.

Figure 1.

Example of a normal N20 SSEP.

Figure 2.

Absent N20 SSEP in our patient on Day 5.

Figure 3.

Electrode positioning for SSEP monitoring.

Our findings are in keeping with another case of a good outcome (barthel index 90/100) post-absent bilateral N20 SSEPs. This patient was, however, not therapeutically cooled and had not sustained a neck injury. ⁷

A possible explanation for the false positive result in this man could be that the attempted hanging caused distension and inflammation of the neck. This may have caused disruption in the nerve transmission pathway, preventing accurate potential detection. The absence of N20 SSEPs in this case therefore may not have been an indicator of severe cerebral injury.

The N9 SSEPs might not have been affected in this way as this conduction pathway does not cross the area of neck swelling (it runs between Erb's point and the wrist).

The resolution of N13 and N20 SSEPs on the subsequent test in this man may be due to the associated reduction in the neck swelling.

It is highly unlikely that the findings can be attributed to operator error as all the tests were conducted by the same team. As such, this calls into question the accuracy of using N20 SSEPs in prognosticating patients with injuries causing swelling and inflammation to the neck.

Further limitations of N20 SSEPs include interpretation reproducibility, moderate inter-observer agreement secondary to noise levels, and the lack of availability in many hospitals. Its prognostic accuracy can also further be affected by SSEP stimulus rate.

In deciding the management and prognosis of this patient, multiple clinical factors and investigations were considered, not all of which suggested a poor prognosis (normal CT and MRI; only one cycle of cardiopulmonary resuscitation). Not only does this case highlight the limitations and uncertainty surrounding the use of N20 SSEPs, but also the hazards and potential repercussions of relying too heavily on a single test when prognosticating patients or deciding the best course of management.

Conclusion

This is a rare presentation where poor neurology on clinical assessment along with absent N20 SSEPs resolved with a good neurological outcome.

We feel that the presence of concomitant neck injuries greatly reduces the validity of N20 SSEP use in prognostication, especially in the absence of N13 SSEPs.

Withdrawing life sustaining treatment in this group of patients based on this finding may be inappropriate. Further research is essential to accurately determine the place of N20 SSEPs in future prognostication.